diff --git a/src/Avalonia.Controls/AppBuilderBase.cs b/src/Avalonia.Controls/AppBuilderBase.cs
index d44b2ab0db..8779ae9122 100644
--- a/src/Avalonia.Controls/AppBuilderBase.cs
+++ b/src/Avalonia.Controls/AppBuilderBase.cs
@@ -14,9 +14,9 @@ namespace Avalonia.Controls
     public abstract class AppBuilderBase<TAppBuilder> where TAppBuilder : AppBuilderBase<TAppBuilder>, new()
     {
         private static bool s_setupWasAlreadyCalled;
-        private Action _optionsInitializers;
-        private Func<Application> _appFactory;
-        private IApplicationLifetime _lifetime;
+        private Action? _optionsInitializers;
+        private Func<Application>? _appFactory;
+        private IApplicationLifetime? _lifetime;
         
         /// <summary>
         /// Gets or sets the <see cref="IRuntimePlatform"/> instance.
@@ -31,32 +31,32 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the <see cref="Application"/> instance being initialized.
         /// </summary>
-        public Application Instance { get; private set; }
+        public Application? Instance { get; private set; }
         
         /// <summary>
         /// Gets the type of the Instance (even if it's not created yet)
         /// </summary>
-        public Type ApplicationType { get; private set; }
+        public Type? ApplicationType { get; private set; }
         
         /// <summary>
         /// Gets or sets a method to call the initialize the windowing subsystem.
         /// </summary>
-        public Action WindowingSubsystemInitializer { get; private set; }
+        public Action? WindowingSubsystemInitializer { get; private set; }
 
         /// <summary>
         /// Gets the name of the currently selected windowing subsystem.
         /// </summary>
-        public string WindowingSubsystemName { get; private set; }
+        public string? WindowingSubsystemName { get; private set; }
 
         /// <summary>
         /// Gets or sets a method to call the initialize the windowing subsystem.
         /// </summary>
-        public Action RenderingSubsystemInitializer { get; private set; }
+        public Action? RenderingSubsystemInitializer { get; private set; }
 
         /// <summary>
         /// Gets the name of the currently selected rendering subsystem.
         /// </summary>
-        public string RenderingSubsystemName { get; private set; }
+        public string? RenderingSubsystemName { get; private set; }
 
         /// <summary>
         /// Gets or sets a method to call after the <see cref="Application"/> is setup.
@@ -126,7 +126,7 @@ namespace Avalonia.Controls
         /// <typeparam name="TMainWindow">The window type.</typeparam>
         /// <param name="dataContextProvider">A delegate that will be called to create a data context for the window (optional).</param>
         [Obsolete("Use either lifetimes or AppMain overload. See see https://github.com/AvaloniaUI/Avalonia/wiki/Application-lifetimes for details")]
-        public void Start<TMainWindow>(Func<object> dataContextProvider = null)
+        public void Start<TMainWindow>(Func<object>? dataContextProvider = null)
             where TMainWindow : Window, new()
         {
             AfterSetup(builder =>
@@ -134,7 +134,7 @@ namespace Avalonia.Controls
                 var window = new TMainWindow();
                 if (dataContextProvider != null)
                     window.DataContext = dataContextProvider();
-                ((IClassicDesktopStyleApplicationLifetime)builder.Instance.ApplicationLifetime)
+                ((IClassicDesktopStyleApplicationLifetime)builder.Instance!.ApplicationLifetime!)
                     .MainWindow = window;
             });
             
@@ -155,7 +155,7 @@ namespace Avalonia.Controls
         public void Start(AppMainDelegate main, string[] args)
         {
             Setup();
-            main(Instance, args);
+            main(Instance!, args);
         }
 
         /// <summary>
@@ -226,8 +226,8 @@ namespace Avalonia.Controls
             var platformClassName = assemblyName.Replace("Avalonia.", string.Empty) + "Platform";
             var platformClassFullName = assemblyName + "." + platformClassName;
             var platformClass = assembly.GetType(platformClassFullName);
-            var init = platformClass.GetRuntimeMethod("Initialize", Type.EmptyTypes);
-            init.Invoke(null, null);
+            var init = platformClass!.GetRuntimeMethod("Initialize", Type.EmptyTypes);
+            init!.Invoke(null, null);
         };
 
         public TAppBuilder UseAvaloniaModules() => AfterSetup(builder => SetupAvaloniaModules());
@@ -251,7 +251,7 @@ namespace Avalonia.Controls
                                              where constructor.GetParameters().Length == 0 && !constructor.IsStatic
                                              select constructor).Single() into constructor
                                      select (Action)(() => constructor.Invoke(Array.Empty<object>()));
-            Delegate.Combine(moduleInitializers.ToArray()).DynamicInvoke();
+            Delegate.Combine(moduleInitializers.ToArray())!.DynamicInvoke();
         }
 
         /// <summary>
@@ -292,6 +292,11 @@ namespace Avalonia.Controls
                 throw new InvalidOperationException("No rendering system configured.");
             }
 
+            if (_appFactory == null)
+            {
+                throw new InvalidOperationException("No Application factory configured.");
+            }
+
             if (s_setupWasAlreadyCalled && CheckSetup)
             {
                 throw new InvalidOperationException("Setup was already called on one of AppBuilder instances");
diff --git a/src/Avalonia.Controls/Application.cs b/src/Avalonia.Controls/Application.cs
index 584c3db23b..9f7f54d293 100644
--- a/src/Avalonia.Controls/Application.cs
+++ b/src/Avalonia.Controls/Application.cs
@@ -177,13 +177,13 @@ namespace Avalonia
         /// </summary>
         public IApplicationLifetime? ApplicationLifetime { get; set; }
 
-        event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesAdded
+        event Action<IReadOnlyList<IStyle>>? IGlobalStyles.GlobalStylesAdded
         {
             add => _stylesAdded += value;
             remove => _stylesAdded -= value;
         }
 
-        event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesRemoved
+        event Action<IReadOnlyList<IStyle>>? IGlobalStyles.GlobalStylesRemoved
         {
             add => _stylesRemoved += value;
             remove => _stylesRemoved -= value;
diff --git a/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs b/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs
index edddf31d45..76e2d3a161 100644
--- a/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs
+++ b/src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs
@@ -15,26 +15,26 @@ namespace Avalonia.Controls.ApplicationLifetimes
     public class ClassicDesktopStyleApplicationLifetime : IClassicDesktopStyleApplicationLifetime, IDisposable
     {
         private int _exitCode;
-        private CancellationTokenSource _cts;
+        private CancellationTokenSource? _cts;
         private bool _isShuttingDown;
         private HashSet<Window> _windows = new HashSet<Window>();
 
-        private static ClassicDesktopStyleApplicationLifetime _activeLifetime;
+        private static ClassicDesktopStyleApplicationLifetime? _activeLifetime;
         static ClassicDesktopStyleApplicationLifetime()
         {
             Window.WindowOpenedEvent.AddClassHandler(typeof(Window), OnWindowOpened);
             Window.WindowClosedEvent.AddClassHandler(typeof(Window), WindowClosedEvent);
         }
 
-        private static void WindowClosedEvent(object sender, RoutedEventArgs e)
+        private static void WindowClosedEvent(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Remove((Window)sender);
-            _activeLifetime?.HandleWindowClosed((Window)sender);
+            _activeLifetime?._windows.Remove((Window)sender!);
+            _activeLifetime?.HandleWindowClosed((Window)sender!);
         }
 
-        private static void OnWindowOpened(object sender, RoutedEventArgs e)
+        private static void OnWindowOpened(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Add((Window)sender);
+            _activeLifetime?._windows.Add((Window)sender!);
         }
 
         public ClassicDesktopStyleApplicationLifetime()
@@ -46,24 +46,24 @@ namespace Avalonia.Controls.ApplicationLifetimes
         }
 
         /// <inheritdoc/>
-        public event EventHandler<ControlledApplicationLifetimeStartupEventArgs> Startup;
+        public event EventHandler<ControlledApplicationLifetimeStartupEventArgs>? Startup;
 
         /// <inheritdoc/>
-        public event EventHandler<ShutdownRequestedEventArgs> ShutdownRequested;
+        public event EventHandler<ShutdownRequestedEventArgs>? ShutdownRequested;
 
         /// <inheritdoc/>
-        public event EventHandler<ControlledApplicationLifetimeExitEventArgs> Exit;
+        public event EventHandler<ControlledApplicationLifetimeExitEventArgs>? Exit;
 
         /// <summary>
         /// Gets the arguments passed to the AppBuilder Start method.
         /// </summary>
-        public string[] Args { get; set; }
+        public string[]? Args { get; set; }
         
         /// <inheritdoc/>
         public ShutdownMode ShutdownMode { get; set; }
         
         /// <inheritdoc/>
-        public Window MainWindow { get; set; }
+        public Window? MainWindow { get; set; }
 
         public IReadOnlyList<Window> Windows => _windows.ToList();
 
@@ -183,7 +183,7 @@ namespace Avalonia.Controls.ApplicationLifetimes
             return true;
         }
         
-        private void OnShutdownRequested(object sender, ShutdownRequestedEventArgs e) => DoShutdown(e);
+        private void OnShutdownRequested(object? sender, ShutdownRequestedEventArgs e) => DoShutdown(e);
     }
     
     public class ClassicDesktopStyleApplicationLifetimeOptions
diff --git a/src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs b/src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs
index a83229b732..2bd5c1238d 100644
--- a/src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs
+++ b/src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs
@@ -20,7 +20,7 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// <see cref="ClassicDesktopStyleApplicationLifetimeExtensions.StartWithClassicDesktopLifetime{T}(T, string[], ShutdownMode)"/>
         /// method.
         /// </summary>
-        string[] Args { get; }
+        string[]? Args { get; }
         
         /// <summary>
         /// Gets or sets the <see cref="ShutdownMode"/>. This property indicates whether the application is shutdown explicitly or implicitly. 
@@ -38,7 +38,7 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// <value>
         /// The main window.
         /// </value>
-        Window MainWindow { get; set; }
+        Window? MainWindow { get; set; }
         
         IReadOnlyList<Window> Windows { get; }
 
@@ -58,6 +58,6 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// will try to close each non-owned open window, invoking the <see cref="Window.Closing"/> event on each and allowing
         /// each window to cancel the shutdown of the application. Windows cannot however prevent OS shutdown.
         /// </remarks>
-        event EventHandler<ShutdownRequestedEventArgs> ShutdownRequested;
+        event EventHandler<ShutdownRequestedEventArgs>? ShutdownRequested;
     }
 }
diff --git a/src/Avalonia.Controls/AutoCompleteBox.cs b/src/Avalonia.Controls/AutoCompleteBox.cs
index 0e946126ea..930e250334 100644
--- a/src/Avalonia.Controls/AutoCompleteBox.cs
+++ b/src/Avalonia.Controls/AutoCompleteBox.cs
@@ -69,7 +69,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <value>The text that is used to determine which items to display in
         /// the <see cref="T:Avalonia.Controls.AutoCompleteBox" />.</value>
-        public string Parameter { get; private set; }
+        public string? Parameter { get; private set; }
 
         /// <summary>
         /// Initializes a new instance of the
@@ -79,7 +79,7 @@ namespace Avalonia.Controls
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.SearchText" />
         /// property, which is used to filter items for the
         /// <see cref="T:Avalonia.Controls.AutoCompleteBox" /> control.</param>
-        public PopulatingEventArgs(string parameter)
+        public PopulatingEventArgs(string? parameter)
         {
             Parameter = parameter;
         }
@@ -98,7 +98,7 @@ namespace Avalonia.Controls
     /// <typeparam name="T">The type used for filtering the
     /// <see cref="T:Avalonia.Controls.AutoCompleteBox" />. This type can
     /// be either a string or an object.</typeparam>
-    public delegate bool AutoCompleteFilterPredicate<T>(string search, T item);
+    public delegate bool AutoCompleteFilterPredicate<T>(string? search, T item);
 
     /// <summary>
     /// Specifies how text in the text box portion of the
@@ -245,7 +245,7 @@ namespace Avalonia.Controls
     /// <see cref="T:Avalonia.Controls.AutoCompleteBox" />.
     /// This type can be either a string or an object.
     /// </typeparam>
-    public delegate string AutoCompleteSelector<T>(string search, T item);
+    public delegate string AutoCompleteSelector<T>(string? search, T item);
 
     /// <summary>
     /// Represents a control that provides a text box for user input and a
@@ -275,19 +275,19 @@ namespace Avalonia.Controls
         /// </summary>
         private const string ElementTextBox = "PART_TextBox";
 
-        private IEnumerable _itemsEnumerable;
+        private IEnumerable? _itemsEnumerable;
 
         /// <summary>
         /// Gets or sets a local cached copy of the items data.
         /// </summary>
-        private List<object> _items;
+        private List<object>? _items;
 
         /// <summary>
         /// Gets or sets the observable collection that contains references to
         /// all of the items in the generated view of data that is provided to
         /// the selection-style control adapter.
         /// </summary>
-        private AvaloniaList<object> _view;
+        private AvaloniaList<object>? _view;
 
         /// <summary>
         /// Gets or sets a value to ignore a number of pending change handlers.
@@ -338,7 +338,7 @@ namespace Avalonia.Controls
         /// Gets or sets the DispatcherTimer used for the MinimumPopulateDelay
         /// condition for auto completion.
         /// </summary>
-        private DispatcherTimer _delayTimer;
+        private DispatcherTimer? _delayTimer;
 
         /// <summary>
         /// Gets or sets a value indicating whether a read-only dependency
@@ -351,47 +351,47 @@ namespace Avalonia.Controls
         /// <summary>
         /// The TextBox template part.
         /// </summary>
-        private TextBox _textBox;
-        private IDisposable _textBoxSubscriptions;
+        private TextBox? _textBox;
+        private IDisposable? _textBoxSubscriptions;
 
         /// <summary>
         /// The SelectionAdapter.
         /// </summary>
-        private ISelectionAdapter _adapter;
+        private ISelectionAdapter? _adapter;
 
         /// <summary>
         /// A control that can provide updated string values from a binding.
         /// </summary>
-        private BindingEvaluator<string> _valueBindingEvaluator;
+        private BindingEvaluator<string>? _valueBindingEvaluator;
 
         /// <summary>
         /// A weak subscription for the collection changed event.
         /// </summary>
-        private IDisposable _collectionChangeSubscription;
+        private IDisposable? _collectionChangeSubscription;
 
-        private Func<string, CancellationToken, Task<IEnumerable<object>>> _asyncPopulator;
-        private CancellationTokenSource _populationCancellationTokenSource;
+        private Func<string?, CancellationToken, Task<IEnumerable<object>>>? _asyncPopulator;
+        private CancellationTokenSource? _populationCancellationTokenSource;
 
         private bool _itemTemplateIsFromValueMemberBinding = true;
         private bool _settingItemTemplateFromValueMemberBinding;
 
-        private object _selectedItem;
+        private object? _selectedItem;
         private bool _isDropDownOpen;
         private bool _isFocused = false;
 
-        private string _text = string.Empty;
-        private string _searchText = string.Empty;
+        private string? _text = string.Empty;
+        private string? _searchText = string.Empty;
 
-        private AutoCompleteFilterPredicate<object> _itemFilter;
-        private AutoCompleteFilterPredicate<string> _textFilter = AutoCompleteSearch.GetFilter(AutoCompleteFilterMode.StartsWith);
+        private AutoCompleteFilterPredicate<object?>? _itemFilter;
+        private AutoCompleteFilterPredicate<string?>? _textFilter = AutoCompleteSearch.GetFilter(AutoCompleteFilterMode.StartsWith);
 
-        private AutoCompleteSelector<object> _itemSelector;
-        private AutoCompleteSelector<string> _textSelector;
+        private AutoCompleteSelector<object>? _itemSelector;
+        private AutoCompleteSelector<string?>? _textSelector;
 
         public static readonly RoutedEvent<SelectionChangedEventArgs> SelectionChangedEvent =
             RoutedEvent.Register<SelectionChangedEventArgs>(nameof(SelectionChanged), RoutingStrategies.Bubble, typeof(AutoCompleteBox));
 
-        public static readonly StyledProperty<string> WatermarkProperty =
+        public static readonly StyledProperty<string?> WatermarkProperty =
             TextBox.WatermarkProperty.AddOwner<AutoCompleteBox>();
 
         /// <summary>
@@ -479,8 +479,8 @@ namespace Avalonia.Controls
         /// <value>The identifier the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.SelectedItem" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, object> SelectedItemProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, object>(
+        public static readonly DirectProperty<AutoCompleteBox, object?> SelectedItemProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, object?>(
                 nameof(SelectedItem),
                 o => o.SelectedItem,
                 (o, v) => o.SelectedItem = v,
@@ -495,7 +495,7 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.Text" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, string> TextProperty =
+        public static readonly DirectProperty<AutoCompleteBox, string?> TextProperty =
             TextBlock.TextProperty.AddOwnerWithDataValidation<AutoCompleteBox>(
                 o => o.Text,
                 (o, v) => o.Text = v,
@@ -510,8 +510,8 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.SearchText" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, string> SearchTextProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, string>(
+        public static readonly DirectProperty<AutoCompleteBox, string?> SearchTextProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, string?>(
                 nameof(SearchText),
                 o => o.SearchText,
                 unsetValue: string.Empty);
@@ -535,8 +535,8 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.ItemFilter" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteFilterPredicate<object>> ItemFilterProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteFilterPredicate<object>>(
+        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteFilterPredicate<object?>?> ItemFilterProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteFilterPredicate<object?>?>(
                 nameof(ItemFilter),
                 o => o.ItemFilter,
                 (o, v) => o.ItemFilter = v);
@@ -549,8 +549,8 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.TextFilter" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteFilterPredicate<string>> TextFilterProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteFilterPredicate<string>>(
+        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteFilterPredicate<string?>?> TextFilterProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteFilterPredicate<string?>?>(
                 nameof(TextFilter),
                 o => o.TextFilter,
                 (o, v) => o.TextFilter = v,
@@ -564,8 +564,8 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.ItemSelector" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteSelector<object>> ItemSelectorProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteSelector<object>>(
+        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteSelector<object>?> ItemSelectorProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteSelector<object>?>(
                 nameof(ItemSelector),
                 o => o.ItemSelector,
                 (o, v) => o.ItemSelector = v);
@@ -578,8 +578,8 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.TextSelector" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteSelector<string>> TextSelectorProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteSelector<string>>(
+        public static readonly DirectProperty<AutoCompleteBox, AutoCompleteSelector<string?>?> TextSelectorProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, AutoCompleteSelector<string?>?>(
                 nameof(TextSelector),
                 o => o.TextSelector,
                 (o, v) => o.TextSelector = v);
@@ -592,14 +592,14 @@ namespace Avalonia.Controls
         /// <value>The identifier for the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.ItemsSource" />
         /// dependency property.</value>
-        public static readonly DirectProperty<AutoCompleteBox, IEnumerable> ItemsProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, IEnumerable>(
+        public static readonly DirectProperty<AutoCompleteBox, IEnumerable?> ItemsProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, IEnumerable?>(
                 nameof(Items),
                 o => o.Items,
                 (o, v) => o.Items = v);
 
-        public static readonly DirectProperty<AutoCompleteBox, Func<string, CancellationToken, Task<IEnumerable<object>>>> AsyncPopulatorProperty =
-            AvaloniaProperty.RegisterDirect<AutoCompleteBox, Func<string, CancellationToken, Task<IEnumerable<object>>>>(
+        public static readonly DirectProperty<AutoCompleteBox, Func<string?, CancellationToken, Task<IEnumerable<object>>>?> AsyncPopulatorProperty =
+            AvaloniaProperty.RegisterDirect<AutoCompleteBox, Func<string?, CancellationToken, Task<IEnumerable<object>>>?>(
                 nameof(AsyncPopulator),
                 o => o.AsyncPopulator,
                 (o, v) => o.AsyncPopulator = v);
@@ -640,7 +640,7 @@ namespace Avalonia.Controls
         /// <param name="e">The event data.</param>
         private void OnControlIsEnabledChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            bool isEnabled = (bool)e.NewValue;
+            bool isEnabled = (bool)e.NewValue!;
             if (!isEnabled)
             {
                 IsDropDownOpen = false;
@@ -655,7 +655,7 @@ namespace Avalonia.Controls
         /// <param name="e">Event arguments.</param>
         private void OnMinimumPopulateDelayChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var newValue = (TimeSpan)e.NewValue;
+            var newValue = (TimeSpan)e.NewValue!;
 
             // Stop any existing timer
             if (_delayTimer != null)
@@ -695,8 +695,8 @@ namespace Avalonia.Controls
                 return;
             }
 
-            bool oldValue = (bool)e.OldValue;
-            bool newValue = (bool)e.NewValue;
+            bool oldValue = (bool)e.OldValue!;
+            bool newValue = (bool)e.NewValue!;
 
             if (newValue)
             {
@@ -750,7 +750,7 @@ namespace Avalonia.Controls
         /// <param name="e">Event arguments.</param>
         private void OnTextPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            TextUpdated((string)e.NewValue, false);
+            TextUpdated((string?)e.NewValue, false);
         }
 
         private void OnSearchTextPropertyChanged(AvaloniaPropertyChangedEventArgs e)
@@ -778,7 +778,7 @@ namespace Avalonia.Controls
         /// <param name="e">Event arguments.</param>
         private void OnFilterModePropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            AutoCompleteFilterMode mode = (AutoCompleteFilterMode)e.NewValue;
+            AutoCompleteFilterMode mode = (AutoCompleteFilterMode)e.NewValue!;
 
             // Sets the filter predicate for the new value
             TextFilter = AutoCompleteSearch.GetFilter(mode);
@@ -790,7 +790,7 @@ namespace Avalonia.Controls
         /// <param name="e">Event arguments.</param>
         private void OnItemFilterPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            AutoCompleteFilterPredicate<object> value = e.NewValue as AutoCompleteFilterPredicate<object>;
+            var value = e.NewValue as AutoCompleteFilterPredicate<object>;
 
             // If null, revert to the "None" predicate
             if (value == null)
@@ -810,7 +810,7 @@ namespace Avalonia.Controls
         /// <param name="e">Event arguments.</param>
         private void OnItemsPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            OnItemsChanged((IEnumerable)e.NewValue);
+            OnItemsChanged((IEnumerable?)e.NewValue);
         }
 
         private void OnItemTemplatePropertyChanged(AvaloniaPropertyChangedEventArgs e)
@@ -818,7 +818,7 @@ namespace Avalonia.Controls
             if (!_settingItemTemplateFromValueMemberBinding)
                 _itemTemplateIsFromValueMemberBinding = false;
         }
-        private void OnValueMemberBindingChanged(IBinding value)
+        private void OnValueMemberBindingChanged(IBinding? value)
         {
             if(_itemTemplateIsFromValueMemberBinding)
             {
@@ -828,7 +828,8 @@ namespace Avalonia.Controls
                         (o, _) =>
                         {
                             var control = new ContentControl();
-                            control.Bind(ContentControl.ContentProperty, value);
+                            if (value is not null)
+                                control.Bind(ContentControl.ContentProperty, value);
                             return control;
                         });
 
@@ -975,7 +976,7 @@ namespace Avalonia.Controls
         /// <value>The <see cref="T:Avalonia.Data.IBinding" /> object used
         /// when binding to a collection property.</value>
         [AssignBinding]
-        public IBinding ValueMemberBinding
+        public IBinding? ValueMemberBinding
         {
             get { return _valueBindingEvaluator?.ValueBinding; }
             set
@@ -998,7 +999,7 @@ namespace Avalonia.Controls
         /// then displayed in the text box, the SelectedItem property will be
         /// a null reference.
         /// </remarks>
-        public object SelectedItem
+        public object? SelectedItem
         {
             get { return _selectedItem; }
             set { SetAndRaise(SelectedItemProperty, ref _selectedItem, value); }
@@ -1010,7 +1011,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <value>The text in the text box portion of the
         /// <see cref="T:Avalonia.Controls.AutoCompleteBox" /> control.</value>
-        public string Text
+        public string? Text
         {
             get { return _text; }
             set { SetAndRaise(TextProperty, ref _text, value); }
@@ -1029,7 +1030,7 @@ namespace Avalonia.Controls
         /// Text property, but is set after the TextChanged event occurs
         /// and before the Populating event.
         /// </remarks>
-        public string SearchText
+        public string? SearchText
         {
             get { return _searchText; }
             private set
@@ -1071,7 +1072,7 @@ namespace Avalonia.Controls
             set { SetValue(FilterModeProperty, value); }
         }
 
-        public string Watermark
+        public string? Watermark
         {
             get { return GetValue(WatermarkProperty); }
             set { SetValue(WatermarkProperty, value); }
@@ -1091,7 +1092,7 @@ namespace Avalonia.Controls
         /// The filter mode is automatically set to Custom if you set the
         /// ItemFilter property.
         /// </remarks>
-        public AutoCompleteFilterPredicate<object> ItemFilter
+        public AutoCompleteFilterPredicate<object?>? ItemFilter
         {
             get { return _itemFilter; }
             set { SetAndRaise(ItemFilterProperty, ref _itemFilter, value); }
@@ -1111,7 +1112,7 @@ namespace Avalonia.Controls
         /// The search mode is automatically set to Custom if you set the
         /// TextFilter property.
         /// </remarks>
-        public AutoCompleteFilterPredicate<string> TextFilter
+        public AutoCompleteFilterPredicate<string?>? TextFilter
         {
             get { return _textFilter; }
             set { SetAndRaise(TextFilterProperty, ref _textFilter, value); }
@@ -1127,7 +1128,7 @@ namespace Avalonia.Controls
         /// text and one of the items specified by the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.ItemsSource" />.
         /// </value>
-        public AutoCompleteSelector<object> ItemSelector
+        public AutoCompleteSelector<object>? ItemSelector
         {
             get { return _itemSelector; }
             set { SetAndRaise(ItemSelectorProperty, ref _itemSelector, value); }
@@ -1145,13 +1146,13 @@ namespace Avalonia.Controls
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.ItemsSource" />
         /// in a text-based way.
         /// </value>
-        public AutoCompleteSelector<string> TextSelector
+        public AutoCompleteSelector<string?>? TextSelector
         {
             get { return _textSelector; }
             set { SetAndRaise(TextSelectorProperty, ref _textSelector, value); }
         }
 
-        public Func<string, CancellationToken, Task<IEnumerable<object>>> AsyncPopulator
+        public Func<string?, CancellationToken, Task<IEnumerable<object>>>? AsyncPopulator
         {
             get { return _asyncPopulator; }
             set { SetAndRaise(AsyncPopulatorProperty, ref _asyncPopulator, value); }
@@ -1165,7 +1166,7 @@ namespace Avalonia.Controls
         /// <value>The collection that is used to generate the items of the
         /// drop-down portion of the
         /// <see cref="T:Avalonia.Controls.AutoCompleteBox" /> control.</value>
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get { return _itemsEnumerable; }
             set { SetAndRaise(ItemsProperty, ref _itemsEnumerable, value); }
@@ -1174,12 +1175,12 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the drop down popup control.
         /// </summary>
-        private Popup DropDownPopup { get; set; }
+        private Popup? DropDownPopup { get; set; }
 
         /// <summary>
         /// Gets or sets the Text template part.
         /// </summary>
-        private TextBox TextBox
+        private TextBox? TextBox
         {
             get { return _textBox; }
             set
@@ -1243,7 +1244,7 @@ namespace Avalonia.Controls
         /// use with AutoCompleteBox or deriving from AutoCompleteBox to
         /// create a custom control.
         /// </remarks>
-        protected ISelectionAdapter SelectionAdapter
+        protected ISelectionAdapter? SelectionAdapter
         {
             get { return _adapter; }
             set
@@ -1279,10 +1280,10 @@ namespace Avalonia.Controls
         /// A <see cref="T:Avalonia.Controls.ISelectionAdapter" /> object,
         /// if possible. Otherwise, null.
         /// </returns>
-        protected virtual ISelectionAdapter GetSelectionAdapterPart(INameScope nameScope)
+        protected virtual ISelectionAdapter? GetSelectionAdapterPart(INameScope nameScope)
         {
-            ISelectionAdapter adapter = null;
-            SelectingItemsControl selector = nameScope.Find<SelectingItemsControl>(ElementSelector);
+            ISelectionAdapter? adapter = null;
+            SelectingItemsControl? selector = nameScope.Find<SelectingItemsControl>(ElementSelector);
             if (selector != null)
             {
                 // Check if it is already an IItemsSelector
@@ -1316,7 +1317,7 @@ namespace Avalonia.Controls
 
             // Set the template parts. Individual part setters remove and add
             // any event handlers.
-            Popup popup = e.NameScope.Find<Popup>(ElementPopup);
+            Popup? popup = e.NameScope.Find<Popup>(ElementPopup);
             if (popup != null)
             {
                 DropDownPopup = popup;
@@ -1358,7 +1359,7 @@ namespace Avalonia.Controls
         /// that contains the event data.</param>
         protected override void OnKeyDown(KeyEventArgs e)
         {
-            Contract.Requires<ArgumentNullException>(e != null);
+            _ = e ?? throw new ArgumentNullException(nameof(e));
 
             base.OnKeyDown(e);
 
@@ -1453,7 +1454,7 @@ namespace Avalonia.Controls
         /// otherwise, false.</returns>
         protected bool HasFocus()
         {
-            IVisual focused = FocusManager.Instance.Current;
+            IVisual? focused = FocusManager.Instance?.Current;
 
             while (focused != null)
             {
@@ -1464,11 +1465,11 @@ namespace Avalonia.Controls
 
                 // This helps deal with popups that may not be in the same
                 // visual tree
-                IVisual parent = focused.GetVisualParent();
+                IVisual? parent = focused.GetVisualParent();
                 if (parent == null)
                 {
                     // Try the logical parent.
-                    IControl element = focused as IControl;
+                    IControl? element = focused as IControl;
                     if (element != null)
                     {
                         parent = element.Parent;
@@ -1519,7 +1520,7 @@ namespace Avalonia.Controls
         /// Occurs when the text in the text box portion of the
         /// <see cref="T:Avalonia.Controls.AutoCompleteBox" /> changes.
         /// </summary>
-        public event EventHandler TextChanged;
+        public event EventHandler? TextChanged;
 
         /// <summary>
         /// Occurs when the
@@ -1535,7 +1536,7 @@ namespace Avalonia.Controls
         /// In this case, if you want possible matches to appear, you must
         /// provide the logic for populating the selection adapter.
         /// </remarks>
-        public event EventHandler<PopulatingEventArgs> Populating;
+        public event EventHandler<PopulatingEventArgs>? Populating;
 
         /// <summary>
         /// Occurs when the
@@ -1544,35 +1545,35 @@ namespace Avalonia.Controls
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.Text" />
         /// property.
         /// </summary>
-        public event EventHandler<PopulatedEventArgs> Populated;
+        public event EventHandler<PopulatedEventArgs>? Populated;
 
         /// <summary>
         /// Occurs when the value of the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.IsDropDownOpen" />
         /// property is changing from false to true.
         /// </summary>
-        public event EventHandler<CancelEventArgs> DropDownOpening;
+        public event EventHandler<CancelEventArgs>? DropDownOpening;
 
         /// <summary>
         /// Occurs when the value of the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.IsDropDownOpen" />
         /// property has changed from false to true and the drop-down is open.
         /// </summary>
-        public event EventHandler DropDownOpened;
+        public event EventHandler? DropDownOpened;
 
         /// <summary>
         /// Occurs when the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.IsDropDownOpen" />
         /// property is changing from true to false.
         /// </summary>
-        public event EventHandler<CancelEventArgs> DropDownClosing;
+        public event EventHandler<CancelEventArgs>? DropDownClosing;
 
         /// <summary>
         /// Occurs when the
         /// <see cref="P:Avalonia.Controls.AutoCompleteBox.IsDropDownOpen" />
         /// property was changed from true to false and the drop-down is open.
         /// </summary>
-        public event EventHandler DropDownClosed;
+        public event EventHandler? DropDownClosed;
 
         /// <summary>
         /// Occurs when the selected item in the drop-down portion of the
@@ -1740,7 +1741,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void DropDownPopup_Closed(object sender, EventArgs e)
+        private void DropDownPopup_Closed(object? sender, EventArgs e)
         {
             // Force the drop down dependency property to be false.
             if (IsDropDownOpen)
@@ -1760,7 +1761,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event arguments.</param>
-        private void PopulateDropDown(object sender, EventArgs e)
+        private void PopulateDropDown(object? sender, EventArgs e)
         {
             if (_delayTimer != null)
             {
@@ -1786,7 +1787,7 @@ namespace Avalonia.Controls
                 PopulateComplete();
             }
         }
-        private bool TryPopulateAsync(string searchText)
+        private bool TryPopulateAsync(string? searchText)
         {
             _populationCancellationTokenSource?.Cancel(false);
             _populationCancellationTokenSource?.Dispose();
@@ -1804,12 +1805,12 @@ namespace Avalonia.Controls
 
             return true;
         }
-        private async Task PopulateAsync(string searchText, CancellationToken cancellationToken)
+        private async Task PopulateAsync(string? searchText, CancellationToken cancellationToken)
         {
 
             try
             {
-                IEnumerable<object> result = await _asyncPopulator.Invoke(searchText, cancellationToken);
+                IEnumerable<object> result = await _asyncPopulator!.Invoke(searchText, cancellationToken);
                 var resultList = result.ToList();
 
                 if (cancellationToken.IsCancellationRequested)
@@ -1878,9 +1879,9 @@ namespace Avalonia.Controls
         /// <param name="clearDataContext">A value indicating whether to clear
         /// the data context after the lookup is performed.</param>
         /// <returns>Formatted Value.</returns>
-        private string FormatValue(object value, bool clearDataContext)
+        private string? FormatValue(object? value, bool clearDataContext)
         {
-            string result = FormatValue(value);
+            string? result = FormatValue(value);
             if(clearDataContext && _valueBindingEvaluator != null)
             {
                 _valueBindingEvaluator.ClearDataContext();
@@ -1902,7 +1903,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// Override this method to provide a custom string conversion.
         /// </remarks>
-        protected virtual string FormatValue(object value)
+        protected virtual string? FormatValue(object? value)
         {
             if (_valueBindingEvaluator != null)
             {
@@ -1923,7 +1924,7 @@ namespace Avalonia.Controls
             Dispatcher.UIThread.Post(() =>
             {
                 // Call the central updated text method as a user-initiated action
-                TextUpdated(_textBox.Text, true);
+                TextUpdated(_textBox!.Text, true);
             });
         }
 
@@ -1933,7 +1934,7 @@ namespace Avalonia.Controls
         /// text changed events when there is a change.
         /// </summary>
         /// <param name="value">The new string value.</param>
-        private void UpdateTextValue(string value)
+        private void UpdateTextValue(string? value)
         {
             UpdateTextValue(value, null);
         }
@@ -1949,7 +1950,7 @@ namespace Avalonia.Controls
         /// underlying text dependency property is updated. In a non-user
         /// interaction, the text box value is updated. When user initiated is
         /// null, all values are updated.</param>
-        private void UpdateTextValue(string value, bool? userInitiated)
+        private void UpdateTextValue(string? value, bool? userInitiated)
         {
             bool callTextChanged = false;
             // Update the Text dependency property
@@ -1987,7 +1988,7 @@ namespace Avalonia.Controls
         /// <param name="userInitiated">A value indicating whether the update
         /// is a user-initiated action. This should be a True value when the
         /// TextUpdated method is called from a TextBox event handler.</param>
-        private void TextUpdated(string newText, bool userInitiated)
+        private void TextUpdated(string? newText, bool userInitiated)
         {
             // Only process this event if it is coming from someone outside
             // setting the Text dependency property directly.
@@ -2087,7 +2088,7 @@ namespace Avalonia.Controls
             bool objectFiltering = FilterMode == AutoCompleteFilterMode.Custom && TextFilter == null;
 
             int view_index = 0;
-            int view_count = _view.Count;
+            int view_count = _view!.Count;
             List<object> items = _items;
             foreach (object item in items)
             {
@@ -2096,7 +2097,7 @@ namespace Avalonia.Controls
                 {
                     if (stringFiltering)
                     {
-                        inResults = TextFilter(text, FormatValue(item));
+                        inResults = TextFilter!(text, FormatValue(item));
                     }
                     else
                     {
@@ -2166,7 +2167,7 @@ namespace Avalonia.Controls
         /// adapter's ItemsSource to the view if appropriate.
         /// </summary>
         /// <param name="newValue">The new enumerable reference.</param>
-        private void OnItemsChanged(IEnumerable newValue)
+        private void OnItemsChanged(IEnumerable? newValue)
         {
             // Remove handler for oldValue.CollectionChanged (if present)
             _collectionChangeSubscription?.Dispose();
@@ -2198,28 +2199,28 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The object that raised the event.</param>
         /// <param name="e">The event data.</param>
-        private void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void ItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             // Update the cache
             if (e.Action == NotifyCollectionChangedAction.Remove && e.OldItems != null)
             {
                 for (int index = 0; index < e.OldItems.Count; index++)
                 {
-                    _items.RemoveAt(e.OldStartingIndex);
+                    _items!.RemoveAt(e.OldStartingIndex);
                 }
             }
-            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems != null && _items.Count >= e.NewStartingIndex)
+            if (e.Action == NotifyCollectionChangedAction.Add && e.NewItems != null && _items!.Count >= e.NewStartingIndex)
             {
                 for (int index = 0; index < e.NewItems.Count; index++)
                 {
-                    _items.Insert(e.NewStartingIndex + index, e.NewItems[index]);
+                    _items.Insert(e.NewStartingIndex + index, e.NewItems[index]!);
                 }
             }
             if (e.Action == NotifyCollectionChangedAction.Replace && e.NewItems != null && e.OldItems != null)
             {
                 for (int index = 0; index < e.NewItems.Count; index++)
                 {
-                    _items[e.NewStartingIndex] = e.NewItems[index];
+                    _items![e.NewStartingIndex] = e.NewItems[index]!;
                 }
             }
 
@@ -2228,7 +2229,7 @@ namespace Avalonia.Controls
             {
                 for (int index = 0; index < e.OldItems.Count; index++)
                 {
-                    _view.Remove(e.OldItems[index]);
+                    _view!.Remove(e.OldItems[index]!);
                 }
             }
 
@@ -2270,7 +2271,7 @@ namespace Avalonia.Controls
             RefreshView();
 
             // Fire the Populated event containing the read-only view data.
-            PopulatedEventArgs populated = new PopulatedEventArgs(new ReadOnlyCollection<object>(_view));
+            PopulatedEventArgs populated = new PopulatedEventArgs(new ReadOnlyCollection<object>(_view!));
             OnPopulated(populated);
 
             if (SelectionAdapter != null && SelectionAdapter.Items != _view)
@@ -2278,7 +2279,7 @@ namespace Avalonia.Controls
                 SelectionAdapter.Items = _view;
             }
 
-            bool isDropDownOpen = _userCalledPopulate && (_view.Count > 0);
+            bool isDropDownOpen = _userCalledPopulate && (_view!.Count > 0);
             if (isDropDownOpen != IsDropDownOpen)
             {
                 _ignorePropertyChange = true;
@@ -2306,20 +2307,20 @@ namespace Avalonia.Controls
         private void UpdateTextCompletion(bool userInitiated)
         {
             // By default this method will clear the selected value
-            object newSelectedItem = null;
-            string text = Text;
+            object? newSelectedItem = null;
+            string? text = Text;
 
             // Text search is StartsWith explicit and only when enabled, in
             // line with WPF's ComboBox lookup. When in use it will associate
             // a Value with the Text if it is found in ItemsSource. This is
             // only valid when there is data and the user initiated the action.
-            if (_view.Count > 0)
+            if (_view!.Count > 0)
             {
                 if (IsTextCompletionEnabled && TextBox != null && userInitiated)
                 {
                     int currentLength = TextBox.Text?.Length ?? 0;
                     int selectionStart = TextBoxSelectionStart;
-                    if (selectionStart == text.Length && selectionStart > _textSelectionStart)
+                    if (selectionStart == text?.Length && selectionStart > _textSelectionStart)
                     {
                         // When the FilterMode dependency property is set to
                         // either StartsWith or StartsWithCaseSensitive, the
@@ -2327,7 +2328,7 @@ namespace Avalonia.Controls
                         // performance on the lookup. It assumes that the
                         // FilterMode the user has selected is an acceptable
                         // case sensitive matching function for their scenario.
-                        object top = FilterMode == AutoCompleteFilterMode.StartsWith || FilterMode == AutoCompleteFilterMode.StartsWithCaseSensitive
+                        object? top = FilterMode == AutoCompleteFilterMode.StartsWith || FilterMode == AutoCompleteFilterMode.StartsWithCaseSensitive
                             ? _view[0]
                             : TryGetMatch(text, _view, AutoCompleteSearch.GetFilter(AutoCompleteFilterMode.StartsWith));
 
@@ -2335,18 +2336,18 @@ namespace Avalonia.Controls
                         if (top != null)
                         {
                             newSelectedItem = top;
-                            string topString = FormatValue(top, true);
+                            string? topString = FormatValue(top, true);
 
                             // Only replace partially when the two words being the same
-                            int minLength = Math.Min(topString.Length, Text.Length);
-                            if (AutoCompleteSearch.Equals(Text.Substring(0, minLength), topString.Substring(0, minLength)))
+                            int minLength = Math.Min(topString?.Length ?? 0, Text?.Length ?? 0);
+                            if (AutoCompleteSearch.Equals(Text?.Substring(0, minLength), topString?.Substring(0, minLength)))
                             {
                                 // Update the text
                                 UpdateTextValue(topString);
 
                                 // Select the text past the user's caret
                                 TextBox.SelectionStart = currentLength;
-                                TextBox.SelectionEnd = topString.Length;
+                                TextBox.SelectionEnd = topString?.Length ?? 0;
                             }
                         }
                     }
@@ -2392,8 +2393,11 @@ namespace Avalonia.Controls
         /// <param name="predicate">The predicate to use for the partial or
         /// exact match.</param>
         /// <returns>Returns the object or null.</returns>
-        private object TryGetMatch(string searchText, AvaloniaList<object> view, AutoCompleteFilterPredicate<string> predicate)
+        private object? TryGetMatch(string? searchText, AvaloniaList<object> view, AutoCompleteFilterPredicate<string?>? predicate)
         {
+            if (predicate is null)
+                return null;
+
             if (view != null && view.Count > 0)
             {
                 foreach (object o in view)
@@ -2428,9 +2432,9 @@ namespace Avalonia.Controls
         /// that is displayed in the text box part.
         /// </summary>
         /// <param name="newItem">The new item.</param>
-        private void OnSelectedItemChanged(object newItem)
+        private void OnSelectedItemChanged(object? newItem)
         {
-            string text;
+            string? text;
 
             if (newItem == null)
             {
@@ -2461,9 +2465,9 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The selection changed event data.</param>
-        private void OnAdapterSelectionChanged(object sender, SelectionChangedEventArgs e)
+        private void OnAdapterSelectionChanged(object? sender, SelectionChangedEventArgs e)
         {
-            SelectedItem = _adapter.SelectedItem;
+            SelectedItem = _adapter!.SelectedItem;
         }
 
         //TODO Check UpdateTextCompletion
@@ -2472,7 +2476,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void OnAdapterSelectionComplete(object sender, RoutedEventArgs e)
+        private void OnAdapterSelectionComplete(object? sender, RoutedEventArgs e)
         {
             IsDropDownOpen = false;
 
@@ -2482,7 +2486,7 @@ namespace Avalonia.Controls
             // Text should not be selected
             ClearTextBoxSelection();
 
-            TextBox.Focus();
+            TextBox!.Focus();
         }
 
         /// <summary>
@@ -2490,7 +2494,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void OnAdapterSelectionCanceled(object sender, RoutedEventArgs e)
+        private void OnAdapterSelectionCanceled(object? sender, RoutedEventArgs e)
         {
             UpdateTextValue(SearchText);
 
@@ -2510,7 +2514,7 @@ namespace Avalonia.Controls
             /// </summary>
             /// <param name="FilterMode">The built-in search mode.</param>
             /// <returns>Returns the string-based comparison function.</returns>
-            public static AutoCompleteFilterPredicate<string> GetFilter(AutoCompleteFilterMode FilterMode)
+            public static AutoCompleteFilterPredicate<string?>? GetFilter(AutoCompleteFilterMode FilterMode)
             {
                 switch (FilterMode)
                 {
@@ -2566,9 +2570,11 @@ namespace Avalonia.Controls
             /// <param name="value">The string value to search for.</param>
             /// <param name="comparison">The string comparison type.</param>
             /// <returns>Returns true when the substring is found.</returns>
-            private static bool Contains(string s, string value, StringComparison comparison)
+            private static bool Contains(string? s, string? value, StringComparison comparison)
             {
-                return s.IndexOf(value, comparison) >= 0;
+                if (s is not null && value is not null)
+                    return s.IndexOf(value, comparison) >= 0;
+                return false;
             }
 
             /// <summary>
@@ -2577,9 +2583,11 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool StartsWith(string text, string value)
+            public static bool StartsWith(string? text, string? value)
             {
-                return value.StartsWith(text, StringComparison.CurrentCultureIgnoreCase);
+                if (value is not null && text is not null)
+                    return value.StartsWith(text, StringComparison.CurrentCultureIgnoreCase);
+                return false;
             }
 
             /// <summary>
@@ -2588,9 +2596,11 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool StartsWithCaseSensitive(string text, string value)
+            public static bool StartsWithCaseSensitive(string? text, string? value)
             {
-                return value.StartsWith(text, StringComparison.CurrentCulture);
+                if (value is not null && text is not null)
+                    return value.StartsWith(text, StringComparison.CurrentCulture);
+                return false;
             }
 
             /// <summary>
@@ -2599,9 +2609,11 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool StartsWithOrdinal(string text, string value)
+            public static bool StartsWithOrdinal(string? text, string? value)
             {
-                return value.StartsWith(text, StringComparison.OrdinalIgnoreCase);
+                if (value is not null && text is not null)
+                    return value.StartsWith(text, StringComparison.OrdinalIgnoreCase);
+                return false;
             }
 
             /// <summary>
@@ -2610,9 +2622,11 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool StartsWithOrdinalCaseSensitive(string text, string value)
+            public static bool StartsWithOrdinalCaseSensitive(string? text, string? value)
             {
-                return value.StartsWith(text, StringComparison.Ordinal);
+                if (value is not null && text is not null)
+                    return value.StartsWith(text, StringComparison.Ordinal);
+                return false;
             }
 
             /// <summary>
@@ -2622,7 +2636,7 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool Contains(string text, string value)
+            public static bool Contains(string? text, string? value)
             {
                 return Contains(value, text, StringComparison.CurrentCultureIgnoreCase);
             }
@@ -2633,7 +2647,7 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool ContainsCaseSensitive(string text, string value)
+            public static bool ContainsCaseSensitive(string? text, string? value)
             {
                 return Contains(value, text, StringComparison.CurrentCulture);
             }
@@ -2644,7 +2658,7 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool ContainsOrdinal(string text, string value)
+            public static bool ContainsOrdinal(string? text, string? value)
             {
                 return Contains(value, text, StringComparison.OrdinalIgnoreCase);
             }
@@ -2655,7 +2669,7 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool ContainsOrdinalCaseSensitive(string text, string value)
+            public static bool ContainsOrdinalCaseSensitive(string? text, string? value)
             {
                 return Contains(value, text, StringComparison.Ordinal);
             }
@@ -2666,9 +2680,9 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool Equals(string text, string value)
+            public static bool Equals(string? text, string? value)
             {
-                return value.Equals(text, StringComparison.CurrentCultureIgnoreCase);
+                return string.Equals(value, text, StringComparison.CurrentCultureIgnoreCase);
             }
 
             /// <summary>
@@ -2677,9 +2691,9 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool EqualsCaseSensitive(string text, string value)
+            public static bool EqualsCaseSensitive(string? text, string? value)
             {
-                return value.Equals(text, StringComparison.CurrentCulture);
+                return string.Equals(value, text, StringComparison.CurrentCulture);
             }
 
             /// <summary>
@@ -2688,9 +2702,9 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool EqualsOrdinal(string text, string value)
+            public static bool EqualsOrdinal(string? text, string? value)
             {
-                return value.Equals(text, StringComparison.OrdinalIgnoreCase);
+                return string.Equals(value, text, StringComparison.OrdinalIgnoreCase);
             }
 
             /// <summary>
@@ -2699,9 +2713,9 @@ namespace Avalonia.Controls
             /// <param name="text">The AutoCompleteBox prefix text.</param>
             /// <param name="value">The item's string value.</param>
             /// <returns>Returns true if the condition is met.</returns>
-            public static bool EqualsOrdinalCaseSensitive(string text, string value)
+            public static bool EqualsOrdinalCaseSensitive(string? text, string? value)
             {
-                return value.Equals(text, StringComparison.Ordinal);
+                return string.Equals(value, text, StringComparison.Ordinal);
             }
         }
 
@@ -2715,7 +2729,7 @@ namespace Avalonia.Controls
             /// <summary>
             /// Gets or sets the string value binding used by the control.
             /// </summary>
-            private IBinding _binding;
+            private IBinding? _binding;
 
             #region public T Value
 
@@ -2739,13 +2753,14 @@ namespace Avalonia.Controls
             /// <summary>
             /// Gets or sets the value binding.
             /// </summary>
-            public IBinding ValueBinding
+            public IBinding? ValueBinding
             {
                 get { return _binding; }
                 set
                 {
                     _binding = value;
-                    AvaloniaObjectExtensions.Bind(this, ValueProperty, value);
+                    if (value is not null)
+                        AvaloniaObjectExtensions.Bind(this, ValueProperty, value);
                 }
             }
 
@@ -2760,7 +2775,7 @@ namespace Avalonia.Controls
             /// setting the initial binding to the provided parameter.
             /// </summary>
             /// <param name="binding">The initial string value binding.</param>
-            public BindingEvaluator(IBinding binding)
+            public BindingEvaluator(IBinding? binding)
                 : this()
             {
                 ValueBinding = binding;
@@ -2802,7 +2817,7 @@ namespace Avalonia.Controls
             /// <param name="o">The object to use as the data context.</param>
             /// <returns>Returns the evaluated T value of the bound dependency
             /// property.</returns>
-            public T GetDynamicValue(object o)
+            public T GetDynamicValue(object? o)
             {
                 DataContext = o;
                 return Value;
diff --git a/src/Avalonia.Controls/Avalonia.Controls.csproj b/src/Avalonia.Controls/Avalonia.Controls.csproj
index e2c6a714aa..543a513d57 100644
--- a/src/Avalonia.Controls/Avalonia.Controls.csproj
+++ b/src/Avalonia.Controls/Avalonia.Controls.csproj
@@ -18,4 +18,5 @@
   <Import Project="..\..\build\Rx.props" />
   <Import Project="..\..\build\JetBrains.Annotations.props" />
   <Import Project="..\..\build\ApiDiff.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
 </Project>
diff --git a/src/Avalonia.Controls/Border.cs b/src/Avalonia.Controls/Border.cs
index ee67f303f3..ee3be1d5b3 100644
--- a/src/Avalonia.Controls/Border.cs
+++ b/src/Avalonia.Controls/Border.cs
@@ -17,14 +17,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BackgroundProperty =
-            AvaloniaProperty.Register<Border, IBrush>(nameof(Background));
+        public static readonly StyledProperty<IBrush?> BackgroundProperty =
+            AvaloniaProperty.Register<Border, IBrush?>(nameof(Background));
 
         /// <summary>
         /// Defines the <see cref="BorderBrush"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BorderBrushProperty =
-            AvaloniaProperty.Register<Border, IBrush>(nameof(BorderBrush));
+        public static readonly StyledProperty<IBrush?> BorderBrushProperty =
+            AvaloniaProperty.Register<Border, IBrush?>(nameof(BorderBrush));
 
         /// <summary>
         /// Defines the <see cref="BorderThickness"/> property.
@@ -91,7 +91,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a brush with which to paint the background.
         /// </summary>
-        public IBrush Background
+        public IBrush? Background
         {
             get { return GetValue(BackgroundProperty); }
             set { SetValue(BackgroundProperty, value); }
@@ -100,7 +100,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a brush with which to paint the border.
         /// </summary>
-        public IBrush BorderBrush
+        public IBrush? BorderBrush
         {
             get { return GetValue(BorderBrushProperty); }
             set { SetValue(BorderBrushProperty, value); }
diff --git a/src/Avalonia.Controls/Button.cs b/src/Avalonia.Controls/Button.cs
index ce41b90bb0..a2efc7fba0 100644
--- a/src/Avalonia.Controls/Button.cs
+++ b/src/Avalonia.Controls/Button.cs
@@ -42,21 +42,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Command"/> property.
         /// </summary>
-        public static readonly DirectProperty<Button, ICommand> CommandProperty =
-            AvaloniaProperty.RegisterDirect<Button, ICommand>(nameof(Command),
+        public static readonly DirectProperty<Button, ICommand?> CommandProperty =
+            AvaloniaProperty.RegisterDirect<Button, ICommand?>(nameof(Command),
                 button => button.Command, (button, command) => button.Command = command, enableDataValidation: true);
 
         /// <summary>
         /// Defines the <see cref="HotKey"/> property.
         /// </summary>
-        public static readonly StyledProperty<KeyGesture> HotKeyProperty =
+        public static readonly StyledProperty<KeyGesture?> HotKeyProperty =
             HotKeyManager.HotKeyProperty.AddOwner<Button>();
 
         /// <summary>
         /// Defines the <see cref="CommandParameter"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> CommandParameterProperty =
-            AvaloniaProperty.Register<Button, object>(nameof(CommandParameter));
+        public static readonly StyledProperty<object?> CommandParameterProperty =
+            AvaloniaProperty.Register<Button, object?>(nameof(CommandParameter));
 
         /// <summary>
         /// Defines the <see cref="IsDefault"/> property.
@@ -88,9 +88,9 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<FlyoutBase> FlyoutProperty =
             AvaloniaProperty.Register<Button, FlyoutBase>(nameof(Flyout));
 
-        private ICommand _command;
+        private ICommand? _command;
         private bool _commandCanExecute = true;
-        private KeyGesture _hotkey;
+        private KeyGesture? _hotkey;
 
         /// <summary>
         /// Initializes static members of the <see cref="Button"/> class.
@@ -112,7 +112,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Raised when the user clicks the button.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Click
+        public event EventHandler<RoutedEventArgs>? Click
         {
             add => AddHandler(ClickEvent, value);
             remove => RemoveHandler(ClickEvent, value);
@@ -130,7 +130,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets an <see cref="ICommand"/> to be invoked when the button is clicked.
         /// </summary>
-        public ICommand Command
+        public ICommand? Command
         {
             get => _command;
             set => SetAndRaise(CommandProperty, ref _command, value);
@@ -139,7 +139,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets an <see cref="KeyGesture"/> associated with this control
         /// </summary>
-        public KeyGesture HotKey
+        public KeyGesture? HotKey
         {
             get => GetValue(HotKeyProperty);
             set => SetValue(HotKeyProperty, value);
@@ -148,7 +148,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a parameter to be passed to the <see cref="Command"/>.
         /// </summary>
-        public object CommandParameter
+        public object? CommandParameter
         {
             get => GetValue(CommandParameterProperty);
             set => SetValue(CommandParameterProperty, value);
@@ -477,7 +477,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void CanExecuteChanged(object sender, EventArgs e)
+        private void CanExecuteChanged(object? sender, EventArgs e)
         {
             var canExecute = Command == null || Command.CanExecute(CommandParameter);
 
@@ -529,7 +529,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void RootDefaultKeyDown(object sender, KeyEventArgs e)
+        private void RootDefaultKeyDown(object? sender, KeyEventArgs e)
         {
             if (e.Key == Key.Enter && IsVisible && IsEnabled)
             {
@@ -542,7 +542,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void RootCancelKeyDown(object sender, KeyEventArgs e)
+        private void RootCancelKeyDown(object? sender, KeyEventArgs e)
         {
             if (e.Key == Key.Escape && IsVisible && IsEnabled)
             {
diff --git a/src/Avalonia.Controls/ButtonSpinner.cs b/src/Avalonia.Controls/ButtonSpinner.cs
index f1c1a55a17..31aba024ae 100644
--- a/src/Avalonia.Controls/ButtonSpinner.cs
+++ b/src/Avalonia.Controls/ButtonSpinner.cs
@@ -42,11 +42,11 @@ namespace Avalonia.Controls
             UpdatePseudoClasses(ButtonSpinnerLocation);
         }
 
-        private Button _decreaseButton;
+        private Button? _decreaseButton;
         /// <summary>
         /// Gets or sets the DecreaseButton template part.
         /// </summary>
-        private Button DecreaseButton
+        private Button? DecreaseButton
         {
             get { return _decreaseButton; }
             set
@@ -63,11 +63,11 @@ namespace Avalonia.Controls
             }
         }
 
-        private Button _increaseButton;
+        private Button? _increaseButton;
         /// <summary>
         /// Gets or sets the IncreaseButton template part.
         /// </summary>
-        private Button IncreaseButton
+        private Button? IncreaseButton
         {
             get
             {
@@ -241,8 +241,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is ButtonSpinner spinner)
             {
-                var oldValue = (bool)e.OldValue;
-                var newValue = (bool)e.NewValue;
+                var oldValue = (bool)e.OldValue!;
+                var newValue = (bool)e.NewValue!;
                 spinner.OnAllowSpinChanged(oldValue, newValue);
             }
         }
@@ -268,7 +268,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void OnButtonClick(object sender, RoutedEventArgs e)
+        private void OnButtonClick(object? sender, RoutedEventArgs e)
         {
             if (AllowSpin)
             {
diff --git a/src/Avalonia.Controls/Calendar/Calendar.cs b/src/Avalonia.Controls/Calendar/Calendar.cs
index 2c8a7c0831..6c83308b39 100644
--- a/src/Avalonia.Controls/Calendar/Calendar.cs
+++ b/src/Avalonia.Controls/Calendar/Calendar.cs
@@ -240,11 +240,11 @@ namespace Avalonia.Controls
         private bool _isShiftPressed;
         private bool _displayDateIsChanging = false;
 
-        internal CalendarDayButton FocusButton { get; set; }
-        internal CalendarButton FocusCalendarButton { get; set; }
+        internal CalendarDayButton? FocusButton { get; set; }
+        internal CalendarButton? FocusCalendarButton { get; set; }
 
-        internal Panel Root { get; set; }
-        internal CalendarItem MonthControl
+        internal Panel? Root { get; set; }
+        internal CalendarItem? MonthControl
         {
             get
             {
@@ -280,7 +280,7 @@ namespace Avalonia.Controls
         private void OnFirstDayOfWeekChanged(AvaloniaPropertyChangedEventArgs e)
         {
 
-            if (IsValidFirstDayOfWeek(e.NewValue))
+            if (IsValidFirstDayOfWeek(e.NewValue!))
             {
                 UpdateMonths();
             }
@@ -373,9 +373,9 @@ namespace Avalonia.Controls
         /// <param name="e">The DependencyPropertyChangedEventArgs.</param>
         private void OnDisplayModePropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            CalendarMode mode = (CalendarMode)e.NewValue;
-            CalendarMode oldMode = (CalendarMode)e.OldValue;
-            CalendarItem monthControl = MonthControl;
+            CalendarMode mode = (CalendarMode)e.NewValue!;
+            CalendarMode oldMode = (CalendarMode)e.OldValue!;
+            CalendarItem? monthControl = MonthControl;
 
             if (monthControl != null)
             {
@@ -459,7 +459,7 @@ namespace Avalonia.Controls
         }
         private void OnSelectionModeChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            if (IsValidSelectionMode(e.NewValue))
+            if (IsValidSelectionMode(e.NewValue!))
             {
                 _displayDateIsChanging = true;
                 SelectedDate = null;
@@ -656,7 +656,7 @@ namespace Avalonia.Controls
                     {
                         FocusButton.IsCurrent = false;
                     }
-                    FocusButton = FindDayButtonFromDay(LastSelectedDate.Value);
+                    FocusButton = FindDayButtonFromDay(LastSelectedDate!.Value);
                     if (FocusButton != null)
                     {
                         FocusButton.IsCurrent = HasFocusInternal;
@@ -754,11 +754,11 @@ namespace Avalonia.Controls
 
         private void OnDisplayDateChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            UpdateDisplayDate(this, (DateTime)e.NewValue, (DateTime)e.OldValue);
+            UpdateDisplayDate(this, (DateTime)e.NewValue!, (DateTime)e.OldValue!);
         }
         private static void UpdateDisplayDate(Calendar c, DateTime addedDate, DateTime removedDate)
         {
-            Contract.Requires<ArgumentNullException>(c != null);
+            _ = c ?? throw new ArgumentNullException(nameof(c));
 
             // If DisplayDate < DisplayDateStart, DisplayDate = DisplayDateStart
             if (DateTime.Compare(addedDate, c.DisplayDateRangeStart) < 0)
@@ -871,7 +871,7 @@ namespace Avalonia.Controls
             if (cal.SelectedDates.Count > 0)
             {
                 selectedDateMin = cal.SelectedDates[0];
-                Debug.Assert(DateTime.Compare(cal.SelectedDate.Value, selectedDateMin) == 0, "The SelectedDate should be the minimum selected date!");
+                Debug.Assert(DateTime.Compare(cal.SelectedDate!.Value, selectedDateMin) == 0, "The SelectedDate should be the minimum selected date!");
             }
             else
             {
@@ -959,7 +959,7 @@ namespace Avalonia.Controls
             if (cal.SelectedDates.Count > 0)
             {
                 selectedDateMax = cal.SelectedDates[0];
-                Debug.Assert(DateTime.Compare(cal.SelectedDate.Value, selectedDateMax) == 0, "The SelectedDate should be the maximum SelectedDate!");
+                Debug.Assert(DateTime.Compare(cal.SelectedDate!.Value, selectedDateMax) == 0, "The SelectedDate should be the maximum SelectedDate!");
             }
             else
             {
@@ -1003,9 +1003,9 @@ namespace Avalonia.Controls
         /// </summary>
         internal bool CalendarDatePickerDisplayDateFlag { get; set; }
 
-        internal CalendarDayButton FindDayButtonFromDay(DateTime day)
+        internal CalendarDayButton? FindDayButtonFromDay(DateTime day)
         {
-            CalendarItem monthControl = MonthControl;
+            CalendarItem? monthControl = MonthControl;
 
             // REMOVE_RTM: should be updated if we support MultiCalendar
             int count = RowsPerMonth * ColumnsPerMonth;
@@ -1054,7 +1054,7 @@ namespace Avalonia.Controls
         internal void OnHeaderClick()
         {
             Debug.Assert(DisplayMode == CalendarMode.Year || DisplayMode == CalendarMode.Decade, "The DisplayMode should be Year or Decade");
-            CalendarItem monthControl = MonthControl;
+            CalendarItem? monthControl = MonthControl;
             if (monthControl != null && monthControl.MonthView != null && monthControl.YearView != null)
             {
                 monthControl.MonthView.IsVisible = false;
@@ -1065,7 +1065,7 @@ namespace Avalonia.Controls
 
         internal void ResetStates()
         {
-            CalendarItem monthControl = MonthControl;
+            CalendarItem? monthControl = MonthControl;
             int count = RowsPerMonth * ColumnsPerMonth;
             if (monthControl != null)
             {
@@ -1083,7 +1083,7 @@ namespace Avalonia.Controls
 
         internal void UpdateMonths()
         {
-            CalendarItem monthControl = MonthControl;
+            CalendarItem? monthControl = MonthControl;
             if (monthControl != null)
             {
                 switch (DisplayMode)
@@ -1163,6 +1163,8 @@ namespace Avalonia.Controls
         {
             if (HoverEnd != null && HoverStart != null)
             {
+                Debug.Assert(MonthControl is not null);
+
                 int startIndex, endIndex, i;
                 CalendarItem monthControl = MonthControl;
 
@@ -1173,7 +1175,7 @@ namespace Avalonia.Controls
 
                     for (i = startIndex; i <= endIndex; i++)
                     {
-                        if (monthControl.MonthView.Children[i] is CalendarDayButton b)
+                        if (monthControl.MonthView!.Children[i] is CalendarDayButton b)
                         {
                             b.IsSelected = true;
                             var d = b.DataContext as DateTime?;
@@ -1201,6 +1203,8 @@ namespace Avalonia.Controls
         {
             if (HoverEnd != null && HoverStart != null)
             {
+                Debug.Assert(MonthControl is not null);
+
                 CalendarItem monthControl = MonthControl;
 
                 if (HoverEndIndex != null && HoverStartIndex != null)
@@ -1212,7 +1216,7 @@ namespace Avalonia.Controls
                     {
                         for (i = startIndex; i <= endIndex; i++)
                         {
-                            if (monthControl.MonthView.Children[i] is CalendarDayButton b)
+                            if (monthControl.MonthView!.Children[i] is CalendarDayButton b)
                             {
                                 var d = b.DataContext as DateTime?;
 
@@ -1231,7 +1235,7 @@ namespace Avalonia.Controls
                         // It is SingleRange
                         for (i = startIndex; i <= endIndex; i++)
                         {
-                            ((CalendarDayButton)monthControl.MonthView.Children[i]).IsSelected = false;
+                            ((CalendarDayButton)monthControl.MonthView!.Children[i]).IsSelected = false;
                         }
                     }
                 }
@@ -1239,6 +1243,11 @@ namespace Avalonia.Controls
         }
         internal void SortHoverIndexes(out int startIndex, out int endIndex)
         {
+            Debug.Assert(HoverStart.HasValue);
+            Debug.Assert(HoverEnd.HasValue);
+            Debug.Assert(HoverStartIndex.HasValue);
+            Debug.Assert(HoverEndIndex.HasValue);
+
             if (DateTimeHelper.CompareDays(HoverEnd.Value, HoverStart.Value) > 0)
             {
                 startIndex = HoverStartIndex.Value;
@@ -1373,6 +1382,8 @@ namespace Avalonia.Controls
         }
         private void OnMonthClick()
         {
+            Debug.Assert(MonthControl is not null);
+
             CalendarItem monthControl = MonthControl;
             if (monthControl != null && monthControl.YearView != null && monthControl.MonthView != null)
             {
@@ -1400,7 +1411,7 @@ namespace Avalonia.Controls
             }
         }
 
-        public event EventHandler<SelectionChangedEventArgs> SelectedDatesChanged;
+        public event EventHandler<SelectionChangedEventArgs>? SelectedDatesChanged;
 
         /// <summary>
         /// Occurs when the
@@ -1410,19 +1421,19 @@ namespace Avalonia.Controls
         /// <remarks>
         /// This event occurs after DisplayDate is assigned its new value.
         /// </remarks>
-        public event EventHandler<CalendarDateChangedEventArgs> DisplayDateChanged;
+        public event EventHandler<CalendarDateChangedEventArgs>? DisplayDateChanged;
 
         /// <summary>
         /// Occurs when the
         /// <see cref="P:System.Windows.Controls.Calendar.DisplayMode" />
         /// property is changed.
         /// </summary>
-        public event EventHandler<CalendarModeChangedEventArgs> DisplayModeChanged;
+        public event EventHandler<CalendarModeChangedEventArgs>? DisplayModeChanged;
 
         /// <summary>
         /// Inherited code: Requires comment.
         /// </summary>
-        internal event EventHandler<PointerReleasedEventArgs> DayButtonMouseUp;
+        internal event EventHandler<PointerReleasedEventArgs>? DayButtonMouseUp;
 
         /// <summary>
         /// This method adds the days that were selected by Keyboard to the
@@ -1461,7 +1472,7 @@ namespace Avalonia.Controls
                     SelectedDates.ClearInternal();
                     if (shift)
                     {
-                        CalendarDayButton b;
+                        CalendarDayButton? b;
                         _isShiftPressed = true;
                         if (HoverStart == null)
                         {
@@ -1513,6 +1524,8 @@ namespace Avalonia.Controls
                             }
                             else
                             {
+                                Debug.Assert(HoverEndInternal is not null);
+
                                 // For Home, End, PageUp and PageDown Keys there
                                 // is no easy way to predict the index value
                                 b = FindDayButtonFromDay(HoverEndInternal.Value);
@@ -1524,6 +1537,7 @@ namespace Avalonia.Controls
                             }
                         }
 
+                        Debug.Assert(HoverEnd is not null);
                         OnDayClick(HoverEnd.Value);
                         HighlightDays();
                     }
@@ -1557,7 +1571,7 @@ namespace Avalonia.Controls
             base.OnPointerReleased(e);
             if (!HasFocusInternal && e.InitialPressMouseButton == MouseButton.Left)
             {
-                FocusManager.Instance.Focus(this);
+                FocusManager.Instance?.Focus(this);
             }
         }
 
@@ -1876,8 +1890,8 @@ namespace Avalonia.Controls
                                 // since DisplayDate is not equal to
                                 // DateTime.MaxValue we are sure selectedDate is\
                                 // not null
-                                selectedDate = DateTimeHelper.AddMonths(selectedDate.Value, 1).Value;
-                                selectedDate = DateTimeHelper.AddDays(selectedDate.Value, -1).Value;
+                                selectedDate = DateTimeHelper.AddMonths(selectedDate.Value, 1)!.Value;
+                                selectedDate = DateTimeHelper.AddDays(selectedDate.Value, -1)!.Value;
                             }
                             else
                             {
@@ -2098,7 +2112,7 @@ namespace Avalonia.Controls
 
             if (Root != null)
             {
-                CalendarItem month = e.NameScope.Find<CalendarItem>(PART_ElementMonth);
+                CalendarItem? month = e.NameScope.Find<CalendarItem>(PART_ElementMonth);
 
                 if (month != null)
                 {
diff --git a/src/Avalonia.Controls/Calendar/CalendarButton.cs b/src/Avalonia.Controls/Calendar/CalendarButton.cs
index 76af933b55..0a8e4dfae8 100644
--- a/src/Avalonia.Controls/Calendar/CalendarButton.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarButton.cs
@@ -45,7 +45,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the Calendar associated with this button.
         /// </summary>
-        internal Calendar Owner { get; set; }
+        internal Calendar? Owner { get; set; }
 
         /// <summary>
         /// Gets or sets a value indicating whether the button is focused.
@@ -120,7 +120,7 @@ namespace Avalonia.Controls.Primitives
         /// stylus touches the tablet PC) while the mouse pointer is over a
         /// UIElement.
         /// </summary>
-        public event EventHandler<PointerPressedEventArgs> CalendarLeftMouseButtonDown;
+        public event EventHandler<PointerPressedEventArgs>? CalendarLeftMouseButtonDown;
 
         /// <summary>
         /// Occurs when the left mouse button is released (or the tip of the
@@ -128,7 +128,7 @@ namespace Avalonia.Controls.Primitives
         /// stylus) is over a UIElement (or while a UIElement holds mouse
         /// capture).
         /// </summary>
-        public event EventHandler<PointerReleasedEventArgs> CalendarLeftMouseButtonUp;
+        public event EventHandler<PointerReleasedEventArgs>? CalendarLeftMouseButtonUp;
 
         /// <summary>
         /// Provides class handling for the MouseLeftButtonDown event that
diff --git a/src/Avalonia.Controls/Calendar/CalendarDatePicker.cs b/src/Avalonia.Controls/Calendar/CalendarDatePicker.cs
index cd9c80d3e0..c1f487c32d 100644
--- a/src/Avalonia.Controls/Calendar/CalendarDatePicker.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarDatePicker.cs
@@ -123,15 +123,15 @@ namespace Avalonia.Controls
         private const string ElementPopup = "PART_Popup";
         private const string ElementCalendar = "PART_Calendar";
 
-        private Calendar _calendar;
+        private Calendar? _calendar;
         private string _defaultText;
-        private Button _dropDownButton;
+        private Button? _dropDownButton;
         //private Canvas _outsideCanvas;
         //private Canvas _outsidePopupCanvas;
-        private Popup _popUp;
-        private TextBox _textBox;
-        private IDisposable _textBoxTextChangedSubscription;
-        private IDisposable _buttonPointerPressedSubscription;
+        private Popup? _popUp;
+        private TextBox? _textBox;
+        private IDisposable? _textBoxTextChangedSubscription;
+        private IDisposable? _buttonPointerPressedSubscription;
 
         private DateTime? _onOpenSelectedDate;
         private bool _settingSelectedDate;
@@ -141,7 +141,7 @@ namespace Avalonia.Controls
         private DateTime? _displayDateEnd;
         private bool _isDropDownOpen;
         private DateTime? _selectedDate;
-        private string _text;
+        private string? _text;
         private bool _suspendTextChangeHandler = false;
         private bool _isPopupClosing = false;
         private bool _ignoreButtonClick = false;
@@ -153,7 +153,7 @@ namespace Avalonia.Controls
         /// A collection of dates that cannot be selected. The default value is
         /// an empty collection.
         /// </value>
-        public CalendarBlackoutDatesCollection BlackoutDates { get; private set; }
+        public CalendarBlackoutDatesCollection? BlackoutDates { get; private set; }
 
         public static readonly DirectProperty<CalendarDatePicker, DateTime> DisplayDateProperty =
             AvaloniaProperty.RegisterDirect<CalendarDatePicker, DateTime>(
@@ -200,12 +200,12 @@ namespace Avalonia.Controls
                 defaultValue: "d",
                 validate: IsValidDateFormatString);
 
-        public static readonly DirectProperty<CalendarDatePicker, string> TextProperty =
-            AvaloniaProperty.RegisterDirect<CalendarDatePicker, string>(
+        public static readonly DirectProperty<CalendarDatePicker, string?> TextProperty =
+            AvaloniaProperty.RegisterDirect<CalendarDatePicker, string?>(
                 nameof(Text),
                 o => o.Text,
                 (o, v) => o.Text = v);
-        public static readonly StyledProperty<string> WatermarkProperty =
+        public static readonly StyledProperty<string?> WatermarkProperty =
             TextBox.WatermarkProperty.AddOwner<CalendarDatePicker>();
         public static readonly StyledProperty<bool> UseFloatingWatermarkProperty =
             TextBox.UseFloatingWatermarkProperty.AddOwner<CalendarDatePicker>();
@@ -361,13 +361,13 @@ namespace Avalonia.Controls
         /// <exception cref="T:System.ArgumentOutOfRangeException">
         /// The text entered parses to a date that is not selectable.
         /// </exception>
-        public string Text
+        public string? Text
         {
             get { return _text; }
             set { SetAndRaise(TextProperty, ref _text, value); }
         }
 
-        public string Watermark
+        public string? Watermark
         {
             get { return GetValue(WatermarkProperty); }
             set { SetValue(WatermarkProperty, value); }
@@ -401,26 +401,26 @@ namespace Avalonia.Controls
         /// Occurs when the drop-down
         /// <see cref="T:Avalonia.Controls.Calendar" /> is closed.
         /// </summary>
-        public event EventHandler CalendarClosed;
+        public event EventHandler? CalendarClosed;
 
         /// <summary>
         /// Occurs when the drop-down
         /// <see cref="T:Avalonia.Controls.Calendar" /> is opened.
         /// </summary>
-        public event EventHandler CalendarOpened;
+        public event EventHandler? CalendarOpened;
 
         /// <summary>
         /// Occurs when <see cref="P:Avalonia.Controls.DatePicker.Text" />
         /// is assigned a value that cannot be interpreted as a date.
         /// </summary>
-        public event EventHandler<CalendarDatePickerDateValidationErrorEventArgs> DateValidationError;
+        public event EventHandler<CalendarDatePickerDateValidationErrorEventArgs>? DateValidationError;
 
         /// <summary>
         /// Occurs when the
         /// <see cref="P:Avalonia.Controls.CalendarDatePicker.SelectedDate" />
         /// property is changed.
         /// </summary>
-        public event EventHandler<SelectionChangedEventArgs> SelectedDateChanged;
+        public event EventHandler<SelectionChangedEventArgs>? SelectedDateChanged;
 
         static CalendarDatePicker()
         {
@@ -579,14 +579,14 @@ namespace Avalonia.Controls
         
         private void OnIsDropDownOpenChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldValue = (bool)e.OldValue;
-            var value = (bool)e.NewValue;
+            var oldValue = (bool)e.OldValue!;
+            var value = (bool)e.NewValue!;
 
             if (_popUp != null && _popUp.Child != null)
             {
                 if (value != oldValue)
                 {
-                    if (_calendar.DisplayMode != CalendarMode.Month)
+                    if (_calendar!.DisplayMode != CalendarMode.Month)
                     {
                         _calendar.DisplayMode = CalendarMode.Month;
                     }
@@ -660,7 +660,7 @@ namespace Avalonia.Controls
 
                     if (date != null)
                     {
-                        string s = DateTimeToString((DateTime)date);
+                        string? s = DateTimeToString((DateTime)date);
                         Text = s;
                     }
                 }
@@ -679,8 +679,8 @@ namespace Avalonia.Controls
         }
         private void OnTextChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldValue = (string)e.OldValue;
-            var value = (string)e.NewValue;
+            var oldValue = (string?)e.OldValue;
+            var value = (string?)e.NewValue;
 
             if (!_suspendTextChangeHandler)
             {
@@ -731,7 +731,7 @@ namespace Avalonia.Controls
         }
         private void OnDateSelected(DateTime? addedDate, DateTime? removedDate)
         {
-            EventHandler<SelectionChangedEventArgs> handler = this.SelectedDateChanged;
+            EventHandler<SelectionChangedEventArgs>? handler = this.SelectedDateChanged;
             if (null != handler)
             {
                 Collection<DateTime> addedItems = new Collection<DateTime>();
@@ -759,23 +759,23 @@ namespace Avalonia.Controls
             CalendarOpened?.Invoke(this, e);
         }
 
-        private void Calendar_DayButtonMouseUp(object sender, PointerReleasedEventArgs e)
+        private void Calendar_DayButtonMouseUp(object? sender, PointerReleasedEventArgs e)
         {
             Focus();
             IsDropDownOpen = false;
         }      
-        private void Calendar_DisplayDateChanged(object sender, CalendarDateChangedEventArgs e)
+        private void Calendar_DisplayDateChanged(object? sender, CalendarDateChangedEventArgs e)
         {
             if (e.AddedDate != this.DisplayDate)
             {
-                SetValue(DisplayDateProperty, (DateTime) e.AddedDate);
+                SetValue(DisplayDateProperty, (DateTime) e.AddedDate!);
             }
         }
-        private void Calendar_SelectedDatesChanged(object sender, SelectionChangedEventArgs e)
+        private void Calendar_SelectedDatesChanged(object? sender, SelectionChangedEventArgs e)
         {
             Debug.Assert(e.AddedItems.Count < 2, "There should be less than 2 AddedItems!");
 
-            if (e.AddedItems.Count > 0 && SelectedDate.HasValue && DateTime.Compare((DateTime)e.AddedItems[0], SelectedDate.Value) != 0)
+            if (e.AddedItems.Count > 0 && SelectedDate.HasValue && DateTime.Compare((DateTime)e.AddedItems[0]!, SelectedDate.Value) != 0)
             {
                 SelectedDate = (DateTime?)e.AddedItems[0];
             }
@@ -796,7 +796,7 @@ namespace Avalonia.Controls
                 }
             }
         }
-        private void Calendar_PointerReleased(object sender, PointerReleasedEventArgs e)
+        private void Calendar_PointerReleased(object? sender, PointerReleasedEventArgs e)
         {
              
             if (e.InitialPressMouseButton == MouseButton.Left)
@@ -804,10 +804,9 @@ namespace Avalonia.Controls
                 e.Handled = true;
             }
         }
-        private void Calendar_KeyDown(object sender, KeyEventArgs e)
+        private void Calendar_KeyDown(object? sender, KeyEventArgs e)
         {
-            Calendar c = sender as Calendar;
-            Contract.Requires<ArgumentNullException>(c != null);
+            Calendar? c = sender as Calendar ?? throw new ArgumentException("Sender must be Calendar.", nameof(sender));
 
             if (!e.Handled && (e.Key == Key.Enter || e.Key == Key.Space || e.Key == Key.Escape) && c.DisplayMode == CalendarMode.Month)
             {
@@ -820,11 +819,11 @@ namespace Avalonia.Controls
                 }
             }
         }
-        private void TextBox_GotFocus(object sender, RoutedEventArgs e)
+        private void TextBox_GotFocus(object? sender, RoutedEventArgs e)
         {
             IsDropDownOpen = false;
         }
-        private void TextBox_KeyDown(object sender, KeyEventArgs e)
+        private void TextBox_KeyDown(object? sender, KeyEventArgs e)
         {
             if (!e.Handled)
             {
@@ -840,11 +839,11 @@ namespace Avalonia.Controls
                 _suspendTextChangeHandler = false;
             }
         }
-        private void DropDownButton_PointerPressed(object sender, PointerPressedEventArgs e)
+        private void DropDownButton_PointerPressed(object? sender, PointerPressedEventArgs e)
         {
             _ignoreButtonClick = _isPopupClosing;
         }
-        private void DropDownButton_Click(object sender, RoutedEventArgs e)
+        private void DropDownButton_Click(object? sender, RoutedEventArgs e)
         {
             if (!_ignoreButtonClick)
             {
@@ -855,7 +854,7 @@ namespace Avalonia.Controls
                 _ignoreButtonClick = false;
             }
         }
-        private void PopUp_Closed(object sender, EventArgs e)
+        private void PopUp_Closed(object? sender, EventArgs e)
         {
             IsDropDownOpen = false;
 
@@ -891,7 +890,7 @@ namespace Avalonia.Controls
         private void OpenPopUp()
         {
             _onOpenSelectedDate = SelectedDate;
-            _popUp.IsOpen = true;
+            _popUp!.IsOpen = true;
         }
 
         /// <summary>
@@ -914,7 +913,7 @@ namespace Avalonia.Controls
             {
                 newSelectedDate = DateTime.Parse(text, DateTimeHelper.GetCurrentDateFormat());
 
-                if (Calendar.IsValidDateSelection(this._calendar, newSelectedDate))
+                if (Calendar.IsValidDateSelection(this._calendar!, newSelectedDate))
                 {
                     return newSelectedDate;
                 }
@@ -941,7 +940,7 @@ namespace Avalonia.Controls
             }
             return null;
         }
-        private string DateTimeToString(DateTime d)
+        private string? DateTimeToString(DateTime d)
         {
             DateTimeFormatInfo dtfi = DateTimeHelper.GetCurrentDateFormat();
 
@@ -982,7 +981,7 @@ namespace Avalonia.Controls
         {
             SetSelectedDate();
             IsDropDownOpen = true;
-            _calendar.Focus();
+            _calendar!.Focus();
         }
         private void SetSelectedDate()
         {
@@ -1001,7 +1000,7 @@ namespace Avalonia.Controls
                         // ex: SelectedDate = DateTime(1008,12,19) but when
                         // "12/19/08" is parsed it is interpreted as
                         // DateTime(2008,12,19)
-                        string selectedDate = DateTimeToString(SelectedDate.Value);
+                        string? selectedDate = DateTimeToString(SelectedDate.Value);
                         if (selectedDate == s)
                         {
                             return;
@@ -1053,7 +1052,7 @@ namespace Avalonia.Controls
                     // SelectedDate value:
                     if (SelectedDate != null)
                     {
-                        string newtext = this.DateTimeToString(SelectedDate.Value);
+                        string? newtext = this.DateTimeToString(SelectedDate.Value);
                         SetValue(TextProperty, newtext);
                         return SelectedDate;
                     }
diff --git a/src/Avalonia.Controls/Calendar/CalendarDateRange.cs b/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
index 47a0ad047b..88bc5ed7bd 100644
--- a/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarDateRange.cs
@@ -65,7 +65,7 @@ namespace Avalonia.Controls
         /// <returns>Inherited code: Requires comment 2.</returns>
         internal bool ContainsAny(CalendarDateRange range)
         {
-            Contract.Requires<ArgumentNullException>(range != null);
+            _ = range ?? throw new ArgumentNullException(nameof(range));
 
             int start = DateTime.Compare(Start, range.Start);
 
diff --git a/src/Avalonia.Controls/Calendar/CalendarDayButton.cs b/src/Avalonia.Controls/Calendar/CalendarDayButton.cs
index d5748bb9e4..2ba4e36260 100644
--- a/src/Avalonia.Controls/Calendar/CalendarDayButton.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarDayButton.cs
@@ -40,7 +40,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the Calendar associated with this button.
         /// </summary>
-        internal Calendar Owner { get; set; }
+        internal Calendar? Owner { get; set; }
         internal int Index { get; set; }
 
         /// <summary>
@@ -177,7 +177,7 @@ namespace Avalonia.Controls.Primitives
         /// stylus touches the tablet PC) while the mouse pointer is over a
         /// UIElement.
         /// </summary>
-        public event EventHandler<PointerPressedEventArgs> CalendarDayButtonMouseDown;
+        public event EventHandler<PointerPressedEventArgs>? CalendarDayButtonMouseDown;
 
         /// <summary>
         /// Occurs when the left mouse button is released (or the tip of the
@@ -185,7 +185,7 @@ namespace Avalonia.Controls.Primitives
         /// stylus) is over a UIElement (or while a UIElement holds mouse
         /// capture).
         /// </summary>
-        public event EventHandler<PointerReleasedEventArgs> CalendarDayButtonMouseUp;
+        public event EventHandler<PointerReleasedEventArgs>? CalendarDayButtonMouseUp;
 
         /// <summary>
         /// Provides class handling for the MouseLeftButtonDown event that
diff --git a/src/Avalonia.Controls/Calendar/CalendarItem.cs b/src/Avalonia.Controls/Calendar/CalendarItem.cs
index e830717b95..128f406317 100644
--- a/src/Avalonia.Controls/Calendar/CalendarItem.cs
+++ b/src/Avalonia.Controls/Calendar/CalendarItem.cs
@@ -33,10 +33,10 @@ namespace Avalonia.Controls.Primitives
         private const string PART_ElementMonthView = "MonthView";
         private const string PART_ElementYearView = "YearView";
 
-        private Button _headerButton;
-        private Button _nextButton;
-        private Button _previousButton;
-        private ITemplate<IControl> _dayTitleTemplate;
+        private Button? _headerButton;
+        private Button? _nextButton;
+        private Button? _previousButton;
+        private ITemplate<IControl>? _dayTitleTemplate;
         
         private DateTime _currentMonth;
         private bool _isMouseLeftButtonDown = false;
@@ -45,11 +45,11 @@ namespace Avalonia.Controls.Primitives
 
         private System.Globalization.Calendar _calendar = new System.Globalization.GregorianCalendar();
 
-        private PointerPressedEventArgs _downEventArg;
-        private PointerPressedEventArgs _downEventArgYearView;
+        private PointerPressedEventArgs? _downEventArg;
+        private PointerPressedEventArgs? _downEventArgYearView;
 
-        internal Calendar Owner { get; set; }
-        internal CalendarDayButton CurrentButton { get; set; }
+        internal Calendar? Owner { get; set; }
+        internal CalendarDayButton? CurrentButton { get; set; }
 
         public static readonly StyledProperty<IBrush> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
         public IBrush HeaderBackground
@@ -57,13 +57,13 @@ namespace Avalonia.Controls.Primitives
             get { return GetValue(HeaderBackgroundProperty); }
             set { SetValue(HeaderBackgroundProperty, value); }
         }
-        public static readonly DirectProperty<CalendarItem, ITemplate<IControl>> DayTitleTemplateProperty =
-                AvaloniaProperty.RegisterDirect<CalendarItem, ITemplate<IControl>>(
+        public static readonly DirectProperty<CalendarItem, ITemplate<IControl>?> DayTitleTemplateProperty =
+                AvaloniaProperty.RegisterDirect<CalendarItem, ITemplate<IControl>?>(
                     nameof(DayTitleTemplate),
                     o => o.DayTitleTemplate,
                     (o,v) => o.DayTitleTemplate = v,
                     defaultBindingMode: BindingMode.OneTime);
-        public ITemplate<IControl> DayTitleTemplate
+        public ITemplate<IControl>? DayTitleTemplate
         {
             get { return _dayTitleTemplate; }
             set { SetAndRaise(DayTitleTemplateProperty, ref _dayTitleTemplate, value); }
@@ -73,7 +73,7 @@ namespace Avalonia.Controls.Primitives
         /// Gets the button that allows switching between month mode, year mode,
         /// and decade mode. 
         /// </summary>
-        internal Button HeaderButton
+        internal Button? HeaderButton
         {
             get { return _headerButton; }
             private set
@@ -94,7 +94,7 @@ namespace Avalonia.Controls.Primitives
         /// Gets the button that displays the next page of the calendar when it
         /// is clicked.
         /// </summary>
-        internal Button NextButton
+        internal Button? NextButton
         {
             get { return _nextButton; }
             private set
@@ -125,7 +125,7 @@ namespace Avalonia.Controls.Primitives
         /// Gets the button that displays the previous page of the calendar when
         /// it is clicked.
         /// </summary>
-        internal Button PreviousButton
+        internal Button? PreviousButton
         {
             get { return _previousButton; }
             private set
@@ -156,11 +156,11 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the Grid that hosts the content when in month mode.
         /// </summary>
-        internal Grid MonthView { get; set; }
+        internal Grid? MonthView { get; set; }
         /// <summary>
         /// Gets the Grid that hosts the content when in year or decade mode.
         /// </summary>
-        internal Grid YearView { get; set; }
+        internal Grid? YearView { get; set; }
         
         private void PopulateGrids()
         {
@@ -294,7 +294,7 @@ namespace Avalonia.Controls.Primitives
         {
             for (int childIndex = 0; childIndex < Calendar.ColumnsPerMonth; childIndex++)
             {
-                var daytitle = MonthView.Children[childIndex];
+                var daytitle = MonthView!.Children[childIndex];
                 if (daytitle != null)
                 {
                     if (Owner != null)
@@ -495,8 +495,7 @@ namespace Avalonia.Controls.Primitives
 
             for (int childIndex = Calendar.ColumnsPerMonth; childIndex < count; childIndex++)
             {
-                CalendarDayButton childButton = MonthView.Children[childIndex] as CalendarDayButton;
-                Contract.Requires<ArgumentNullException>(childButton != null);
+                CalendarDayButton childButton = (CalendarDayButton)MonthView!.Children[childIndex];
 
                 childButton.Index = childIndex;
                 SetButtonState(childButton, dateToAdd);
@@ -532,8 +531,7 @@ namespace Avalonia.Controls.Primitives
                     childIndex++;
                     for (int i = childIndex; i < count; i++)
                     {
-                        childButton = MonthView.Children[i] as CalendarDayButton;
-                        Contract.Requires<ArgumentNullException>(childButton != null);
+                        childButton = (CalendarDayButton)MonthView.Children[i];
                         // button needs a content to occupy the necessary space
                         // for the content presenter
                         childButton.Content = i.ToString(DateTimeHelper.GetCurrentDateFormat());
@@ -626,10 +624,9 @@ namespace Avalonia.Controls.Primitives
         private void SetMonthButtonsForYearMode()
         {
             int count = 0;
-            foreach (object child in YearView.Children)
+            foreach (object child in YearView!.Children)
             {
-                CalendarButton childButton = child as CalendarButton;
-                Contract.Requires<ArgumentNullException>(childButton != null);
+                CalendarButton childButton = (CalendarButton)child;
                 // There should be no time component. Time is 12:00 AM
                 DateTime day = new DateTime(_currentMonth.Year, count + 1, 1);
                 childButton.DataContext = day;
@@ -703,7 +700,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null && calendarButton != null && calendarButton.DataContext != null)
             {
-                Owner.FocusCalendarButton.IsCalendarButtonFocused = false;
+                Owner.FocusCalendarButton!.IsCalendarButtonFocused = false;
                 Owner.FocusCalendarButton = calendarButton;
                 calendarButton.IsCalendarButtonFocused = Owner.HasFocusInternal;
 
@@ -722,10 +719,9 @@ namespace Avalonia.Controls.Primitives
         {
             int year;
             int count = -1;
-            foreach (object child in YearView.Children)
+            foreach (object child in YearView!.Children)
             {
-                CalendarButton childButton = child as CalendarButton;
-                Contract.Requires<ArgumentNullException>(childButton != null);
+                CalendarButton childButton = (CalendarButton)child;
                 year = decade + count;
 
                 if (year <= DateTime.MaxValue.Year && year >= DateTime.MinValue.Year)
@@ -797,7 +793,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        internal void HeaderButton_Click(object sender, RoutedEventArgs e)
+        internal void HeaderButton_Click(object? sender, RoutedEventArgs e)
         {
             if (Owner != null)
             {
@@ -805,7 +801,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     Owner.Focus();
                 }
-                Button b = (Button)sender;
+                Button b = (Button)sender!;
                 DateTime d;
 
                 if (b.IsEnabled)
@@ -833,7 +829,7 @@ namespace Avalonia.Controls.Primitives
                 }
             }
         }
-        internal void PreviousButton_Click(object sender, RoutedEventArgs e)
+        internal void PreviousButton_Click(object? sender, RoutedEventArgs e)
         {
             if (Owner != null)
             {
@@ -842,14 +838,14 @@ namespace Avalonia.Controls.Primitives
                     Owner.Focus();
                 }
 
-                Button b = (Button)sender;
+                Button b = (Button)sender!;
                 if (b.IsEnabled)
                 {
                     Owner.OnPreviousClick();
                 }
             }
         }
-        internal void NextButton_Click(object sender, RoutedEventArgs e)
+        internal void NextButton_Click(object? sender, RoutedEventArgs e)
         {
             if (Owner != null)
             {
@@ -857,7 +853,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     Owner.Focus();
                 }
-                Button b = (Button)sender;
+                Button b = (Button)sender!;
 
                 if (b.IsEnabled)
                 {
@@ -866,7 +862,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        internal void Cell_MouseEnter(object sender, PointerEventArgs e)
+        internal void Cell_MouseEnter(object? sender, PointerEventArgs e)
         {
             if (Owner != null)
             {
@@ -878,7 +874,7 @@ namespace Avalonia.Controls.Primitives
                     {
                         case CalendarSelectionMode.SingleDate:
                             {
-                                DateTime selectedDate = (DateTime)b.DataContext;
+                                DateTime selectedDate = (DateTime)b.DataContext!;
                                 Owner.CalendarDatePickerDisplayDateFlag = true;
                                 if (Owner.SelectedDates.Count == 0)
                                 {
@@ -906,7 +902,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
         
-        internal void Cell_MouseLeftButtonDown(object sender, PointerPressedEventArgs e)
+        internal void Cell_MouseLeftButtonDown(object? sender, PointerPressedEventArgs e)
         {
             if (Owner != null)
             {
@@ -917,14 +913,14 @@ namespace Avalonia.Controls.Primitives
 
                 bool ctrl, shift;
                 CalendarExtensions.GetMetaKeyState(e.KeyModifiers, out ctrl, out shift);
-                CalendarDayButton b = sender as CalendarDayButton;
+                CalendarDayButton b = (CalendarDayButton)sender!;
 
                 if (b != null)
                 {
                     _isControlPressed = ctrl;
                     if (b.IsEnabled && !b.IsBlackout)
                     {
-                        DateTime selectedDate = (DateTime)b.DataContext;
+                        DateTime selectedDate = (DateTime)b.DataContext!;
                         Contract.Requires<ArgumentNullException>(selectedDate != null);
                         _isMouseLeftButtonDown = true;
                         // null check is added for unit tests
@@ -1027,7 +1023,7 @@ namespace Avalonia.Controls.Primitives
             if (Owner != null)
             {
                 Owner.HoverEndIndex = b.Index;
-                Owner.HoverEnd = (DateTime)b.DataContext;
+                Owner.HoverEnd = (DateTime)b.DataContext!;
 
                 if (Owner.HoverEnd != null && Owner.HoverStart != null)
                 {
@@ -1041,11 +1037,11 @@ namespace Avalonia.Controls.Primitives
                 }
             }
         }
-        internal void Cell_MouseLeftButtonUp(object sender, PointerReleasedEventArgs e)
+        internal void Cell_MouseLeftButtonUp(object? sender, PointerReleasedEventArgs e)
         {
             if (Owner != null)
             {
-                CalendarDayButton b = sender as CalendarDayButton;
+                CalendarDayButton? b = sender as CalendarDayButton;
                 if (b != null && !b.IsBlackout)
                 {
                     Owner.OnDayButtonMouseUp(e);
@@ -1094,14 +1090,13 @@ namespace Avalonia.Controls.Primitives
                 }
             }
         }
-        private void Cell_Click(object sender, RoutedEventArgs e)
+        private void Cell_Click(object? sender, RoutedEventArgs e)
         {
             if (Owner != null)
             {
                 if (_isControlPressed && Owner.SelectionMode == CalendarSelectionMode.MultipleRange)
                 {
-                    CalendarDayButton b = sender as CalendarDayButton;
-                    Contract.Requires<ArgumentNullException>(b != null);
+                    CalendarDayButton b = (CalendarDayButton)sender!;
 
                     if (b.IsSelected)
                     {
@@ -1118,10 +1113,9 @@ namespace Avalonia.Controls.Primitives
             _isControlPressed = false;
         }
 
-        private void Month_CalendarButtonMouseDown(object sender, PointerPressedEventArgs e)
+        private void Month_CalendarButtonMouseDown(object? sender, PointerPressedEventArgs e)
         {
-            CalendarButton b = sender as CalendarButton;
-            Contract.Requires<ArgumentNullException>(b != null);
+            CalendarButton b = (CalendarButton)sender!;
 
             _isMouseLeftButtonDownYearView = true;
 
@@ -1133,13 +1127,13 @@ namespace Avalonia.Controls.Primitives
             UpdateYearViewSelection(b);
         }
 
-        internal void Month_CalendarButtonMouseUp(object sender, PointerReleasedEventArgs e)
+        internal void Month_CalendarButtonMouseUp(object? sender, PointerReleasedEventArgs e)
         {
             _isMouseLeftButtonDownYearView = false;
 
             if (Owner != null)
             {
-                DateTime newmonth = (DateTime)((CalendarButton)sender).DataContext;
+                DateTime newmonth = (DateTime)((CalendarButton)sender!).DataContext!;
 
                 if (Owner.DisplayMode == CalendarMode.Year)
                 {
@@ -1155,12 +1149,11 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void Month_MouseEnter(object sender, PointerEventArgs e)
+        private void Month_MouseEnter(object? sender, PointerEventArgs e)
         {
             if (_isMouseLeftButtonDownYearView)
             {
-                CalendarButton b = sender as CalendarButton;
-                Contract.Requires<ArgumentNullException>(b != null);
+                CalendarButton b = (CalendarButton)sender!;
                 UpdateYearViewSelection(b);
             }
         }
diff --git a/src/Avalonia.Controls/Calendar/DateTimeHelper.cs b/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
index 033bfc5726..eb90f6c399 100644
--- a/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
+++ b/src/Avalonia.Controls/Calendar/DateTimeHelper.cs
@@ -5,6 +5,7 @@
 
 using System;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Globalization;
 
 namespace Avalonia.Controls
@@ -73,6 +74,7 @@ namespace Avalonia.Controls
             return newD;
         }
 
+        [return: NotNullIfNotNull("d")]
         public static DateTime? DiscardTime(DateTime? d)
         {
             if (d == null)
diff --git a/src/Avalonia.Controls/Calendar/SelectedDatesCollection.cs b/src/Avalonia.Controls/Calendar/SelectedDatesCollection.cs
index 3573c67057..c7c35718e0 100644
--- a/src/Avalonia.Controls/Calendar/SelectedDatesCollection.cs
+++ b/src/Avalonia.Controls/Calendar/SelectedDatesCollection.cs
@@ -49,7 +49,7 @@ namespace Avalonia.Controls.Primitives
 
         private void InvokeCollectionChanged(System.Collections.IList removedItems, System.Collections.IList addedItems)
         {
-            _owner.OnSelectedDatesCollectionChanged(new SelectionChangedEventArgs(null, removedItems, addedItems));
+            _owner.OnSelectedDatesCollectionChanged(new SelectionChangedEventArgs(SelectingItemsControl.SelectionChangedEvent, removedItems, addedItems));
         }
 
         /// <summary>
@@ -119,7 +119,7 @@ namespace Avalonia.Controls.Primitives
                 }
             }
 
-            _owner.OnSelectedDatesCollectionChanged(new SelectionChangedEventArgs(null, _owner.RemovedItems, _addedItems));
+            _owner.OnSelectedDatesCollectionChanged(new SelectionChangedEventArgs(SelectingItemsControl.SelectionChangedEvent, _owner.RemovedItems, _addedItems));
             _owner.RemovedItems.Clear();
             _owner.UpdateMonths();
             _isRangeAdded = false;
diff --git a/src/Avalonia.Controls/Canvas.cs b/src/Avalonia.Controls/Canvas.cs
index 5cc823a6af..fabf8978c7 100644
--- a/src/Avalonia.Controls/Canvas.cs
+++ b/src/Avalonia.Controls/Canvas.cs
@@ -135,7 +135,7 @@ namespace Avalonia.Controls
         /// <param name="from">The control from which movement begins.</param>
         /// <param name="wrap">Whether to wrap around when the first or last item is reached.</param>
         /// <returns>The control.</returns>
-        IInputElement INavigableContainer.GetControl(NavigationDirection direction, IInputElement from, bool wrap)
+        IInputElement? INavigableContainer.GetControl(NavigationDirection direction, IInputElement? from, bool wrap)
         {
             // TODO: Implement this
             return null;
diff --git a/src/Avalonia.Controls/Chrome/TitleBar.cs b/src/Avalonia.Controls/Chrome/TitleBar.cs
index fbddb06952..52fbee6952 100644
--- a/src/Avalonia.Controls/Chrome/TitleBar.cs
+++ b/src/Avalonia.Controls/Chrome/TitleBar.cs
@@ -36,7 +36,7 @@ namespace Avalonia.Controls.Chrome
                     }
                 }
 
-                IsVisible = window.PlatformImpl.NeedsManagedDecorations;
+                IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
             }
         }
 
diff --git a/src/Avalonia.Controls/ComboBox.cs b/src/Avalonia.Controls/ComboBox.cs
index 274696d501..d38cd3a0fd 100644
--- a/src/Avalonia.Controls/ComboBox.cs
+++ b/src/Avalonia.Controls/ComboBox.cs
@@ -46,8 +46,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="SelectionBoxItem"/> property.
         /// </summary>
-        public static readonly DirectProperty<ComboBox, object> SelectionBoxItemProperty =
-            AvaloniaProperty.RegisterDirect<ComboBox, object>(nameof(SelectionBoxItem), o => o.SelectionBoxItem);
+        public static readonly DirectProperty<ComboBox, object?> SelectionBoxItemProperty =
+            AvaloniaProperty.RegisterDirect<ComboBox, object?>(nameof(SelectionBoxItem), o => o.SelectionBoxItem);
 
         /// <summary>
         /// Defines the <see cref="VirtualizationMode"/> property.
@@ -58,14 +58,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="PlaceholderText"/> property.
         /// </summary>
-        public static readonly StyledProperty<string> PlaceholderTextProperty =
-            AvaloniaProperty.Register<ComboBox, string>(nameof(PlaceholderText));
+        public static readonly StyledProperty<string?> PlaceholderTextProperty =
+            AvaloniaProperty.Register<ComboBox, string?>(nameof(PlaceholderText));
 
         /// <summary>
         /// Defines the <see cref="PlaceholderForeground"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> PlaceholderForegroundProperty =
-            AvaloniaProperty.Register<ComboBox, IBrush>(nameof(PlaceholderForeground));
+        public static readonly StyledProperty<IBrush?> PlaceholderForegroundProperty =
+            AvaloniaProperty.Register<ComboBox, IBrush?>(nameof(PlaceholderForeground));
 
         /// <summary>
         /// Defines the <see cref="HorizontalContentAlignment"/> property.
@@ -80,8 +80,8 @@ namespace Avalonia.Controls
             ContentControl.VerticalContentAlignmentProperty.AddOwner<ComboBox>();
 
         private bool _isDropDownOpen;
-        private Popup _popup;
-        private object _selectionBoxItem;
+        private Popup? _popup;
+        private object? _selectionBoxItem;
         private readonly CompositeDisposable _subscriptionsOnOpen = new CompositeDisposable();
 
         /// <summary>
@@ -117,7 +117,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the item to display as the control's content.
         /// </summary>
-        protected object SelectionBoxItem
+        protected object? SelectionBoxItem
         {
             get { return _selectionBoxItem; }
             set { SetAndRaise(SelectionBoxItemProperty, ref _selectionBoxItem, value); }
@@ -126,7 +126,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the PlaceHolder text.
         /// </summary>
-        public string PlaceholderText
+        public string? PlaceholderText
         {
             get { return GetValue(PlaceholderTextProperty); }
             set { SetValue(PlaceholderTextProperty, value); }
@@ -135,7 +135,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the Brush that renders the placeholder text.
         /// </summary>
-        public IBrush PlaceholderForeground
+        public IBrush? PlaceholderForeground
         {
             get { return GetValue(PlaceholderForegroundProperty); }
             set { SetValue(PlaceholderForegroundProperty, value); }
@@ -262,9 +262,9 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         protected override void OnPointerReleased(PointerReleasedEventArgs e)
         {
-            if (!e.Handled)
+            if (!e.Handled && e.Source is IVisual source)
             {
-                if (_popup?.IsInsidePopup((IVisual)e.Source) == true)
+                if (_popup?.IsInsidePopup(source) == true)
                 {
                     if (UpdateSelectionFromEventSource(e.Source))
                     {
@@ -304,7 +304,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void PopupClosed(object sender, EventArgs e)
+        private void PopupClosed(object? sender, EventArgs e)
         {
             _subscriptionsOnOpen.Clear();
 
@@ -314,7 +314,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void PopupOpened(object sender, EventArgs e)
+        private void PopupOpened(object? sender, EventArgs e)
         {
             TryFocusSelectedItem();
 
@@ -326,7 +326,7 @@ namespace Avalonia.Controls
                 toplevel.AddDisposableHandler(PointerWheelChangedEvent, (s, ev) =>
                 {
                     //eat wheel scroll event outside dropdown popup while it's open
-                    if (IsDropDownOpen && (ev.Source as IVisual).GetVisualRoot() == toplevel)
+                    if (IsDropDownOpen && (ev.Source as IVisual)?.GetVisualRoot() == toplevel)
                     {
                         ev.Handled = true;
                     }
@@ -360,12 +360,12 @@ namespace Avalonia.Controls
             var selectedIndex = SelectedIndex;
             if (IsDropDownOpen && selectedIndex != -1)
             {
-                var container = ItemContainerGenerator.ContainerFromIndex(selectedIndex);
+                var container = ItemContainerGenerator!.ContainerFromIndex(selectedIndex);
 
                 if (container == null && SelectedIndex != -1)
                 {
                     ScrollIntoView(Selection.SelectedIndex);
-                    container = ItemContainerGenerator.ContainerFromIndex(selectedIndex);
+                    container = ItemContainerGenerator!.ContainerFromIndex(selectedIndex);
                 }
 
                 if (container != null && CanFocus(container))
@@ -377,7 +377,7 @@ namespace Avalonia.Controls
 
         private bool CanFocus(IControl control) => control.Focusable && control.IsEffectivelyEnabled && control.IsVisible;
 
-        private void UpdateSelectionBoxItem(object item)
+        private void UpdateSelectionBoxItem(object? item)
         {
             var contentControl = item as IContentControl;
 
@@ -415,7 +415,7 @@ namespace Avalonia.Controls
 
         private void SelectFocusedItem()
         {
-            foreach (ItemContainerInfo dropdownItem in ItemContainerGenerator.Containers)
+            foreach (ItemContainerInfo dropdownItem in ItemContainerGenerator!.Containers)
             {
                 if (dropdownItem.ContainerControl.IsFocused)
                 {
diff --git a/src/Avalonia.Controls/ContentControl.cs b/src/Avalonia.Controls/ContentControl.cs
index 1329bd91c7..ac7d24be92 100644
--- a/src/Avalonia.Controls/ContentControl.cs
+++ b/src/Avalonia.Controls/ContentControl.cs
@@ -17,14 +17,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Content"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> ContentProperty =
-            AvaloniaProperty.Register<ContentControl, object>(nameof(Content));
+        public static readonly StyledProperty<object?> ContentProperty =
+            AvaloniaProperty.Register<ContentControl, object?>(nameof(Content));
 
         /// <summary>
         /// Defines the <see cref="ContentTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ContentTemplateProperty =
-            AvaloniaProperty.Register<ContentControl, IDataTemplate>(nameof(ContentTemplate));
+        public static readonly StyledProperty<IDataTemplate?> ContentTemplateProperty =
+            AvaloniaProperty.Register<ContentControl, IDataTemplate?>(nameof(ContentTemplate));
 
         /// <summary>
         /// Defines the <see cref="HorizontalContentAlignment"/> property.
@@ -48,7 +48,7 @@ namespace Avalonia.Controls
         /// </summary>
         [Content]
         [DependsOn(nameof(ContentTemplate))]
-        public object Content
+        public object? Content
         {
             get { return GetValue(ContentProperty); }
             set { SetValue(ContentProperty, value); }
@@ -57,7 +57,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the data template used to display the content of the control.
         /// </summary>
-        public IDataTemplate ContentTemplate
+        public IDataTemplate? ContentTemplate
         {
             get { return GetValue(ContentTemplateProperty); }
             set { SetValue(ContentTemplateProperty, value); }
@@ -66,7 +66,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the presenter from the control's template.
         /// </summary>
-        public IContentPresenter Presenter
+        public IContentPresenter? Presenter
         {
             get;
             private set;
diff --git a/src/Avalonia.Controls/ContextMenu.cs b/src/Avalonia.Controls/ContextMenu.cs
index d94bff37df..d5cadc16e9 100644
--- a/src/Avalonia.Controls/ContextMenu.cs
+++ b/src/Avalonia.Controls/ContextMenu.cs
@@ -368,7 +368,7 @@ namespace Avalonia.Controls
             });
         }
 
-        private void PopupOpened(object sender, EventArgs e)
+        private void PopupOpened(object? sender, EventArgs e)
         {
             _previousFocus = FocusManager.Instance?.Current;
             Focus();
@@ -376,12 +376,12 @@ namespace Avalonia.Controls
             _popupHostChangedHandler?.Invoke(_popup!.Host);
         }
 
-        private void PopupClosing(object sender, CancelEventArgs e)
+        private void PopupClosing(object? sender, CancelEventArgs e)
         {
             e.Cancel = CancelClosing();
         }
 
-        private void PopupClosed(object sender, EventArgs e)
+        private void PopupClosed(object? sender, EventArgs e)
         {
             foreach (var i in LogicalChildren)
             {
@@ -411,7 +411,7 @@ namespace Avalonia.Controls
             _popupHostChangedHandler?.Invoke(null);
         }
 
-        private void PopupKeyUp(object sender, KeyEventArgs e)
+        private void PopupKeyUp(object? sender, KeyEventArgs e)
         {
             if (IsOpen)
             {
@@ -426,7 +426,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private static void ControlContextRequested(object sender, ContextRequestedEventArgs e)
+        private static void ControlContextRequested(object? sender, ContextRequestedEventArgs e)
         {
             if (sender is Control control
                 && control.ContextMenu is ContextMenu contextMenu
@@ -439,7 +439,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private static void ControlDetachedFromVisualTree(object sender, VisualTreeAttachmentEventArgs e)
+        private static void ControlDetachedFromVisualTree(object? sender, VisualTreeAttachmentEventArgs e)
         {
             if (sender is Control control
                 && control.ContextMenu is ContextMenu contextMenu)
diff --git a/src/Avalonia.Controls/Control.cs b/src/Avalonia.Controls/Control.cs
index 441421181c..807e992d55 100644
--- a/src/Avalonia.Controls/Control.cs
+++ b/src/Avalonia.Controls/Control.cs
@@ -112,7 +112,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when the user has completed a context input gesture, such as a right-click.
         /// </summary>
-        public event EventHandler<ContextRequestedEventArgs> ContextRequested
+        public event EventHandler<ContextRequestedEventArgs>? ContextRequested
         {
             add => AddHandler(ContextRequestedEvent, value);
             remove => RemoveHandler(ContextRequestedEvent, value);
diff --git a/src/Avalonia.Controls/ControlExtensions.cs b/src/Avalonia.Controls/ControlExtensions.cs
index c3cab0a729..1718679294 100644
--- a/src/Avalonia.Controls/ControlExtensions.cs
+++ b/src/Avalonia.Controls/ControlExtensions.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Controls
         /// <param name="control">The control.</param>
         public static void BringIntoView(this IControl control)
         {
-            Contract.Requires<ArgumentNullException>(control != null);
+            _ = control ?? throw new ArgumentNullException(nameof(control));
 
             control.BringIntoView(new Rect(control.Bounds.Size));
         }
@@ -29,7 +29,7 @@ namespace Avalonia.Controls
         /// <param name="rect">The area of the control to being into view.</param>
         public static void BringIntoView(this IControl control, Rect rect)
         {
-            Contract.Requires<ArgumentNullException>(control != null);
+            _ = control ?? throw new ArgumentNullException(nameof(control));
 
             if (control.IsEffectivelyVisible)
             {
@@ -51,10 +51,10 @@ namespace Avalonia.Controls
         /// <param name="control">The control to look in.</param>
         /// <param name="name">The name of the control to find.</param>
         /// <returns>The control or null if not found.</returns>
-        public static T FindControl<T>(this IControl control, string name) where T : class, IControl
+        public static T? FindControl<T>(this IControl control, string name) where T : class, IControl
         {
-            Contract.Requires<ArgumentNullException>(control != null);
-            Contract.Requires<ArgumentNullException>(name != null);
+            _ = control ?? throw new ArgumentNullException(nameof(control));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
 
             var nameScope = control.FindNameScope();
 
@@ -66,6 +66,29 @@ namespace Avalonia.Controls
             return nameScope.Find<T>(name);
         }
 
+        /// <summary>
+        /// Finds the named control in the scope of the specified control and throws if not found.
+        /// </summary>
+        /// <typeparam name="T">The type of the control to find.</typeparam>
+        /// <param name="control">The control to look in.</param>
+        /// <param name="name">The name of the control to find.</param>
+        /// <returns>The control.</returns>
+        public static T GetControl<T>(this IControl control, string name) where T : class, IControl
+        {
+            _ = control ?? throw new ArgumentNullException(nameof(control));
+            _ = name ?? throw new ArgumentNullException(nameof(name));
+
+            var nameScope = control.FindNameScope();
+
+            if (nameScope == null)
+            {
+                throw new InvalidOperationException("Could not find parent name scope.");
+            }
+
+            return nameScope.Find<T>(name) ??
+                throw new ArgumentException($"Could not find control named '{name}'.");
+        }
+
         /// <summary>
         /// Sets a pseudoclass depending on an observable trigger.
         /// </summary>
@@ -75,7 +98,7 @@ namespace Avalonia.Controls
         /// <returns>A disposable used to cancel the subscription.</returns>
         public static IDisposable Set(this IPseudoClasses classes, string name, IObservable<bool> trigger)
         {
-            Contract.Requires<ArgumentNullException>(classes != null);
+            _ = classes ?? throw new ArgumentNullException(nameof(classes));
 
             return trigger.Subscribe(x => classes.Set(name, x));
         }
diff --git a/src/Avalonia.Controls/Converters/MarginMultiplierConverter.cs b/src/Avalonia.Controls/Converters/MarginMultiplierConverter.cs
index 46affcbe33..b0c30ea11f 100644
--- a/src/Avalonia.Controls/Converters/MarginMultiplierConverter.cs
+++ b/src/Avalonia.Controls/Converters/MarginMultiplierConverter.cs
@@ -16,7 +16,7 @@ namespace Avalonia.Controls.Converters
 
         public bool Bottom { get; set; } = false;
 
-        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             if (value is int scalarDepth)
             {
@@ -38,7 +38,7 @@ namespace Avalonia.Controls.Converters
             
         }
 
-        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             throw new System.NotImplementedException();
         }
diff --git a/src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs b/src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs
index 65f95808ff..6ab2f4c517 100644
--- a/src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs
+++ b/src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs
@@ -11,7 +11,7 @@ namespace Avalonia.Controls.Converters
     {
         public static readonly MenuScrollingVisibilityConverter Instance = new MenuScrollingVisibilityConverter();
 
-        public object Convert(IList<object> values, Type targetType, object parameter, CultureInfo culture)
+        public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
         {
             if (parameter == null ||
                 values == null ||
diff --git a/src/Avalonia.Controls/Converters/PlatformKeyGestureConverter.cs b/src/Avalonia.Controls/Converters/PlatformKeyGestureConverter.cs
index 2af8dba479..9a657cce68 100644
--- a/src/Avalonia.Controls/Converters/PlatformKeyGestureConverter.cs
+++ b/src/Avalonia.Controls/Converters/PlatformKeyGestureConverter.cs
@@ -13,7 +13,7 @@ namespace Avalonia.Controls.Converters
     /// </summary>
     public class PlatformKeyGestureConverter : IValueConverter
     {
-        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? Convert(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             if (value is null)
             {
@@ -29,7 +29,7 @@ namespace Avalonia.Controls.Converters
             }
         }
 
-        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
+        public object? ConvertBack(object? value, Type targetType, object? parameter, CultureInfo culture)
         {
             throw new NotImplementedException();
         }
diff --git a/src/Avalonia.Controls/DataValidationErrors.cs b/src/Avalonia.Controls/DataValidationErrors.cs
index 3c64691816..00a06101c5 100644
--- a/src/Avalonia.Controls/DataValidationErrors.cs
+++ b/src/Avalonia.Controls/DataValidationErrors.cs
@@ -21,8 +21,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the DataValidationErrors.Errors attached property.
         /// </summary>
-        public static readonly AttachedProperty<IEnumerable<object>> ErrorsProperty =
-            AvaloniaProperty.RegisterAttached<DataValidationErrors, Control, IEnumerable<object>>("Errors");
+        public static readonly AttachedProperty<IEnumerable<object>?> ErrorsProperty =
+            AvaloniaProperty.RegisterAttached<DataValidationErrors, Control, IEnumerable<object>?>("Errors");
 
         /// <summary>
         /// Defines the DataValidationErrors.HasErrors attached property.
@@ -34,15 +34,15 @@ namespace Avalonia.Controls
             AvaloniaProperty.Register<DataValidationErrors, IDataTemplate>(nameof(ErrorTemplate));
 
 
-        private Control _owner;
+        private Control? _owner;
 
-        public static readonly DirectProperty<DataValidationErrors, Control> OwnerProperty =
-            AvaloniaProperty.RegisterDirect<DataValidationErrors, Control>(
+        public static readonly DirectProperty<DataValidationErrors, Control?> OwnerProperty =
+            AvaloniaProperty.RegisterDirect<DataValidationErrors, Control?>(
                 nameof(Owner),
                 o => o.Owner,
                 (o, v) => o.Owner = v);
 
-        public Control Owner
+        public Control? Owner
         {
             get { return _owner; }
             set { SetAndRaise(OwnerProperty, ref _owner, value); }
@@ -75,7 +75,7 @@ namespace Avalonia.Controls
         private static void ErrorsChanged(AvaloniaPropertyChangedEventArgs e)
         {
             var control = (Control)e.Sender;
-            var errors = (IEnumerable<object>)e.NewValue;
+            var errors = (IEnumerable<object>?)e.NewValue;
 
             var hasErrors = false;
             if (errors != null && errors.Any())
@@ -87,18 +87,18 @@ namespace Avalonia.Controls
         {
             var control = (Control)e.Sender;
             var classes = (IPseudoClasses)control.Classes;
-            classes.Set(":error", (bool)e.NewValue);
+            classes.Set(":error", (bool)e.NewValue!);
         }
 
-        public static IEnumerable<object> GetErrors(Control control)
+        public static IEnumerable<object>? GetErrors(Control control)
         {
             return control.GetValue(ErrorsProperty);
         }
-        public static void SetErrors(Control control, IEnumerable<object> errors)
+        public static void SetErrors(Control control, IEnumerable<object>? errors)
         {
             control.SetValue(ErrorsProperty, errors);
         }
-        public static void SetError(Control control, Exception error)
+        public static void SetError(Control control, Exception? error)
         {
             SetErrors(control, UnpackException(error));
         }
@@ -111,7 +111,7 @@ namespace Avalonia.Controls
             return control.GetValue(HasErrorsProperty);
         }
 
-        private static IEnumerable<object> UnpackException(Exception exception)
+        private static IEnumerable<object>? UnpackException(Exception? exception)
         {
             if (exception != null)
             {
@@ -132,8 +132,9 @@ namespace Avalonia.Controls
 
         private static object GetExceptionData(Exception exception)
         {
-            if (exception is DataValidationException dataValidationException)
-                return dataValidationException.ErrorData;
+            if (exception is DataValidationException dataValidationException &&
+                dataValidationException.ErrorData is object data)
+                return data;
 
             return exception;
         }
diff --git a/src/Avalonia.Controls/DateTimePickers/DatePicker.cs b/src/Avalonia.Controls/DateTimePickers/DatePicker.cs
index 5893a02b04..8c6ac17280 100644
--- a/src/Avalonia.Controls/DateTimePickers/DatePicker.cs
+++ b/src/Avalonia.Controls/DateTimePickers/DatePicker.cs
@@ -93,15 +93,15 @@ namespace Avalonia.Controls
                 defaultBindingMode: BindingMode.TwoWay);
 
         // Template Items
-        private Button _flyoutButton;
-        private TextBlock _dayText;
-        private TextBlock _monthText;
-        private TextBlock _yearText;
-        private Grid _container;
-        private Rectangle _spacer1;
-        private Rectangle _spacer2;
-        private Popup _popup;
-        private DatePickerPresenter _presenter;
+        private Button? _flyoutButton;
+        private TextBlock? _dayText;
+        private TextBlock? _monthText;
+        private TextBlock? _yearText;
+        private Grid? _container;
+        private Rectangle? _spacer1;
+        private Rectangle? _spacer2;
+        private Popup? _popup;
+        private DatePickerPresenter? _presenter;
 
         private bool _areControlsAvailable;
 
@@ -256,7 +256,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Raised when the <see cref="SelectedDate"/> changes
         /// </summary>
-        public event EventHandler<DatePickerSelectedValueChangedEventArgs> SelectedDateChanged;
+        public event EventHandler<DatePickerSelectedValueChangedEventArgs>? SelectedDateChanged;
 
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
@@ -307,16 +307,16 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnDismissPicker(object sender, EventArgs e)
+        private void OnDismissPicker(object? sender, EventArgs e)
         {
-            _popup.Close();
+            _popup!.Close();
             Focus();
         }
 
-        private void OnConfirmed(object sender, EventArgs e)
+        private void OnConfirmed(object? sender, EventArgs e)
         {
-            _popup.Close();
-            SelectedDate = _presenter.Date;
+            _popup!.Close();
+            SelectedDate = _presenter!.Date;
         }
 
         private void SetGrid()
@@ -325,7 +325,7 @@ namespace Avalonia.Controls
                 return;
 
             var fmt = CultureInfo.CurrentCulture.DateTimeFormat.ShortDatePattern;
-            var columns = new List<(TextBlock, int)>
+            var columns = new List<(TextBlock?, int)>
             {
                 (_monthText, MonthVisible ? fmt.IndexOf("m", StringComparison.OrdinalIgnoreCase) : -1),
                 (_yearText, YearVisible ? fmt.IndexOf("y", StringComparison.OrdinalIgnoreCase) : -1),
@@ -366,11 +366,11 @@ namespace Avalonia.Controls
             var isSpacer1Visible = columnIndex > 1;
             var isSpacer2Visible = columnIndex > 2;
             // ternary conditional operator is used to make sure grid cells will be validated
-            Grid.SetColumn(_spacer1, isSpacer1Visible ? 1 : 0);
-            Grid.SetColumn(_spacer2, isSpacer2Visible ? 3 : 0);
+            Grid.SetColumn(_spacer1!, isSpacer1Visible ? 1 : 0);
+            Grid.SetColumn(_spacer2!, isSpacer2Visible ? 3 : 0);
 
-            _spacer1.IsVisible = isSpacer1Visible;
-            _spacer2.IsVisible = isSpacer2Visible;
+            _spacer1!.IsVisible = isSpacer1Visible;
+            _spacer2!.IsVisible = isSpacer2Visible;
         }
 
         private void SetSelectedDateText()
@@ -382,37 +382,37 @@ namespace Avalonia.Controls
             {
                 PseudoClasses.Set(":hasnodate", false);
                 var selDate = SelectedDate.Value;
-                _monthText.Text = selDate.ToString(MonthFormat);
-                _yearText.Text = selDate.ToString(YearFormat);
-                _dayText.Text = selDate.ToString(DayFormat);
+                _monthText!.Text = selDate.ToString(MonthFormat);
+                _yearText!.Text = selDate.ToString(YearFormat);
+                _dayText!.Text = selDate.ToString(DayFormat);
             }
             else
             {
                 PseudoClasses.Set(":hasnodate", true);
-                _monthText.Text = "month";
-                _yearText.Text = "year";
-                _dayText.Text = "day";
+                _monthText!.Text = "month";
+                _yearText!.Text = "year";
+                _dayText!.Text = "day";
             }
         }
 
-        private void OnFlyoutButtonClicked(object sender, RoutedEventArgs e)
+        private void OnFlyoutButtonClicked(object? sender, RoutedEventArgs e)
         {
             if (_presenter == null)
                 throw new InvalidOperationException("No DatePickerPresenter found");
 
             _presenter.Date = SelectedDate ?? DateTimeOffset.Now;
 
-            _popup.IsOpen = true;
+            _popup!.IsOpen = true;
 
             var deltaY = _presenter.GetOffsetForPopup();
 
             // The extra 5 px I think is related to default popup placement behavior
-            _popup.Host.ConfigurePosition(_popup.PlacementTarget, PlacementMode.AnchorAndGravity, new Point(0, deltaY + 5),
+            _popup!.Host!.ConfigurePosition(_popup.PlacementTarget!, PlacementMode.AnchorAndGravity, new Point(0, deltaY + 5),
                 Primitives.PopupPositioning.PopupAnchor.Bottom, Primitives.PopupPositioning.PopupGravity.Bottom,
                  Primitives.PopupPositioning.PopupPositionerConstraintAdjustment.SlideY);
         }
 
-        protected virtual void OnSelectedDateChanged(object sender, DatePickerSelectedValueChangedEventArgs e)
+        protected virtual void OnSelectedDateChanged(object? sender, DatePickerSelectedValueChangedEventArgs e)
         {
             SelectedDateChanged?.Invoke(sender, e);
         }
diff --git a/src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs b/src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs
index eec4615736..eac6d83074 100644
--- a/src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs
+++ b/src/Avalonia.Controls/DateTimePickers/DatePickerPresenter.cs
@@ -78,23 +78,23 @@ namespace Avalonia.Controls
             x.YearVisible, (x, v) => x.YearVisible = v);
 
         // Template Items
-        private Grid _pickerContainer;
-        private Button _acceptButton;
-        private Button _dismissButton;
-        private Rectangle _spacer1;
-        private Rectangle _spacer2;
-        private Panel _monthHost;
-        private Panel _yearHost;
-        private Panel _dayHost;
-        private DateTimePickerPanel _monthSelector;
-        private DateTimePickerPanel _yearSelector;
-        private DateTimePickerPanel _daySelector;
-        private Button _monthUpButton;
-        private Button _dayUpButton;
-        private Button _yearUpButton;
-        private Button _monthDownButton;
-        private Button _dayDownButton;
-        private Button _yearDownButton;
+        private Grid? _pickerContainer;
+        private Button? _acceptButton;
+        private Button? _dismissButton;
+        private Rectangle? _spacer1;
+        private Rectangle? _spacer2;
+        private Panel? _monthHost;
+        private Panel? _yearHost;
+        private Panel? _dayHost;
+        private DateTimePickerPanel? _monthSelector;
+        private DateTimePickerPanel? _yearSelector;
+        private DateTimePickerPanel? _daySelector;
+        private Button? _monthUpButton;
+        private Button? _dayUpButton;
+        private Button? _yearUpButton;
+        private Button? _monthDownButton;
+        private Button? _dayDownButton;
+        private Button? _yearDownButton;
 
         private DateTimeOffset _date;
         private string _dayFormat = "%d";
@@ -308,9 +308,12 @@ namespace Avalonia.Controls
                     e.Handled = true;
                     break;
                 case Key.Tab:
-                    var nextFocus = KeyboardNavigationHandler.GetNext(FocusManager.Instance.Current, NavigationDirection.Next);
-                    KeyboardDevice.Instance?.SetFocusedElement(nextFocus, NavigationMethod.Tab, KeyModifiers.None);
-                    e.Handled = true;
+                    if (FocusManager.Instance?.Current is IInputElement focus)
+                    {
+                        var nextFocus = KeyboardNavigationHandler.GetNext(focus, NavigationDirection.Next);
+                        KeyboardDevice.Instance?.SetFocusedElement(nextFocus, NavigationMethod.Tab, KeyModifiers.None);
+                        e.Handled = true;
+                    }
                     break;
                 case Key.Enter:
                     Date = _syncDate;
@@ -332,13 +335,13 @@ namespace Avalonia.Controls
 
             _suppressUpdateSelection = true;
 
-            _monthSelector.MaximumValue = 12;
+            _monthSelector!.MaximumValue = 12;
             _monthSelector.MinimumValue = 1;
             _monthSelector.ItemFormat = MonthFormat;
 
-            _daySelector.ItemFormat = DayFormat;
+            _daySelector!.ItemFormat = DayFormat;
 
-            _yearSelector.MaximumValue = MaxYear.Year;
+            _yearSelector!.MaximumValue = MaxYear.Year;
             _yearSelector.MinimumValue = MinYear.Year;
             _yearSelector.ItemFormat = YearFormat;
 
@@ -375,7 +378,7 @@ namespace Avalonia.Controls
         private void SetGrid()
         {
             var fmt = CultureInfo.CurrentCulture.DateTimeFormat.ShortDatePattern;
-            var columns = new List<(Panel, int)>
+            var columns = new List<(Panel?, int)>
             {
                 (_monthHost, MonthVisible ? fmt.IndexOf("m", StringComparison.OrdinalIgnoreCase) : -1),
                 (_yearHost, YearVisible ? fmt.IndexOf("y", StringComparison.OrdinalIgnoreCase) : -1),
@@ -383,7 +386,7 @@ namespace Avalonia.Controls
             };
 
             columns.Sort((x, y) => x.Item2 - y.Item2);
-            _pickerContainer.ColumnDefinitions.Clear();
+            _pickerContainer!.ColumnDefinitions.Clear();
 
             var columnIndex = 0;
 
@@ -416,18 +419,18 @@ namespace Avalonia.Controls
             var isSpacer1Visible = columnIndex > 1;
             var isSpacer2Visible = columnIndex > 2;
             // ternary conditional operator is used to make sure grid cells will be validated
-            Grid.SetColumn(_spacer1, isSpacer1Visible ? 1 : 0);
-            Grid.SetColumn(_spacer2, isSpacer2Visible ? 3 : 0);
+            Grid.SetColumn(_spacer1!, isSpacer1Visible ? 1 : 0);
+            Grid.SetColumn(_spacer2!, isSpacer2Visible ? 3 : 0);
 
-            _spacer1.IsVisible = isSpacer1Visible;
-            _spacer2.IsVisible = isSpacer2Visible;
+            _spacer1!.IsVisible = isSpacer1Visible;
+            _spacer2!.IsVisible = isSpacer2Visible;
         }
 
         private void SetInitialFocus()
         {
-            int monthCol = MonthVisible ? Grid.GetColumn(_monthHost) : int.MaxValue;
-            int dayCol = DayVisible ? Grid.GetColumn(_dayHost) : int.MaxValue;
-            int yearCol = YearVisible ? Grid.GetColumn(_yearHost) : int.MaxValue;
+            int monthCol = MonthVisible ? Grid.GetColumn(_monthHost!) : int.MaxValue;
+            int dayCol = DayVisible ? Grid.GetColumn(_dayHost!) : int.MaxValue;
+            int yearCol = YearVisible ? Grid.GetColumn(_yearHost!) : int.MaxValue;
 
             if (monthCol < dayCol && monthCol < yearCol)
             {
@@ -443,39 +446,39 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnDismissButtonClicked(object sender, RoutedEventArgs e)
+        private void OnDismissButtonClicked(object? sender, RoutedEventArgs e)
         {
             OnDismiss();
         }
 
-        private void OnAcceptButtonClicked(object sender, RoutedEventArgs e)
+        private void OnAcceptButtonClicked(object? sender, RoutedEventArgs e)
         {
             Date = _syncDate;
             OnConfirmed();
         }
 
-        private void OnSelectorButtonClick(object sender, RoutedEventArgs e)
+        private void OnSelectorButtonClick(object? sender, RoutedEventArgs e)
         {
             if (sender == _monthUpButton)
-                _monthSelector.ScrollUp();
+                _monthSelector!.ScrollUp();
             else if (sender == _monthDownButton)
-                _monthSelector.ScrollDown();
+                _monthSelector!.ScrollDown();
             else if (sender == _yearUpButton)
-                _yearSelector.ScrollUp();
+                _yearSelector!.ScrollUp();
             else if (sender == _yearDownButton)
-                _yearSelector.ScrollDown();
+                _yearSelector!.ScrollDown();
             else if (sender == _dayUpButton)
-                _daySelector.ScrollUp();
+                _daySelector!.ScrollUp();
             else if (sender == _dayDownButton)
-                _daySelector.ScrollDown();
+                _daySelector!.ScrollDown();
         }
 
-        private void OnYearChanged(object sender, EventArgs e)
+        private void OnYearChanged(object? sender, EventArgs e)
         {
             if (_suppressUpdateSelection)
                 return;
 
-            int maxDays = _calendar.GetDaysInMonth(_yearSelector.SelectedValue, _syncDate.Month);
+            int maxDays = _calendar.GetDaysInMonth(_yearSelector!.SelectedValue, _syncDate.Month);
             var newDate = new DateTimeOffset(_yearSelector.SelectedValue, _syncDate.Month,
                 _syncDate.Day > maxDays ? maxDays : _syncDate.Day, 0, 0, 0, _syncDate.Offset);
 
@@ -487,7 +490,7 @@ namespace Avalonia.Controls
 
             _suppressUpdateSelection = true;
 
-            _daySelector.FormatDate = newDate.Date;
+            _daySelector!.FormatDate = newDate.Date;
 
             if (_daySelector.MaximumValue != maxDays)
                 _daySelector.MaximumValue = maxDays;
@@ -497,19 +500,19 @@ namespace Avalonia.Controls
             _suppressUpdateSelection = false;
         }
 
-        private void OnDayChanged(object sender, EventArgs e)
+        private void OnDayChanged(object? sender, EventArgs e)
         {
             if (_suppressUpdateSelection)
                 return;
-            _syncDate = new DateTimeOffset(_syncDate.Year, _syncDate.Month, _daySelector.SelectedValue, 0, 0, 0, _syncDate.Offset);
+            _syncDate = new DateTimeOffset(_syncDate.Year, _syncDate.Month, _daySelector!.SelectedValue, 0, 0, 0, _syncDate.Offset);
         }
 
-        private void OnMonthChanged(object sender, EventArgs e)
+        private void OnMonthChanged(object? sender, EventArgs e)
         {
             if (_suppressUpdateSelection)
                 return;
 
-            int maxDays = _calendar.GetDaysInMonth(_syncDate.Year, _monthSelector.SelectedValue);
+            int maxDays = _calendar.GetDaysInMonth(_syncDate.Year, _monthSelector!.SelectedValue);
             var newDate = new DateTimeOffset(_syncDate.Year, _monthSelector.SelectedValue,
                 _syncDate.Day > maxDays ? maxDays : _syncDate.Day, 0, 0, 0, _syncDate.Offset);
 
@@ -521,7 +524,7 @@ namespace Avalonia.Controls
 
             _suppressUpdateSelection = true;
 
-            _daySelector.FormatDate = newDate.Date;
+            _daySelector!.FormatDate = newDate.Date;
             _syncDate = newDate;
 
             if (_daySelector.MaximumValue != maxDays)
@@ -535,7 +538,7 @@ namespace Avalonia.Controls
         internal double GetOffsetForPopup()
         {
             var acceptDismissButtonHeight = _acceptButton != null ? _acceptButton.Bounds.Height : 41;
-            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_monthSelector.ItemHeight / 2);
+            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_monthSelector!.ItemHeight / 2);
         }
     }
 }
diff --git a/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs b/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
index e16e609a15..10b7f9bdf9 100644
--- a/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
+++ b/src/Avalonia.Controls/DateTimePickers/DateTimePickerPanel.cs
@@ -58,7 +58,7 @@ namespace Avalonia.Controls.Primitives
         private Vector _offset;
         private bool _hasInit;
         private bool _suppressUpdateOffset;
-        private ListBoxItem _pressedItem;
+        private ListBoxItem? _pressedItem;
 
         public DateTimePickerPanel()
         {
@@ -271,9 +271,9 @@ namespace Avalonia.Controls.Primitives
 
         public Size Viewport => new Size(0, ItemHeight);
 
-        public event EventHandler ScrollInvalidated;
+        public event EventHandler? ScrollInvalidated;
 
-        public event EventHandler SelectionChanged;
+        public event EventHandler? SelectionChanged;
 
         protected override Size MeasureOverride(Size availableSize)
         {
@@ -523,40 +523,44 @@ namespace Avalonia.Controls.Primitives
             return newValue;
         }
 
-        private void OnItemPointerDown(object sender, PointerPressedEventArgs e)
+        private void OnItemPointerDown(object? sender, PointerPressedEventArgs e)
         {
-            if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
+            if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed &&
+                e.Source is IVisual source)
             {
-                _pressedItem = GetItemFromSource((IVisual)e.Source);
+                _pressedItem = GetItemFromSource(source);
                 e.Handled = true;
             }
         }
 
-        private void OnItemPointerUp(object sender, PointerReleasedEventArgs e)
+        private void OnItemPointerUp(object? sender, PointerReleasedEventArgs e)
         {
             if (e.GetCurrentPoint(this).Properties.PointerUpdateKind == PointerUpdateKind.LeftButtonReleased &&
-                _pressedItem != null)
+                _pressedItem != null &&
+                e.Source is IVisual source &&
+                GetItemFromSource(source) is ListBoxItem item &&
+                item.Tag is int tag)
             {
-                SelectedValue = (int)GetItemFromSource((IVisual)e.Source).Tag;
+                SelectedValue = tag;
                 _pressedItem = null;
                 e.Handled = true;
             }
         }
 
         //Helper to get ListBoxItem from pointerevent source
-        private ListBoxItem GetItemFromSource(IVisual src)
+        private ListBoxItem? GetItemFromSource(IVisual src)
         {
             var item = src;
             while (item != null && !(item is ListBoxItem))
             {
                 item = item.VisualParent;
             }
-            return (ListBoxItem)item;
+            return (ListBoxItem?)item;
         }
 
         public bool BringIntoView(IControl target, Rect targetRect) { return false; }
 
-        public IControl GetControlInDirection(NavigationDirection direction, IControl from) { return null; }
+        public IControl? GetControlInDirection(NavigationDirection direction, IControl from) { return null; }
 
         public void RaiseScrollInvalidated(EventArgs e)
         {
diff --git a/src/Avalonia.Controls/DateTimePickers/PickerPresenterBase.cs b/src/Avalonia.Controls/DateTimePickers/PickerPresenterBase.cs
index 639b187ad8..629b1a1766 100644
--- a/src/Avalonia.Controls/DateTimePickers/PickerPresenterBase.cs
+++ b/src/Avalonia.Controls/DateTimePickers/PickerPresenterBase.cs
@@ -16,10 +16,10 @@ namespace Avalonia.Controls.Primitives
 
         protected virtual void OnDismiss()
         {
-            Dismissed.Invoke(this, EventArgs.Empty);
+            Dismissed?.Invoke(this, EventArgs.Empty);
         }
 
-        public event EventHandler Confirmed;
-        public event EventHandler Dismissed;
+        public event EventHandler? Confirmed;
+        public event EventHandler? Dismissed;
     }
 }
diff --git a/src/Avalonia.Controls/DateTimePickers/TimePicker.cs b/src/Avalonia.Controls/DateTimePickers/TimePicker.cs
index 6b3f66912f..a0a27bd4ed 100644
--- a/src/Avalonia.Controls/DateTimePickers/TimePicker.cs
+++ b/src/Avalonia.Controls/DateTimePickers/TimePicker.cs
@@ -49,18 +49,18 @@ namespace Avalonia.Controls
                 defaultBindingMode: BindingMode.TwoWay);
 
         // Template Items
-        private TimePickerPresenter _presenter;
-        private Button _flyoutButton;
-        private Border _firstPickerHost;
-        private Border _secondPickerHost;
-        private Border _thirdPickerHost;
-        private TextBlock _hourText;
-        private TextBlock _minuteText;
-        private TextBlock _periodText;
-        private Rectangle _firstSplitter;
-        private Rectangle _secondSplitter;
-        private Grid _contentGrid;
-        private Popup _popup;
+        private TimePickerPresenter? _presenter;
+        private Button? _flyoutButton;
+        private Border? _firstPickerHost;
+        private Border? _secondPickerHost;
+        private Border? _thirdPickerHost;
+        private TextBlock? _hourText;
+        private TextBlock? _minuteText;
+        private TextBlock? _periodText;
+        private Rectangle? _firstSplitter;
+        private Rectangle? _secondSplitter;
+        private Grid? _contentGrid;
+        private Popup? _popup;
 
         private TimeSpan? _selectedTime;
         private int _minuteIncrement = 1;
@@ -142,7 +142,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Raised when the <see cref="SelectedTime"/> property changes
         /// </summary>
-        public event EventHandler<TimePickerSelectedValueChangedEventArgs> SelectedTimeChanged;
+        public event EventHandler<TimePickerSelectedValueChangedEventArgs>? SelectedTimeChanged;
 
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
@@ -200,15 +200,15 @@ namespace Avalonia.Controls
             var columnsD = use24HourClock ? "*, Auto, *" : "*, Auto, *, Auto, *";
             _contentGrid.ColumnDefinitions = new ColumnDefinitions(columnsD);
 
-            _thirdPickerHost.IsVisible = !use24HourClock;
-            _secondSplitter.IsVisible = !use24HourClock;
+            _thirdPickerHost!.IsVisible = !use24HourClock;
+            _secondSplitter!.IsVisible = !use24HourClock;
 
-            Grid.SetColumn(_firstPickerHost, 0);
-            Grid.SetColumn(_secondPickerHost, 2);
+            Grid.SetColumn(_firstPickerHost!, 0);
+            Grid.SetColumn(_secondPickerHost!, 2);
 
             Grid.SetColumn(_thirdPickerHost, use24HourClock ? 0 : 4);
 
-            Grid.SetColumn(_firstSplitter, 1);
+            Grid.SetColumn(_firstSplitter!, 1);
             Grid.SetColumn(_secondSplitter, use24HourClock ? 0 : 3);
         }
 
@@ -220,7 +220,7 @@ namespace Avalonia.Controls
             var time = SelectedTime;
             if (time.HasValue)
             {
-                var newTime = SelectedTime.Value;
+                var newTime = SelectedTime!.Value;
 
                 if (ClockIdentifier == "12HourClock")
                 {
@@ -252,30 +252,30 @@ namespace Avalonia.Controls
             SelectedTimeChanged?.Invoke(this, new TimePickerSelectedValueChangedEventArgs(oldTime, newTime));
         }
 
-        private void OnFlyoutButtonClicked(object sender, Interactivity.RoutedEventArgs e)
+        private void OnFlyoutButtonClicked(object? sender, Interactivity.RoutedEventArgs e)
         {
-            _presenter.Time = SelectedTime ?? DateTime.Now.TimeOfDay;
+            _presenter!.Time = SelectedTime ?? DateTime.Now.TimeOfDay;
 
-            _popup.IsOpen = true;
+            _popup!.IsOpen = true;
 
             var deltaY = _presenter.GetOffsetForPopup();
 
             // The extra 5 px I think is related to default popup placement behavior
-            _popup.Host.ConfigurePosition(_popup.PlacementTarget, PlacementMode.AnchorAndGravity, new Point(0, deltaY + 5),
+            _popup.Host!.ConfigurePosition(_popup.PlacementTarget!, PlacementMode.AnchorAndGravity, new Point(0, deltaY + 5),
                 Primitives.PopupPositioning.PopupAnchor.Bottom, Primitives.PopupPositioning.PopupGravity.Bottom,
                  Primitives.PopupPositioning.PopupPositionerConstraintAdjustment.SlideY);
         }
 
-        private void OnDismissPicker(object sender, EventArgs e)
+        private void OnDismissPicker(object? sender, EventArgs e)
         {
-            _popup.Close();
+            _popup!.Close();
             Focus();
         }
 
-        private void OnConfirmed(object sender, EventArgs e)
+        private void OnConfirmed(object? sender, EventArgs e)
         {
-            _popup.Close();
-            SelectedTime = _presenter.Time;
+            _popup!.Close();
+            SelectedTime = _presenter!.Time;
         }
     }
 }
diff --git a/src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs b/src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs
index 920eeeb406..55bc1af3a7 100644
--- a/src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs
+++ b/src/Avalonia.Controls/DateTimePickers/TimePickerPresenter.cs
@@ -40,20 +40,20 @@ namespace Avalonia.Controls
         }
 
         // TemplateItems
-        private Grid _pickerContainer;
-        private Button _acceptButton;
-        private Button _dismissButton;
-        private Rectangle _spacer2;
-        private Panel _periodHost;
-        private DateTimePickerPanel _hourSelector;
-        private DateTimePickerPanel _minuteSelector;
-        private DateTimePickerPanel _periodSelector;
-        private Button _hourUpButton;
-        private Button _minuteUpButton;
-        private Button _periodUpButton;
-        private Button _hourDownButton;
-        private Button _minuteDownButton;
-        private Button _periodDownButton;
+        private Grid? _pickerContainer;
+        private Button? _acceptButton;
+        private Button? _dismissButton;
+        private Rectangle? _spacer2;
+        private Panel? _periodHost;
+        private DateTimePickerPanel? _hourSelector;
+        private DateTimePickerPanel? _minuteSelector;
+        private DateTimePickerPanel? _periodSelector;
+        private Button? _hourUpButton;
+        private Button? _minuteUpButton;
+        private Button? _periodUpButton;
+        private Button? _hourDownButton;
+        private Button? _minuteDownButton;
+        private Button? _periodDownButton;
 
         // Backing Fields
         private TimeSpan _time;
@@ -156,9 +156,12 @@ namespace Avalonia.Controls
                     e.Handled = true;
                     break;
                 case Key.Tab:
-                    var nextFocus = KeyboardNavigationHandler.GetNext(FocusManager.Instance.Current, NavigationDirection.Next);
-                    KeyboardDevice.Instance?.SetFocusedElement(nextFocus, NavigationMethod.Tab, KeyModifiers.None);
-                    e.Handled = true;
+                    if (FocusManager.Instance?.Current is IInputElement focus)
+                    {
+                        var nextFocus = KeyboardNavigationHandler.GetNext(focus, NavigationDirection.Next);
+                        KeyboardDevice.Instance?.SetFocusedElement(nextFocus, NavigationMethod.Tab, KeyModifiers.None);
+                        e.Handled = true;
+                    }
                     break;
                 case Key.Enter:
                     OnConfirmed();
@@ -170,9 +173,9 @@ namespace Avalonia.Controls
 
         protected override void OnConfirmed()
         {
-            var hr = _hourSelector.SelectedValue;
-            var min = _minuteSelector.SelectedValue;
-            var per = _periodSelector.SelectedValue;
+            var hr = _hourSelector!.SelectedValue;
+            var min = _minuteSelector!.SelectedValue;
+            var per = _periodSelector!.SelectedValue;
 
             if (ClockIdentifier == "12HourClock")
             {
@@ -190,20 +193,20 @@ namespace Avalonia.Controls
                 return;
 
             bool clock12 = ClockIdentifier == "12HourClock";
-            _hourSelector.MaximumValue = clock12 ? 12 : 23;
+            _hourSelector!.MaximumValue = clock12 ? 12 : 23;
             _hourSelector.MinimumValue = clock12 ? 1 : 0;
             _hourSelector.ItemFormat = "%h";
             var hr = Time.Hours;
             _hourSelector.SelectedValue = !clock12 ? hr :
                 hr > 12 ? hr - 12 : hr == 0 ? 12 : hr;
 
-            _minuteSelector.MaximumValue = 59;
+            _minuteSelector!.MaximumValue = 59;
             _minuteSelector.MinimumValue = 0;
             _minuteSelector.Increment = MinuteIncrement;
             _minuteSelector.SelectedValue = Time.Minutes;
             _minuteSelector.ItemFormat = "mm";
 
-            _periodSelector.MaximumValue = 1;
+            _periodSelector!.MaximumValue = 1;
             _periodSelector.MinimumValue = 0;
             _periodSelector.SelectedValue = hr >= 12 ? 1 : 0;
 
@@ -216,45 +219,45 @@ namespace Avalonia.Controls
             bool use24HourClock = ClockIdentifier == "24HourClock";
 
             var columnsD = use24HourClock ? "*, Auto, *" : "*, Auto, *, Auto, *";
-            _pickerContainer.ColumnDefinitions = new ColumnDefinitions(columnsD);
+            _pickerContainer!.ColumnDefinitions = new ColumnDefinitions(columnsD);
 
-            _spacer2.IsVisible = !use24HourClock;
-            _periodHost.IsVisible = !use24HourClock;
+            _spacer2!.IsVisible = !use24HourClock;
+            _periodHost!.IsVisible = !use24HourClock;
 
             Grid.SetColumn(_spacer2, use24HourClock ? 0 : 3);
             Grid.SetColumn(_periodHost, use24HourClock ? 0 : 4);
         }
 
-        private void OnDismissButtonClicked(object sender, RoutedEventArgs e)
+        private void OnDismissButtonClicked(object? sender, RoutedEventArgs e)
         {
             OnDismiss();
         }
 
-        private void OnAcceptButtonClicked(object sender, RoutedEventArgs e)
+        private void OnAcceptButtonClicked(object? sender, RoutedEventArgs e)
         {
             OnConfirmed();
         }
 
-        private void OnSelectorButtonClick(object sender, RoutedEventArgs e)
+        private void OnSelectorButtonClick(object? sender, RoutedEventArgs e)
         {
             if (sender == _hourUpButton)
-                _hourSelector.ScrollUp();
+                _hourSelector!.ScrollUp();
             else if (sender == _hourDownButton)
-                _hourSelector.ScrollDown();
+                _hourSelector!.ScrollDown();
             else if (sender == _minuteUpButton)
-                _minuteSelector.ScrollUp();
+                _minuteSelector!.ScrollUp();
             else if (sender == _minuteDownButton)
-                _minuteSelector.ScrollDown();
+                _minuteSelector!.ScrollDown();
             else if (sender == _periodUpButton)
-                _periodSelector.ScrollUp();
+                _periodSelector!.ScrollUp();
             else if (sender == _periodDownButton)
-                _periodSelector.ScrollDown();
+                _periodSelector!.ScrollDown();
         }
 
         internal double GetOffsetForPopup()
         {
             var acceptDismissButtonHeight = _acceptButton != null ? _acceptButton.Bounds.Height : 41;
-            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_hourSelector.ItemHeight / 2);
+            return -(MaxHeight - acceptDismissButtonHeight) / 2 - (_hourSelector!.ItemHeight / 2);
         }
     }
 }
diff --git a/src/Avalonia.Controls/Decorator.cs b/src/Avalonia.Controls/Decorator.cs
index 8959f0e306..bd866ee8d8 100644
--- a/src/Avalonia.Controls/Decorator.cs
+++ b/src/Avalonia.Controls/Decorator.cs
@@ -11,8 +11,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Child"/> property.
         /// </summary>
-        public static readonly StyledProperty<IControl> ChildProperty =
-            AvaloniaProperty.Register<Decorator, IControl>(nameof(Child));
+        public static readonly StyledProperty<IControl?> ChildProperty =
+            AvaloniaProperty.Register<Decorator, IControl?>(nameof(Child));
 
         /// <summary>
         /// Defines the <see cref="Padding"/> property.
@@ -33,7 +33,7 @@ namespace Avalonia.Controls
         /// Gets or sets the decorated control.
         /// </summary>
         [Content]
-        public IControl Child
+        public IControl? Child
         {
             get { return GetValue(ChildProperty); }
             set { SetValue(ChildProperty, value); }
@@ -66,8 +66,8 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         private void ChildChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldChild = (Control)e.OldValue;
-            var newChild = (Control)e.NewValue;
+            var oldChild = (Control?)e.OldValue;
+            var newChild = (Control?)e.NewValue;
 
             if (oldChild != null)
             {
diff --git a/src/Avalonia.Controls/DefinitionBase.cs b/src/Avalonia.Controls/DefinitionBase.cs
index 37b8691ce9..eb09ff397a 100644
--- a/src/Avalonia.Controls/DefinitionBase.cs
+++ b/src/Avalonia.Controls/DefinitionBase.cs
@@ -39,7 +39,7 @@ namespace Avalonia.Controls
                 string sharedSizeGroupId = SharedSizeGroup;
                 if (sharedSizeGroupId != null)
                 {
-                    SharedSizeScope privateSharedSizeScope = PrivateSharedSizeScope;
+                    SharedSizeScope? privateSharedSizeScope = PrivateSharedSizeScope;
                     if (privateSharedSizeScope != null)
                     {
                         _sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
@@ -104,7 +104,7 @@ namespace Avalonia.Controls
         /// </remarks>
         internal static void OnIsSharedSizeScopePropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
-            if ((bool)e.NewValue)
+            if ((bool)e.NewValue!)
             {
                 SharedSizeScope sharedStatesCollection = new SharedSizeScope();
                 d.SetValue(PrivateSharedSizeScopeProperty, sharedStatesCollection);
@@ -131,8 +131,8 @@ namespace Avalonia.Controls
             }
             else
             {
-                GridUnitType oldUnitType = ((GridLength)e.OldValue).GridUnitType;
-                GridUnitType newUnitType = ((GridLength)e.NewValue).GridUnitType;
+                GridUnitType oldUnitType = ((GridLength)e.OldValue!).GridUnitType;
+                GridUnitType newUnitType = ((GridLength)e.NewValue!).GridUnitType;
 
                 if (oldUnitType != newUnitType)
                 {
@@ -300,7 +300,7 @@ namespace Avalonia.Controls
         /// </summary>
         internal abstract double UserMaxSizeValueCache { get; }
 
-        internal Grid Parent { get; set; }
+        internal Grid? Parent { get; set; }
 
         /// <summary>
         /// SetFlags is used to set or unset one or multiple
@@ -326,7 +326,7 @@ namespace Avalonia.Controls
 
             if (definition.Parent != null)
             {
-                string sharedSizeGroupId = (string)e.NewValue;
+                string sharedSizeGroupId = (string)e.NewValue!;
 
                 if (definition._sharedState != null)
                 {
@@ -338,7 +338,7 @@ namespace Avalonia.Controls
 
                 if ((definition._sharedState == null) && (sharedSizeGroupId != null))
                 {
-                    SharedSizeScope privateSharedSizeScope = definition.PrivateSharedSizeScope;
+                    SharedSizeScope? privateSharedSizeScope = definition.PrivateSharedSizeScope;
                     if (privateSharedSizeScope != null)
                     {
                         //  if definition is not registered and both: shared size group id AND private shared scope 
@@ -402,7 +402,7 @@ namespace Avalonia.Controls
 
             if (definition.Parent != null)
             {
-                SharedSizeScope privateSharedSizeScope = (SharedSizeScope)e.NewValue;
+                SharedSizeScope privateSharedSizeScope = (SharedSizeScope)e.NewValue!;
 
                 if (definition._sharedState != null)
                 {
@@ -429,9 +429,9 @@ namespace Avalonia.Controls
         /// <summary>
         /// Private getter of shared state collection dynamic property.
         /// </summary>
-        private SharedSizeScope PrivateSharedSizeScope
+        private SharedSizeScope? PrivateSharedSizeScope
         {
-            get { return (SharedSizeScope)GetValue(PrivateSharedSizeScopeProperty); }
+            get { return (SharedSizeScope?)GetValue(PrivateSharedSizeScopeProperty); }
         }
 
         /// <summary>
@@ -462,7 +462,7 @@ namespace Avalonia.Controls
         private double _sizeCache;                      //  cache used for various purposes (sorting, caching, etc) during calculations
         private double _offset;                         //  offset of the DefinitionBase from left / top corner (assuming LTR case)
 
-        private SharedSizeState _sharedState;           //  reference to shared state object this instance is registered with
+        private SharedSizeState? _sharedState;           //  reference to shared state object this instance is registered with
 
         [System.Flags]
         private enum Flags : byte
@@ -488,7 +488,7 @@ namespace Avalonia.Controls
                 //  check that sharedSizeGroup is not default
                 Debug.Assert(sharedSizeGroup != null);
 
-                SharedSizeState sharedState = _registry[sharedSizeGroup] as SharedSizeState;
+                SharedSizeState? sharedState = _registry[sharedSizeGroup] as SharedSizeState;
                 if (sharedState == null)
                 {
                     sharedState = new SharedSizeState(this, sharedSizeGroup);
@@ -567,7 +567,7 @@ namespace Avalonia.Controls
                 {
                     for (int i = 0, count = _registry.Count; i < count; ++i)
                     {
-                        Grid parentGrid = (Grid)(_registry[i].Parent);
+                        Grid parentGrid = (Grid)(_registry[i].Parent!);
                         parentGrid.Invalidate();
                     }
                     _broadcastInvalidation = false;
@@ -645,7 +645,7 @@ namespace Avalonia.Controls
             /// OnLayoutUpdated handler. Validates that all participating definitions
             /// have updated min size value. Forces another layout update cycle if needed.
             /// </summary>
-            private void OnLayoutUpdated(object sender, EventArgs e)
+            private void OnLayoutUpdated(object? sender, EventArgs e)
             {
                 double sharedMinSize = 0;
 
@@ -707,14 +707,14 @@ namespace Avalonia.Controls
 
                     if(!measureIsValid)
                     {
-                        definitionBase.Parent.InvalidateMeasure();
+                        definitionBase.Parent!.InvalidateMeasure();
                     }
                     else if (!MathUtilities.AreClose(sharedMinSize, definitionBase.SizeCache))
                     {
                         //  if measure is valid then also need to check arrange.
                         //  Note: definitionBase.SizeCache is volatile but at this point 
                         //  it contains up-to-date final size
-                        definitionBase.Parent.InvalidateArrange();
+                        definitionBase.Parent!.InvalidateArrange();
                     }
 
                     // now we can restore the invariant, and clear the layout flag
@@ -724,7 +724,7 @@ namespace Avalonia.Controls
 
                 _minSize = sharedMinSize;
 
-                _layoutUpdatedHost.LayoutUpdated -= _layoutUpdated;
+                _layoutUpdatedHost!.LayoutUpdated -= _layoutUpdated;
                 _layoutUpdatedHost = null;
 
                 _broadcastInvalidation = true;
@@ -743,7 +743,7 @@ namespace Avalonia.Controls
             private readonly EventHandler _layoutUpdated;
 
             //  Control for which layout updated event handler is registered
-            private Control _layoutUpdatedHost;
+            private Control? _layoutUpdatedHost;
 
             //  "true" when broadcasting of invalidation is needed
             private bool _broadcastInvalidation;
@@ -762,8 +762,8 @@ namespace Avalonia.Controls
         /// Private shared size scope property holds a collection of shared state objects for the a given shared size scope.
         /// <see cref="OnIsSharedSizeScopePropertyChanged"/>
         /// </summary>
-        internal static readonly AttachedProperty<SharedSizeScope> PrivateSharedSizeScopeProperty =
-            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, SharedSizeScope>(
+        internal static readonly AttachedProperty<SharedSizeScope?> PrivateSharedSizeScopeProperty =
+            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, SharedSizeScope?>(
                 "PrivateSharedSizeScope",
                 defaultValue: null,
                 inherits: true);
diff --git a/src/Avalonia.Controls/DefinitionList.cs b/src/Avalonia.Controls/DefinitionList.cs
index ad0e2513c4..c8b4fe4db4 100644
--- a/src/Avalonia.Controls/DefinitionList.cs
+++ b/src/Avalonia.Controls/DefinitionList.cs
@@ -36,7 +36,7 @@ namespace Avalonia.Controls
             }
         }
 
-        internal void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        internal void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             var idx = 0;
 
@@ -62,7 +62,7 @@ namespace Avalonia.Controls
 
             for (var i = 0; i < count; i++)
             {
-                var definition = (DefinitionBase) items[i];
+                var definition = (DefinitionBase) items[i]!;
 
                 if (wasRemoved)
                 {
diff --git a/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs b/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs
index e2afbd3bdc..08d559a5c1 100644
--- a/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs
+++ b/src/Avalonia.Controls/Embedding/Offscreen/OffscreenTopLevelImpl.cs
@@ -12,7 +12,7 @@ namespace Avalonia.Controls.Embedding.Offscreen
         private double _scaling = 1;
         private Size _clientSize;
 
-        public IInputRoot InputRoot { get; private set; }
+        public IInputRoot? InputRoot { get; private set; }
         public bool IsDisposed { get; private set; }
 
         public virtual void Dispose()
@@ -47,12 +47,12 @@ namespace Avalonia.Controls.Embedding.Offscreen
             }
         }
         
-        public Action<RawInputEventArgs> Input { get; set; }
-        public Action<Rect> Paint { get; set; }
-        public Action<Size, PlatformResizeReason> Resized { get; set; }
-        public Action<double> ScalingChanged { get; set; }
+        public Action<RawInputEventArgs>? Input { get; set; }
+        public Action<Rect>? Paint { get; set; }
+        public Action<Size, PlatformResizeReason>? Resized { get; set; }
+        public Action<double>? ScalingChanged { get; set; }
 
-        public Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+        public Action<WindowTransparencyLevel>? TransparencyLevelChanged { get; set; }
 
         /// <inheritdoc/>
         public AcrylicPlatformCompensationLevels AcrylicCompensationLevels { get; } = new AcrylicPlatformCompensationLevels(1, 1, 1);
@@ -63,18 +63,18 @@ namespace Avalonia.Controls.Embedding.Offscreen
 
         public virtual PixelPoint PointToScreen(Point point) => PixelPoint.FromPoint(point, 1);
 
-        public virtual void SetCursor(ICursorImpl cursor)
+        public virtual void SetCursor(ICursorImpl? cursor)
         {
         }
 
-        public Action Closed { get; set; }
-        public Action LostFocus { get; set; }
+        public Action? Closed { get; set; }
+        public Action? LostFocus { get; set; }
         public abstract IMouseDevice MouseDevice { get; }
 
         public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) { }
 
         public WindowTransparencyLevel TransparencyLevel { get; private set; }
 
-        public IPopupImpl CreatePopup() => null;
+        public IPopupImpl? CreatePopup() => null;
     }
 }
diff --git a/src/Avalonia.Controls/ExperimentalAcrylicBorder.cs b/src/Avalonia.Controls/ExperimentalAcrylicBorder.cs
index 57861163d6..30bdb4c60e 100644
--- a/src/Avalonia.Controls/ExperimentalAcrylicBorder.cs
+++ b/src/Avalonia.Controls/ExperimentalAcrylicBorder.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Controls
 
         private readonly BorderRenderHelper _borderRenderHelper = new BorderRenderHelper();
 
-        private IDisposable _subscription;
+        private IDisposable? _subscription;
 
         static ExperimentalAcrylicBorder()
         {
@@ -46,11 +46,14 @@ namespace Avalonia.Controls
         {
             base.OnAttachedToVisualTree(e);
 
-            var tl = (e.Root as TopLevel);
+            var tl = (TopLevel)e.Root;
 
             _subscription = tl.GetObservable(TopLevel.ActualTransparencyLevelProperty)
                 .Subscribe(x =>
                 {
+                    if (tl.PlatformImpl is null)
+                        return;
+
                     switch (x)
                     {
                         case WindowTransparencyLevel.Transparent:
diff --git a/src/Avalonia.Controls/Flyouts/FlyoutBase.cs b/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
index a08b82f512..657d0450e6 100644
--- a/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
+++ b/src/Avalonia.Controls/Flyouts/FlyoutBase.cs
@@ -267,7 +267,7 @@ namespace Avalonia.Controls.Primitives
             return true;
         }
 
-        private void PlacementTarget_DetachedFromVisualTree(object sender, VisualTreeAttachmentEventArgs e)
+        private void PlacementTarget_DetachedFromVisualTree(object? sender, VisualTreeAttachmentEventArgs e)
         {
             _ = HideCore(false);
         }
@@ -335,7 +335,7 @@ namespace Avalonia.Controls.Primitives
 
         protected virtual void OnOpened()
         {
-            Opened?.Invoke(this, null);
+            Opened?.Invoke(this, EventArgs.Empty);
         }
 
         protected virtual void OnClosing(CancelEventArgs args)
@@ -345,7 +345,7 @@ namespace Avalonia.Controls.Primitives
 
         protected virtual void OnClosed()
         {
-            Closed?.Invoke(this, null);
+            Closed?.Invoke(this, EventArgs.Empty);
         }
 
         /// <summary>
@@ -368,14 +368,14 @@ namespace Avalonia.Controls.Primitives
             return popup;
         }
 
-        private void OnPopupOpened(object sender, EventArgs e)
+        private void OnPopupOpened(object? sender, EventArgs e)
         {
             IsOpen = true;
 
             _popupHostChangedHandler?.Invoke(Popup!.Host);
         }
 
-        private void OnPopupClosing(object sender, CancelEventArgs e)
+        private void OnPopupClosing(object? sender, CancelEventArgs e)
         {
             if (IsOpen)
             {
@@ -383,7 +383,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void OnPopupClosed(object sender, EventArgs e)
+        private void OnPopupClosed(object? sender, EventArgs e)
         {
             HideCore(false);
 
@@ -391,7 +391,7 @@ namespace Avalonia.Controls.Primitives
         }
 
         // This method is handling both popup logical tree and target logical tree.
-        private void OnPlacementTargetOrPopupKeyUp(object sender, KeyEventArgs e)
+        private void OnPlacementTargetOrPopupKeyUp(object? sender, KeyEventArgs e)
         {
             if (!e.Handled
                 && IsOpen
@@ -530,10 +530,10 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private static void OnControlContextRequested(object sender, ContextRequestedEventArgs e)
+        private static void OnControlContextRequested(object? sender, ContextRequestedEventArgs e)
         {
-            var control = (Control)sender;
             if (!e.Handled
+                && sender is Control control
                 && control.ContextFlyout is FlyoutBase flyout)
             {
                 if (control.ContextMenu != null)
diff --git a/src/Avalonia.Controls/Flyouts/MenuFlyout.cs b/src/Avalonia.Controls/Flyouts/MenuFlyout.cs
index 5b2e323f4f..001a3dc3d2 100644
--- a/src/Avalonia.Controls/Flyouts/MenuFlyout.cs
+++ b/src/Avalonia.Controls/Flyouts/MenuFlyout.cs
@@ -18,7 +18,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Items"/> property
         /// </summary>
-        public static readonly DirectProperty<MenuFlyout, IEnumerable> ItemsProperty =
+        public static readonly DirectProperty<MenuFlyout, IEnumerable?> ItemsProperty =
             ItemsControl.ItemsProperty.AddOwner<MenuFlyout>(x => x.Items,
                 (x, v) => x.Items = v);
 
@@ -35,7 +35,7 @@ namespace Avalonia.Controls
         /// Gets or sets the items of the MenuFlyout
         /// </summary>
         [Content]
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get => _items;
             set => SetAndRaise(ItemsProperty, ref _items, value);
@@ -51,7 +51,7 @@ namespace Avalonia.Controls
         }
 
         private Classes? _classes;
-        private IEnumerable _items;
+        private IEnumerable? _items;
         private IDataTemplate? _itemTemplate;
 
         protected override Control CreatePresenter()
diff --git a/src/Avalonia.Controls/Generators/IItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/IItemContainerGenerator.cs
index 6220a20a82..dd4c6561c2 100644
--- a/src/Avalonia.Controls/Generators/IItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/IItemContainerGenerator.cs
@@ -17,27 +17,27 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>
-        IDataTemplate ItemTemplate { get; set; }
+        IDataTemplate? ItemTemplate { get; set; }
 
         /// <summary>
         /// Gets the ContainerType, or null if its an untyped ContainerGenerator.
         /// </summary>
-        Type ContainerType { get; }
+        Type? ContainerType { get; }
 
         /// <summary>
         /// Signaled whenever new containers are materialized.
         /// </summary>
-        event EventHandler<ItemContainerEventArgs> Materialized;
+        event EventHandler<ItemContainerEventArgs>? Materialized;
 
         /// <summary>
         /// Event raised whenever containers are dematerialized.
         /// </summary>
-        event EventHandler<ItemContainerEventArgs> Dematerialized;
+        event EventHandler<ItemContainerEventArgs>? Dematerialized;
 
         /// <summary>
         /// Event raised whenever containers are recycled.
         /// </summary>
-        event EventHandler<ItemContainerEventArgs> Recycled;
+        event EventHandler<ItemContainerEventArgs>? Recycled;
 
         /// <summary>
         /// Creates a container control for an item.
@@ -90,7 +90,7 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         /// <param name="index">The index.</param>
         /// <returns>The container, or null if no container created.</returns>
-        IControl ContainerFromIndex(int index);
+        IControl? ContainerFromIndex(int index);
 
         /// <summary>
         /// Gets the index of the specified container control.
diff --git a/src/Avalonia.Controls/Generators/ITreeItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/ITreeItemContainerGenerator.cs
index 335e0df5f7..f15987fe79 100644
--- a/src/Avalonia.Controls/Generators/ITreeItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/ITreeItemContainerGenerator.cs
@@ -8,7 +8,7 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Gets the container index for the tree.
         /// </summary>
-        TreeContainerIndex Index { get; }
+        TreeContainerIndex? Index { get; }
 
         /// <summary>
         /// Updates the index based on the parent <see cref="TreeView"/>.
diff --git a/src/Avalonia.Controls/Generators/ItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/ItemContainerGenerator.cs
index f16834f6e1..d02eaffeb2 100644
--- a/src/Avalonia.Controls/Generators/ItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/ItemContainerGenerator.cs
@@ -20,27 +20,25 @@ namespace Avalonia.Controls.Generators
         /// <param name="owner">The owner control.</param>
         public ItemContainerGenerator(IControl owner)
         {
-            Contract.Requires<ArgumentNullException>(owner != null);
-
-            Owner = owner;
+            Owner = owner ?? throw new ArgumentNullException(nameof(owner));
         }
 
         /// <inheritdoc/>
         public IEnumerable<ItemContainerInfo> Containers => _containers.Values;
 
         /// <inheritdoc/>
-        public event EventHandler<ItemContainerEventArgs> Materialized;
+        public event EventHandler<ItemContainerEventArgs>? Materialized;
 
         /// <inheritdoc/>
-        public event EventHandler<ItemContainerEventArgs> Dematerialized;
+        public event EventHandler<ItemContainerEventArgs>? Dematerialized;
 
         /// <inheritdoc/>
-        public event EventHandler<ItemContainerEventArgs> Recycled;
+        public event EventHandler<ItemContainerEventArgs>? Recycled;
 
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>
-        public IDataTemplate ItemTemplate { get; set; }
+        public IDataTemplate? ItemTemplate { get; set; }
 
         /// <summary>
         /// Gets the owner control.
@@ -48,12 +46,12 @@ namespace Avalonia.Controls.Generators
         public IControl Owner { get; }
 
         /// <inheritdoc/>
-        public virtual Type ContainerType => null;
+        public virtual Type? ContainerType => null;
 
         /// <inheritdoc/>
         public ItemContainerInfo Materialize(int index, object item)
         {
-            var container = new ItemContainerInfo(CreateContainer(item), item, index);
+            var container = new ItemContainerInfo(CreateContainer(item)!, item, index);
 
             _containers.Add(container.Index, container);
             Materialized?.Invoke(this, new ItemContainerEventArgs(container));
@@ -104,9 +102,7 @@ namespace Avalonia.Controls.Generators
             {
                 for (var i = startingIndex; i < startingIndex + count; ++i)
                 {
-                    ItemContainerInfo found;
-
-                    if (_containers.TryGetValue(i, out found))
+                    if (_containers.TryGetValue(i, out var found))
                     {
                         result.Add(found);
                     }
@@ -154,9 +150,9 @@ namespace Avalonia.Controls.Generators
         }
 
         /// <inheritdoc/>
-        public IControl ContainerFromIndex(int index)
+        public IControl? ContainerFromIndex(int index)
         {
-            ItemContainerInfo result;
+            ItemContainerInfo? result;
             _containers.TryGetValue(index, out result);
             return result?.ContainerControl;
         }
@@ -180,7 +176,7 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         /// <param name="item">The item.</param>
         /// <returns>The created container control.</returns>
-        protected virtual IControl CreateContainer(object item)
+        protected virtual IControl? CreateContainer(object item)
         {
             var result = item as IControl;
 
diff --git a/src/Avalonia.Controls/Generators/ItemContainerGenerator`1.cs b/src/Avalonia.Controls/Generators/ItemContainerGenerator`1.cs
index d1fbb25cb1..635f3a7d37 100644
--- a/src/Avalonia.Controls/Generators/ItemContainerGenerator`1.cs
+++ b/src/Avalonia.Controls/Generators/ItemContainerGenerator`1.cs
@@ -19,13 +19,10 @@ namespace Avalonia.Controls.Generators
         public ItemContainerGenerator(
             IControl owner, 
             AvaloniaProperty contentProperty,
-            AvaloniaProperty contentTemplateProperty)
+            AvaloniaProperty? contentTemplateProperty)
             : base(owner)
         {
-            Contract.Requires<ArgumentNullException>(owner != null);
-            Contract.Requires<ArgumentNullException>(contentProperty != null);
-
-            ContentProperty = contentProperty;
+            ContentProperty = contentProperty ?? throw new ArgumentNullException(nameof(contentProperty));
             ContentTemplateProperty = contentTemplateProperty;
         }
 
@@ -40,10 +37,10 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Gets the container's ContentTemplate property.
         /// </summary>
-        protected AvaloniaProperty ContentTemplateProperty { get; }
+        protected AvaloniaProperty? ContentTemplateProperty { get; }
 
         /// <inheritdoc/>
-        protected override IControl CreateContainer(object item)
+        protected override IControl? CreateContainer(object item)
         {
             var container = item as T;
 
diff --git a/src/Avalonia.Controls/Generators/MenuItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/MenuItemContainerGenerator.cs
index d3cf70e0f8..bfcb474515 100644
--- a/src/Avalonia.Controls/Generators/MenuItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/MenuItemContainerGenerator.cs
@@ -12,7 +12,7 @@
         }
 
         /// <inheritdoc/>
-        protected override IControl CreateContainer(object item)
+        protected override IControl? CreateContainer(object item)
         {
             var separator = item as Separator;
             return separator != null ? separator : base.CreateContainer(item);
diff --git a/src/Avalonia.Controls/Generators/TabItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/TabItemContainerGenerator.cs
index 840a1f66f1..c6b0bda9af 100644
--- a/src/Avalonia.Controls/Generators/TabItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/TabItemContainerGenerator.cs
@@ -20,7 +20,7 @@ namespace Avalonia.Controls.Generators
 
         protected override IControl CreateContainer(object item)
         {
-            var tabItem = (TabItem)base.CreateContainer(item);
+            var tabItem = (TabItem)base.CreateContainer(item)!;
 
             tabItem.Bind(TabItem.TabStripPlacementProperty, new OwnerBinding<Dock>(
                 tabItem,
@@ -28,7 +28,7 @@ namespace Avalonia.Controls.Generators
 
             if (tabItem.HeaderTemplate == null)
             {
-                tabItem.Bind(TabItem.HeaderTemplateProperty, new OwnerBinding<IDataTemplate>(
+                tabItem.Bind(TabItem.HeaderTemplateProperty, new OwnerBinding<IDataTemplate?>(
                     tabItem,
                     TabControl.ItemTemplateProperty));
             }
@@ -50,7 +50,7 @@ namespace Avalonia.Controls.Generators
 
             if (!(tabItem.Content is IControl))
             {
-                tabItem.Bind(TabItem.ContentTemplateProperty, new OwnerBinding<IDataTemplate>(
+                tabItem.Bind(TabItem.ContentTemplateProperty, new OwnerBinding<IDataTemplate?>(
                     tabItem,
                     TabControl.ContentTemplateProperty));
             }
@@ -62,8 +62,8 @@ namespace Avalonia.Controls.Generators
         {
             private readonly TabItem _item;
             private readonly StyledProperty<T> _ownerProperty;
-            private IDisposable _ownerSubscription;
-            private IDisposable _propertySubscription;
+            private IDisposable? _ownerSubscription;
+            private IDisposable? _propertySubscription;
 
             public OwnerBinding(TabItem item, StyledProperty<T> ownerProperty)
             {
@@ -82,7 +82,7 @@ namespace Avalonia.Controls.Generators
                 _ownerSubscription = null;
             }
 
-            private void OwnerChanged(ILogical c)
+            private void OwnerChanged(ILogical? c)
             {
                 _propertySubscription?.Dispose();
                 _propertySubscription = null;
diff --git a/src/Avalonia.Controls/Generators/TreeContainerIndex.cs b/src/Avalonia.Controls/Generators/TreeContainerIndex.cs
index f0da370f73..da13416700 100644
--- a/src/Avalonia.Controls/Generators/TreeContainerIndex.cs
+++ b/src/Avalonia.Controls/Generators/TreeContainerIndex.cs
@@ -21,12 +21,12 @@ namespace Avalonia.Controls.Generators
         /// <summary>
         /// Signaled whenever new containers are materialized.
         /// </summary>
-        public event EventHandler<ItemContainerEventArgs> Materialized;
+        public event EventHandler<ItemContainerEventArgs>? Materialized;
 
         /// <summary>
         /// Event raised whenever containers are dematerialized.
         /// </summary>
-        public event EventHandler<ItemContainerEventArgs> Dematerialized;
+        public event EventHandler<ItemContainerEventArgs>? Dematerialized;
 
         /// <summary>
         /// Gets the currently materialized containers.
@@ -92,7 +92,7 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         /// <param name="item">The item.</param>
         /// <returns>The container, or null of not found.</returns>
-        public IControl ContainerFromItem(object item)
+        public IControl? ContainerFromItem(object item)
         {
             if (item != null)
             {
@@ -108,7 +108,7 @@ namespace Avalonia.Controls.Generators
         /// </summary>
         /// <param name="container">The container.</param>
         /// <returns>The item, or null of not found.</returns>
-        public object ItemFromContainer(IControl container)
+        public object? ItemFromContainer(IControl? container)
         {
             if (container != null)
             {
diff --git a/src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs b/src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs
index 9e65ef5f81..536a5fdd06 100644
--- a/src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs
+++ b/src/Avalonia.Controls/Generators/TreeItemContainerGenerator.cs
@@ -14,7 +14,7 @@ namespace Avalonia.Controls.Generators
     public class TreeItemContainerGenerator<T> : ItemContainerGenerator<T>, ITreeItemContainerGenerator
         where T : class, IControl, new()
     {
-        private TreeView _treeView;
+        private TreeView? _treeView;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="TreeItemContainerGenerator{T}"/> class.
@@ -32,20 +32,15 @@ namespace Avalonia.Controls.Generators
             AvaloniaProperty isExpandedProperty)
             : base(owner, contentProperty, contentTemplateProperty)
         {
-            Contract.Requires<ArgumentNullException>(owner != null);
-            Contract.Requires<ArgumentNullException>(contentProperty != null);
-            Contract.Requires<ArgumentNullException>(itemsProperty != null);
-            Contract.Requires<ArgumentNullException>(isExpandedProperty != null);
-
-            ItemsProperty = itemsProperty;
-            IsExpandedProperty = isExpandedProperty;
+            ItemsProperty = itemsProperty ?? throw new ArgumentNullException(nameof(itemsProperty));
+            IsExpandedProperty = isExpandedProperty ?? throw new ArgumentNullException(nameof(isExpandedProperty));
             UpdateIndex();
         }
 
         /// <summary>
         /// Gets the container index for the tree.
         /// </summary>
-        public TreeContainerIndex Index { get; private set; }
+        public TreeContainerIndex? Index { get; private set; }
 
         /// <summary>
         /// Gets the item container's Items property.
@@ -58,7 +53,7 @@ namespace Avalonia.Controls.Generators
         protected AvaloniaProperty IsExpandedProperty { get; }
 
         /// <inheritdoc/>
-        protected override IControl CreateContainer(object item)
+        protected override IControl? CreateContainer(object? item)
         {
             var container = item as T;
 
@@ -141,12 +136,12 @@ namespace Avalonia.Controls.Generators
         {
             private readonly IDataTemplate _inner;
             public WrapperTreeDataTemplate(IDataTemplate inner) => _inner = inner;
-            public IControl Build(object param) => _inner.Build(param);
-            public bool Match(object data) => _inner.Match(data);
-            public InstancedBinding ItemsSelector(object item) => null;
+            public IControl? Build(object? param) => _inner.Build(param);
+            public bool Match(object? data) => _inner.Match(data);
+            public InstancedBinding? ItemsSelector(object item) => null;
         }
 
-        private ITreeDataTemplate GetTreeDataTemplate(object item, IDataTemplate primary)
+        private ITreeDataTemplate GetTreeDataTemplate(object item, IDataTemplate? primary)
         {
             var template = Owner.FindDataTemplate(item, primary) ?? FuncDataTemplate.Default;
             var treeTemplate = template as ITreeDataTemplate ?? new WrapperTreeDataTemplate(template);
diff --git a/src/Avalonia.Controls/Grid.cs b/src/Avalonia.Controls/Grid.cs
index 74401c450a..4949e1f839 100644
--- a/src/Avalonia.Controls/Grid.cs
+++ b/src/Avalonia.Controls/Grid.cs
@@ -8,6 +8,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Threading;
 using Avalonia.Layout;
 using Avalonia.Media;
@@ -48,7 +49,7 @@ namespace Avalonia.Controls
         /// <param name="value">Column property value.</param>
         public static void SetColumn(Control element, int value)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ColumnProperty, value);
         }
 
@@ -59,7 +60,7 @@ namespace Avalonia.Controls
         /// <returns>Column property value.</returns>
         public static int GetColumn(Control element)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(ColumnProperty);
         }
 
@@ -70,7 +71,7 @@ namespace Avalonia.Controls
         /// <param name="value">Row property value.</param>
         public static void SetRow(Control element, int value)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(RowProperty, value);
         }
 
@@ -81,7 +82,7 @@ namespace Avalonia.Controls
         /// <returns>Row property value.</returns>
         public static int GetRow(Control element)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(RowProperty);
         }
 
@@ -92,7 +93,7 @@ namespace Avalonia.Controls
         /// <param name="value">ColumnSpan property value.</param>
         public static void SetColumnSpan(Control element, int value)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ColumnSpanProperty, value);
         }
 
@@ -103,7 +104,7 @@ namespace Avalonia.Controls
         /// <returns>ColumnSpan property value.</returns>
         public static int GetColumnSpan(Control element)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(ColumnSpanProperty);
         }
 
@@ -114,7 +115,7 @@ namespace Avalonia.Controls
         /// <param name="value">RowSpan property value.</param>
         public static void SetRowSpan(Control element, int value)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(RowSpanProperty, value);
         }
 
@@ -125,7 +126,7 @@ namespace Avalonia.Controls
         /// <returns>RowSpan property value.</returns>
         public static int GetRowSpan(Control element)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(RowSpanProperty);
         }
 
@@ -136,7 +137,7 @@ namespace Avalonia.Controls
         /// <param name="value">IsSharedSizeScope property value.</param>
         public static void SetIsSharedSizeScope(Control element, bool value)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsSharedSizeScopeProperty, value);
         }
 
@@ -147,7 +148,7 @@ namespace Avalonia.Controls
         /// <returns>IsSharedSizeScope property value.</returns>
         public static bool GetIsSharedSizeScope(Control element)
         {
-            Contract.Requires<ArgumentNullException>(element != null);
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(IsSharedSizeScopeProperty);
         }
 
@@ -573,7 +574,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// <see cref="Panel.ChildrenChanged"/>
         /// </summary>
-        protected override void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected override void ChildrenChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             CellsStructureDirty = true;
             base.ChildrenChanged(sender, e);
@@ -994,7 +995,7 @@ namespace Avalonia.Controls
             }
 
             var children = this.Children;
-            Hashtable spanStore = null;
+            Hashtable? spanStore = null;
             bool ignoreDesiredSizeV = forceInfinityV;
 
             int i = cellsHead;
@@ -1048,7 +1049,7 @@ namespace Avalonia.Controls
                 foreach (DictionaryEntry e in spanStore)
                 {
                     SpanKey key = (SpanKey)e.Key;
-                    double requestedSize = (double)e.Value;
+                    double requestedSize = (double)e.Value!;
 
                     EnsureMinSizeInDefinitionRange(
                         key.U ? DefinitionsU : DefinitionsV,
@@ -1069,7 +1070,7 @@ namespace Avalonia.Controls
         /// <param name="u"><c>true</c> if this is a column span. <c>false</c> if this is a row span.</param>
         /// <param name="value">Value to store. If an entry already exists the biggest value is stored.</param>
         private static void RegisterSpan(
-            ref Hashtable store,
+            ref Hashtable? store,
             int start,
             int count,
             bool u,
@@ -1081,7 +1082,7 @@ namespace Avalonia.Controls
             }
 
             SpanKey key = new SpanKey(start, count, u);
-            object o = store[key];
+            object? o = store[key];
 
             if (o == null
                 || value > (double)o)
@@ -1605,12 +1606,12 @@ namespace Avalonia.Controls
                     while (minCount > 0 && tempDefinitions[minCount - 1].MeasureSize < 0.0)
                     {
                         --minCount;
-                        tempDefinitions[minCount] = null;
+                        tempDefinitions[minCount] = null!;
                     }
                     while (maxCount > 0 && tempDefinitions[defCount + maxCount - 1].MeasureSize < 0.0)
                     {
                         --maxCount;
-                        tempDefinitions[defCount + maxCount] = null;
+                        tempDefinitions[defCount + maxCount] = null!;
                     }
                 }
 
@@ -2321,7 +2322,7 @@ namespace Avalonia.Controls
         /// by adding / removing GridLinesRenderer visual.
         /// Returns a reference to GridLinesRenderer visual or null.
         /// </summary>
-        private GridLinesRenderer EnsureGridLinesRenderer()
+        private GridLinesRenderer? EnsureGridLinesRenderer()
         {
             //
             //  synchronize the state
@@ -2369,16 +2370,16 @@ namespace Avalonia.Controls
                 grid.InvalidateVisual();
             }
 
-            grid.SetFlags((bool)e.NewValue, Flags.ShowGridLinesPropertyValue);
+            grid.SetFlags((bool)e.NewValue!, Flags.ShowGridLinesPropertyValue);
         }
 
         private static void OnCellAttachedPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
-            Visual child = d as Visual;
+            Visual? child = d as Visual;
 
             if (child != null)
             {
-                Grid grid = child.GetVisualParent<Grid>();
+                Grid? grid = child.GetVisualParent<Grid>();
                 if (grid != null
                     && grid.ExtData != null
                     && grid.ListenToNotifications)
@@ -2395,7 +2396,7 @@ namespace Avalonia.Controls
         /// true if one or both of x and y are null, in which case result holds
         /// the relative sort order.
         /// </returns>
-        private static bool CompareNullRefs(object x, object y, out int result)
+        private static bool CompareNullRefs([NotNullWhen(false)] object? x, [NotNullWhen(false)] object? y, out int result)
         {
             result = 2;
 
@@ -2426,7 +2427,7 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsU
         {
-            get { return (ExtData.DefinitionsU); }
+            get { return (ExtData.DefinitionsU!); }
         }
 
         /// <summary>
@@ -2434,7 +2435,7 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsV
         {
-            get { return (ExtData.DefinitionsV); }
+            get { return (ExtData.DefinitionsV!); }
         }
 
         /// <summary>
@@ -2450,7 +2451,7 @@ namespace Avalonia.Controls
                 if (extData.TempDefinitions == null
                     || extData.TempDefinitions.Length < requiredLength)
                 {
-                    WeakReference tempDefinitionsWeakRef = (WeakReference)Thread.GetData(s_tempDefinitionsDataSlot);
+                    WeakReference? tempDefinitionsWeakRef = (WeakReference?)Thread.GetData(s_tempDefinitionsDataSlot);
                     if (tempDefinitionsWeakRef == null)
                     {
                         extData.TempDefinitions = new DefinitionBase[requiredLength];
@@ -2458,7 +2459,7 @@ namespace Avalonia.Controls
                     }
                     else
                     {
-                        extData.TempDefinitions = (DefinitionBase[])tempDefinitionsWeakRef.Target;
+                        extData.TempDefinitions = (DefinitionBase[]?)tempDefinitionsWeakRef.Target;
                         if (extData.TempDefinitions == null
                             || extData.TempDefinitions.Length < requiredLength)
                         {
@@ -2515,7 +2516,7 @@ namespace Avalonia.Controls
         /// </summary>
         private CellCache[] PrivateCells
         {
-            get { return (ExtData.CellCachesCollection); }
+            get { return (ExtData.CellCachesCollection!); }
         }
 
         /// <summary>
@@ -2586,7 +2587,7 @@ namespace Avalonia.Controls
         /// </summary>
         private ExtendedData ExtData
         {
-            get { return (_data); }
+            get { return (_data!); }
         }
 
         /// <summary>
@@ -2608,17 +2609,17 @@ namespace Avalonia.Controls
         }
 
         // Extended data instantiated on demand, for non-trivial case handling only
-        private ExtendedData _data;
+        private ExtendedData? _data;
 
         // Grid validity / property caches dirtiness flags
         private Flags _flags;
-        private GridLinesRenderer _gridLinesRenderer;
+        private GridLinesRenderer? _gridLinesRenderer;
 
         // Keeps track of definition indices.
-        int[] _definitionIndices;
+        int[]? _definitionIndices;
 
         // Stores unrounded values and rounding errors during layout rounding.
-        double[] _roundingErrors;
+        double[]? _roundingErrors;
 
         // 5 is an arbitrary constant chosen to end the measure loop
         private const int c_layoutLoopMaxCount = 5;
@@ -2635,16 +2636,16 @@ namespace Avalonia.Controls
         /// </summary>
         private class ExtendedData
         {
-            internal ColumnDefinitions ColumnDefinitions;  //  collection of column definitions (logical tree support)
-            internal RowDefinitions RowDefinitions;        //  collection of row definitions (logical tree support)
-            internal IReadOnlyList<DefinitionBase> DefinitionsU;    //  collection of column definitions used during calc
-            internal IReadOnlyList<DefinitionBase> DefinitionsV;    //  collection of row definitions used during calc
-            internal CellCache[] CellCachesCollection;              //  backing store for logical children
+            internal ColumnDefinitions? ColumnDefinitions;  //  collection of column definitions (logical tree support)
+            internal RowDefinitions? RowDefinitions;        //  collection of row definitions (logical tree support)
+            internal IReadOnlyList<DefinitionBase>? DefinitionsU;    //  collection of column definitions used during calc
+            internal IReadOnlyList<DefinitionBase>? DefinitionsV;    //  collection of row definitions used during calc
+            internal CellCache[]? CellCachesCollection;              //  backing store for logical children
             internal int CellGroup1;                                //  index of the first cell in first cell group
             internal int CellGroup2;                                //  index of the first cell in second cell group
             internal int CellGroup3;                                //  index of the first cell in third cell group
             internal int CellGroup4;                                //  index of the first cell in forth cell group
-            internal DefinitionBase[] TempDefinitions;              //  temporary array used during layout for various purposes
+            internal DefinitionBase[]? TempDefinitions;              //  temporary array used during layout for various purposes
                                                                     //  TempDefinitions.Length == Max(definitionsU.Length, definitionsV.Length)
         }
 
@@ -2831,9 +2832,9 @@ namespace Avalonia.Controls
             /// <summary>
             /// <see cref="object.Equals(object)"/>
             /// </summary>
-            public override bool Equals(object obj)
+            public override bool Equals(object? obj)
             {
-                SpanKey sk = obj as SpanKey;
+                SpanKey? sk = obj as SpanKey;
                 return (sk != null
                         && sk._start == _start
                         && sk._count == _count
@@ -2866,10 +2867,10 @@ namespace Avalonia.Controls
         /// </summary>
         private class SpanPreferredDistributionOrderComparer : IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
-                DefinitionBase definitionX = x as DefinitionBase;
-                DefinitionBase definitionY = y as DefinitionBase;
+                DefinitionBase? definitionX = x as DefinitionBase;
+                DefinitionBase? definitionY = y as DefinitionBase;
 
                 int result;
 
@@ -2908,10 +2909,10 @@ namespace Avalonia.Controls
         /// </summary>
         private class SpanMaxDistributionOrderComparer : IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
-                DefinitionBase definitionX = x as DefinitionBase;
-                DefinitionBase definitionY = y as DefinitionBase;
+                DefinitionBase? definitionX = x as DefinitionBase;
+                DefinitionBase? definitionY = y as DefinitionBase;
 
                 int result;
 
@@ -2954,17 +2955,16 @@ namespace Avalonia.Controls
 
             internal StarDistributionOrderIndexComparer(IReadOnlyList<DefinitionBase> definitions)
             {
-                Contract.Requires<NullReferenceException>(definitions != null);
-                this.definitions = definitions;
+                this.definitions = definitions ?? throw new ArgumentNullException(nameof(definitions));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
 
-                DefinitionBase definitionX = null;
-                DefinitionBase definitionY = null;
+                DefinitionBase? definitionX = null;
+                DefinitionBase? definitionY = null;
 
                 if (indexX != null)
                 {
@@ -2995,17 +2995,16 @@ namespace Avalonia.Controls
 
             internal DistributionOrderIndexComparer(IReadOnlyList<DefinitionBase> definitions)
             {
-                Contract.Requires<NullReferenceException>(definitions != null);
-                this.definitions = definitions;
+                this.definitions = definitions ?? throw new ArgumentNullException(nameof(definitions));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
 
-                DefinitionBase definitionX = null;
-                DefinitionBase definitionY = null;
+                DefinitionBase? definitionX = null;
+                DefinitionBase? definitionY = null;
 
                 if (indexX != null)
                 {
@@ -3038,11 +3037,10 @@ namespace Avalonia.Controls
 
             internal RoundingErrorIndexComparer(double[] errors)
             {
-                Contract.Requires<NullReferenceException>(errors != null);
-                this.errors = errors;
+                this.errors = errors ?? throw new ArgumentNullException(nameof(errors));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
@@ -3067,10 +3065,10 @@ namespace Avalonia.Controls
         /// </summary>
         private class MinRatioComparer : IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
-                DefinitionBase definitionX = x as DefinitionBase;
-                DefinitionBase definitionY = y as DefinitionBase;
+                DefinitionBase? definitionX = x as DefinitionBase;
+                DefinitionBase? definitionY = y as DefinitionBase;
 
                 int result;
 
@@ -3090,10 +3088,10 @@ namespace Avalonia.Controls
         /// </summary>
         private class MaxRatioComparer : IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
-                DefinitionBase definitionX = x as DefinitionBase;
-                DefinitionBase definitionY = y as DefinitionBase;
+                DefinitionBase? definitionX = x as DefinitionBase;
+                DefinitionBase? definitionY = y as DefinitionBase;
 
                 int result;
 
@@ -3112,10 +3110,10 @@ namespace Avalonia.Controls
         /// </summary>
         private class StarWeightComparer : IComparer
         {
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
-                DefinitionBase definitionX = x as DefinitionBase;
-                DefinitionBase definitionY = y as DefinitionBase;
+                DefinitionBase? definitionX = x as DefinitionBase;
+                DefinitionBase? definitionY = y as DefinitionBase;
 
                 int result;
 
@@ -3137,17 +3135,16 @@ namespace Avalonia.Controls
 
             internal MinRatioIndexComparer(IReadOnlyList<DefinitionBase> definitions)
             {
-                Contract.Requires<NullReferenceException>(definitions != null);
-                this.definitions = definitions;
+                this.definitions = definitions ?? throw new ArgumentNullException(nameof(definitions));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
 
-                DefinitionBase definitionX = null;
-                DefinitionBase definitionY = null;
+                DefinitionBase? definitionX = null;
+                DefinitionBase? definitionY = null;
 
                 if (indexX != null)
                 {
@@ -3178,17 +3175,16 @@ namespace Avalonia.Controls
 
             internal MaxRatioIndexComparer(IReadOnlyList<DefinitionBase> definitions)
             {
-                Contract.Requires<NullReferenceException>(definitions != null);
-                this.definitions = definitions;
+                this.definitions = definitions ?? throw new ArgumentNullException(nameof(definitions));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
 
-                DefinitionBase definitionX = null;
-                DefinitionBase definitionY = null;
+                DefinitionBase? definitionX = null;
+                DefinitionBase? definitionY = null;
 
                 if (indexX != null)
                 {
@@ -3219,17 +3215,16 @@ namespace Avalonia.Controls
 
             internal StarWeightIndexComparer(IReadOnlyList<DefinitionBase> definitions)
             {
-                Contract.Requires<NullReferenceException>(definitions != null);
-                this.definitions = definitions;
+                this.definitions = definitions ?? throw new ArgumentNullException(nameof(definitions));
             }
 
-            public int Compare(object x, object y)
+            public int Compare(object? x, object? y)
             {
                 int? indexX = x as int?;
                 int? indexY = y as int?;
 
-                DefinitionBase definitionX = null;
-                DefinitionBase definitionY = null;
+                DefinitionBase? definitionX = null;
+                DefinitionBase? definitionY = null;
 
                 if (indexX != null)
                 {
diff --git a/src/Avalonia.Controls/GridLength.cs b/src/Avalonia.Controls/GridLength.cs
index ee6a146d61..d7022b80ce 100644
--- a/src/Avalonia.Controls/GridLength.cs
+++ b/src/Avalonia.Controls/GridLength.cs
@@ -135,7 +135,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="o">The object with which to test equality.</param>
         /// <returns>True if the objects are equal, otherwise false.</returns>
-        public override bool Equals(object o)
+        public override bool Equals(object? o)
         {
             if (o == null)
             {
diff --git a/src/Avalonia.Controls/GridSplitter.cs b/src/Avalonia.Controls/GridSplitter.cs
index 26c8464576..216e43e1f0 100644
--- a/src/Avalonia.Controls/GridSplitter.cs
+++ b/src/Avalonia.Controls/GridSplitter.cs
@@ -59,7 +59,7 @@ namespace Avalonia.Controls
         private static readonly Cursor s_columnSplitterCursor = new Cursor(StandardCursorType.SizeWestEast);
         private static readonly Cursor s_rowSplitterCursor = new Cursor(StandardCursorType.SizeNorthSouth);
 
-        private ResizeData _resizeData;
+        private ResizeData? _resizeData;
 
         /// <summary>
         /// Indicates whether the Splitter resizes the Columns, Rows, or Both.
@@ -197,9 +197,9 @@ namespace Avalonia.Controls
         /// </summary>
         private void RemovePreviewAdorner()
         {
-            if (_resizeData.Adorner != null)
+            if (_resizeData?.Adorner != null)
             {
-                AdornerLayer layer = AdornerLayer.GetAdornerLayer(this);
+                AdornerLayer layer = AdornerLayer.GetAdornerLayer(this)!;
                 layer.Children.Remove(_resizeData.Adorner);
             }
         }
@@ -242,7 +242,7 @@ namespace Avalonia.Controls
         /// </summary>
         private bool SetupDefinitionsToResize()
         {
-            int gridSpan = GetValue(_resizeData.ResizeDirection == GridResizeDirection.Columns ?
+            int gridSpan = GetValue(_resizeData!.ResizeDirection == GridResizeDirection.Columns ?
                 Grid.ColumnSpanProperty :
                 Grid.RowSpanProperty);
 
@@ -276,8 +276,8 @@ namespace Avalonia.Controls
 
                 // Get count of rows/columns in the resize direction.
                 int count = _resizeData.ResizeDirection == GridResizeDirection.Columns ?
-                    _resizeData.Grid.ColumnDefinitions.Count :
-                    _resizeData.Grid.RowDefinitions.Count;
+                    _resizeData.Grid!.ColumnDefinitions.Count :
+                    _resizeData.Grid!.RowDefinitions.Count;
 
                 if (index1 >= 0 && index2 < count)
                 {
@@ -322,10 +322,10 @@ namespace Avalonia.Controls
         /// </summary>
         private void SetupPreviewAdorner()
         {
-            if (_resizeData.ShowsPreview)
+            if (_resizeData!.ShowsPreview)
             {
                 // Get the adorner layer and add an adorner to it.
-                var adornerLayer = AdornerLayer.GetAdornerLayer(_resizeData.Grid);
+                var adornerLayer = AdornerLayer.GetAdornerLayer(_resizeData.Grid!);
 
                 var previewContent = PreviewContent;
 
@@ -335,7 +335,7 @@ namespace Avalonia.Controls
                     return;
                 }
 
-                IControl builtPreviewContent = previewContent?.Build();
+                IControl? builtPreviewContent = previewContent?.Build();
 
                 _resizeData.Adorner = new PreviewAdorner(builtPreviewContent);
 
@@ -409,13 +409,13 @@ namespace Avalonia.Controls
                     // Set the Translation of the Adorner to the distance from the thumb.
                     if (_resizeData.ResizeDirection == GridResizeDirection.Columns)
                     {
-                        _resizeData.Adorner.OffsetX = Math.Min(
+                        _resizeData.Adorner!.OffsetX = Math.Min(
                             Math.Max(horizontalChange, _resizeData.MinChange),
                             _resizeData.MaxChange);
                     }
                     else
                     {
-                        _resizeData.Adorner.OffsetY = Math.Min(
+                        _resizeData.Adorner!.OffsetY = Math.Min(
                             Math.Max(verticalChange, _resizeData.MinChange),
                             _resizeData.MaxChange);
                     }
@@ -437,7 +437,7 @@ namespace Avalonia.Controls
                 if (_resizeData.ShowsPreview)
                 {
                     // Update the grid.
-                    MoveSplitter(_resizeData.Adorner.OffsetX, _resizeData.Adorner.OffsetY);
+                    MoveSplitter(_resizeData.Adorner!.OffsetX, _resizeData.Adorner.OffsetY);
                     RemovePreviewAdorner();
                 }
 
@@ -481,14 +481,14 @@ namespace Avalonia.Controls
         private void CancelResize()
         {
             // Restore original column/row lengths.
-            if (_resizeData.ShowsPreview)
+            if (_resizeData!.ShowsPreview)
             {
                 RemovePreviewAdorner();
             }
             else // Reset the columns/rows lengths to the saved values.
             {
-                SetDefinitionLength(_resizeData.Definition1, _resizeData.OriginalDefinition1Length);
-                SetDefinitionLength(_resizeData.Definition2, _resizeData.OriginalDefinition2Length);
+                SetDefinitionLength(_resizeData.Definition1!, _resizeData.OriginalDefinition1Length);
+                SetDefinitionLength(_resizeData.Definition2!, _resizeData.OriginalDefinition2Length);
             }
 
             _resizeData = null;
@@ -536,12 +536,12 @@ namespace Avalonia.Controls
         /// </summary>
         private void GetDeltaConstraints(out double minDelta, out double maxDelta)
         {
-            double definition1Len = GetActualLength(_resizeData.Definition1);
-            double definition1Min = _resizeData.Definition1.UserMinSizeValueCache;
+            double definition1Len = GetActualLength(_resizeData!.Definition1!);
+            double definition1Min = _resizeData.Definition1!.UserMinSizeValueCache;
             double definition1Max = _resizeData.Definition1.UserMaxSizeValueCache;
 
-            double definition2Len = GetActualLength(_resizeData.Definition2);
-            double definition2Min = _resizeData.Definition2.UserMinSizeValueCache;
+            double definition2Len = GetActualLength(_resizeData.Definition2!);
+            double definition2Min = _resizeData.Definition2!.UserMinSizeValueCache;
             double definition2Max = _resizeData.Definition2.UserMaxSizeValueCache;
 
             // Set MinWidths to be greater than width of splitter.
@@ -565,11 +565,11 @@ namespace Avalonia.Controls
         private void SetLengths(double definition1Pixels, double definition2Pixels)
         {
             // For the case where both definition1 and 2 are stars, update all star values to match their current pixel values.
-            if (_resizeData.SplitBehavior == SplitBehavior.Split)
+            if (_resizeData!.SplitBehavior == SplitBehavior.Split)
             {
                 var definitions = _resizeData.ResizeDirection == GridResizeDirection.Columns ?
-                    (IAvaloniaReadOnlyList<DefinitionBase>)_resizeData.Grid.ColumnDefinitions :
-                    (IAvaloniaReadOnlyList<DefinitionBase>)_resizeData.Grid.RowDefinitions;
+                    (IAvaloniaReadOnlyList<DefinitionBase>)_resizeData.Grid!.ColumnDefinitions :
+                    (IAvaloniaReadOnlyList<DefinitionBase>)_resizeData.Grid!.RowDefinitions;
 
                 var definitionsCount = definitions.Count;
 
@@ -595,11 +595,11 @@ namespace Avalonia.Controls
             }
             else if (_resizeData.SplitBehavior == SplitBehavior.Resize1)
             {
-                SetDefinitionLength(_resizeData.Definition1, new GridLength(definition1Pixels));
+                SetDefinitionLength(_resizeData.Definition1!, new GridLength(definition1Pixels));
             }
             else
             {
-                SetDefinitionLength(_resizeData.Definition2, new GridLength(definition2Pixels));
+                SetDefinitionLength(_resizeData.Definition2!, new GridLength(definition2Pixels));
             }
         }
 
@@ -623,8 +623,8 @@ namespace Avalonia.Controls
                 delta = LayoutHelper.RoundLayoutValue(delta, LayoutHelper.GetLayoutScale(this));
             }
             
-            DefinitionBase definition1 = _resizeData.Definition1;
-            DefinitionBase definition2 = _resizeData.Definition2;
+            DefinitionBase? definition1 = _resizeData.Definition1;
+            DefinitionBase? definition2 = _resizeData.Definition2;
 
             if (definition1 != null && definition2 != null)
             {
@@ -691,7 +691,7 @@ namespace Avalonia.Controls
             private readonly TranslateTransform _translation;
             private readonly Decorator _decorator;
             
-            public PreviewAdorner(IControl previewControl)
+            public PreviewAdorner(IControl? previewControl)
             {
                 // Add a decorator to perform translations.
                 _translation = new TranslateTransform();
@@ -762,22 +762,22 @@ namespace Avalonia.Controls
         private class ResizeData
         {
             public bool ShowsPreview;
-            public PreviewAdorner Adorner;
+            public PreviewAdorner? Adorner;
 
             // The constraints to keep the Preview within valid ranges.
             public double MinChange;
             public double MaxChange;
 
             // The grid to Resize.
-            public Grid Grid;
+            public Grid? Grid;
 
             // Cache of Resize Direction and Behavior.
             public GridResizeDirection ResizeDirection;
             public GridResizeBehavior ResizeBehavior;
 
             // The columns/rows to resize.
-            public DefinitionBase Definition1;
-            public DefinitionBase Definition2;
+            public DefinitionBase? Definition1;
+            public DefinitionBase? Definition2;
 
             // Are the columns/rows star lengths.
             public SplitBehavior SplitBehavior;
diff --git a/src/Avalonia.Controls/HotkeyManager.cs b/src/Avalonia.Controls/HotkeyManager.cs
index e89bb177dd..d83b996aa0 100644
--- a/src/Avalonia.Controls/HotkeyManager.cs
+++ b/src/Avalonia.Controls/HotkeyManager.cs
@@ -7,29 +7,29 @@ namespace Avalonia.Controls
 {
     public class HotKeyManager
     {
-        public static readonly AttachedProperty<KeyGesture> HotKeyProperty
-            = AvaloniaProperty.RegisterAttached<Control, KeyGesture>("HotKey", typeof(HotKeyManager));
+        public static readonly AttachedProperty<KeyGesture?> HotKeyProperty
+            = AvaloniaProperty.RegisterAttached<Control, KeyGesture?>("HotKey", typeof(HotKeyManager));
 
         class HotkeyCommandWrapper : ICommand
         {
-            public HotkeyCommandWrapper(ICommandSource control)
+            public HotkeyCommandWrapper(ICommandSource? control)
             {
                 CommandSource = control;
             }
 
-            public readonly ICommandSource CommandSource;
+            public readonly ICommandSource? CommandSource;
 
-            private ICommand GetCommand() => CommandSource.Command;
+            private ICommand? GetCommand() => CommandSource?.Command;
 
-            public bool CanExecute(object parameter) =>
-                CommandSource.Command?.CanExecute(CommandSource.CommandParameter) == true
+            public bool CanExecute(object? parameter) =>
+                CommandSource?.Command?.CanExecute(CommandSource.CommandParameter) == true
                 && CommandSource.IsEffectivelyEnabled;
 
-            public void Execute(object parameter) =>
-                GetCommand()?.Execute(CommandSource.CommandParameter);
+            public void Execute(object? parameter) =>
+                GetCommand()?.Execute(CommandSource?.CommandParameter);
 
 #pragma warning disable 67 // Event not used
-            public event EventHandler CanExecuteChanged;
+            public event EventHandler? CanExecuteChanged;
 #pragma warning restore 67
         }
 
@@ -37,12 +37,12 @@ namespace Avalonia.Controls
         class Manager
         {
             private readonly IControl _control;
-            private TopLevel _root;
-            private IDisposable _parentSub;
-            private IDisposable _hotkeySub;
-            private KeyGesture _hotkey;
+            private TopLevel? _root;
+            private IDisposable? _parentSub;
+            private IDisposable? _hotkeySub;
+            private KeyGesture? _hotkey;
             private readonly HotkeyCommandWrapper _wrapper;
-            private KeyBinding _binding;
+            private KeyBinding? _binding;
 
             public Manager(IControl control)
             {
@@ -56,14 +56,14 @@ namespace Avalonia.Controls
                 _parentSub = AncestorFinder.Create<TopLevel>(_control).Subscribe(OnParentChanged);
             }
 
-            private void OnParentChanged(TopLevel control)
+            private void OnParentChanged(TopLevel? control)
             {
                 Unregister();
                 _root = control;
                 Register();
             }
 
-            private void OnHotkeyChanged(KeyGesture hotkey)
+            private void OnHotkeyChanged(KeyGesture? hotkey)
             {
                 if (hotkey == null)
                     //Subscription will be recreated by static property watcher
@@ -95,8 +95,8 @@ namespace Avalonia.Controls
             void Stop()
             {
                 Unregister();
-                _parentSub.Dispose();
-                _hotkeySub.Dispose();
+                _parentSub?.Dispose();
+                _hotkeySub?.Dispose();
             }
         }
 
@@ -118,6 +118,6 @@ namespace Avalonia.Controls
             });
         }
         public static void SetHotKey(AvaloniaObject target, KeyGesture value) => target.SetValue(HotKeyProperty, value);
-        public static KeyGesture GetHotKey(AvaloniaObject target) => target.GetValue(HotKeyProperty);
+        public static KeyGesture? GetHotKey(AvaloniaObject target) => target.GetValue(HotKeyProperty);
     }
 }
diff --git a/src/Avalonia.Controls/IContentControl.cs b/src/Avalonia.Controls/IContentControl.cs
index 889771b355..d28b0afb25 100644
--- a/src/Avalonia.Controls/IContentControl.cs
+++ b/src/Avalonia.Controls/IContentControl.cs
@@ -12,12 +12,12 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the content to display.
         /// </summary>
-        object Content { get; set; }
+        object? Content { get; set; }
 
         /// <summary>
         /// Gets or sets the data template used to display the content of the control.
         /// </summary>
-        IDataTemplate ContentTemplate { get; set; }
+        IDataTemplate? ContentTemplate { get; set; }
 
         /// <summary>
         /// Gets or sets the horizontal alignment of the content within the control.
@@ -29,4 +29,4 @@ namespace Avalonia.Controls
         /// </summary>
         VerticalAlignment VerticalContentAlignment { get; set; }
     }
-}
\ No newline at end of file
+}
diff --git a/src/Avalonia.Controls/IHeadered.cs b/src/Avalonia.Controls/IHeadered.cs
index c208e21c97..8c1b5cfb1b 100644
--- a/src/Avalonia.Controls/IHeadered.cs
+++ b/src/Avalonia.Controls/IHeadered.cs
@@ -8,6 +8,6 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or set the header.
         /// </summary>
-        object Header { get; set; }
+        object? Header { get; set; }
     }
 }
diff --git a/src/Avalonia.Controls/IVirtualizingPanel.cs b/src/Avalonia.Controls/IVirtualizingPanel.cs
index 8c4dc80934..604a62dcb9 100644
--- a/src/Avalonia.Controls/IVirtualizingPanel.cs
+++ b/src/Avalonia.Controls/IVirtualizingPanel.cs
@@ -16,7 +16,7 @@ namespace Avalonia.Controls
         /// Note that this property may remain null if the panel is added to a control that does
         /// not act as a virtualizing controller.
         /// </remarks>
-        IVirtualizingController Controller { get; set; }
+        IVirtualizingController? Controller { get; set; }
 
         /// <summary>
         /// Gets a value indicating whether the panel is full.
diff --git a/src/Avalonia.Controls/ItemsControl.cs b/src/Avalonia.Controls/ItemsControl.cs
index 1ff49326b6..70f771ca00 100644
--- a/src/Avalonia.Controls/ItemsControl.cs
+++ b/src/Avalonia.Controls/ItemsControl.cs
@@ -32,8 +32,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Items"/> property.
         /// </summary>
-        public static readonly DirectProperty<ItemsControl, IEnumerable> ItemsProperty =
-            AvaloniaProperty.RegisterDirect<ItemsControl, IEnumerable>(nameof(Items), o => o.Items, (o, v) => o.Items = v);
+        public static readonly DirectProperty<ItemsControl, IEnumerable?> ItemsProperty =
+            AvaloniaProperty.RegisterDirect<ItemsControl, IEnumerable?>(nameof(Items), o => o.Items, (o, v) => o.Items = v);
 
         /// <summary>
         /// Defines the <see cref="ItemCount"/> property.
@@ -50,13 +50,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="ItemTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ItemTemplateProperty =
-            AvaloniaProperty.Register<ItemsControl, IDataTemplate>(nameof(ItemTemplate));
+        public static readonly StyledProperty<IDataTemplate?> ItemTemplateProperty =
+            AvaloniaProperty.Register<ItemsControl, IDataTemplate?>(nameof(ItemTemplate));
 
-        private IEnumerable _items = new AvaloniaList<object>();
+        private IEnumerable? _items = new AvaloniaList<object>();
         private int _itemCount;
-        private IItemContainerGenerator _itemContainerGenerator;
-        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+        private IItemContainerGenerator? _itemContainerGenerator;
+        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsControl"/> class.
@@ -79,7 +79,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the <see cref="IItemContainerGenerator"/> for the control.
         /// </summary>
-        public IItemContainerGenerator ItemContainerGenerator
+        public IItemContainerGenerator? ItemContainerGenerator
         {
             get
             {
@@ -104,7 +104,7 @@ namespace Avalonia.Controls
         /// Gets or sets the items to display.
         /// </summary>
         [Content]
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get { return _items; }
             set { SetAndRaise(ItemsProperty, ref _items, value); }
@@ -131,7 +131,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>
-        public IDataTemplate ItemTemplate
+        public IDataTemplate? ItemTemplate
         {
             get { return GetValue(ItemTemplateProperty); }
             set { SetValue(ItemTemplateProperty, value); }
@@ -140,13 +140,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the items presenter control.
         /// </summary>
-        public IItemsPresenter Presenter
+        public IItemsPresenter? Presenter
         {
             get;
             protected set;
         }
 
-        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
             remove => _childIndexChanged -= value;
@@ -161,7 +161,7 @@ namespace Avalonia.Controls
             }
 
             Presenter = presenter;
-            ItemContainerGenerator.Clear();
+            ItemContainerGenerator?.Clear();
 
             if (Presenter is IChildIndexProvider innerProvider)
             {
@@ -189,11 +189,11 @@ namespace Avalonia.Controls
         /// <param name="items">The collection.</param>
         /// <param name="index">The index.</param>
         /// <returns>The item at the given index or null if the index is out of bounds.</returns>
-        protected static object ElementAt(IEnumerable items, int index)
+        protected static object? ElementAt(IEnumerable? items, int index)
         {
             if (index != -1 && index < items.Count())
             {
-                return items.ElementAt(index) ?? null;
+                return items!.ElementAt(index) ?? null;
             }
             else
             {
@@ -207,7 +207,7 @@ namespace Avalonia.Controls
         /// <param name="items">The collection.</param>
         /// <param name="item">The item.</param>
         /// <returns>The index of the item or -1 if the item was not found.</returns>
-        protected static int IndexOf(IEnumerable items, object item)
+        protected static int IndexOf(IEnumerable? items, object item)
         {
             if (items != null && item != null)
             {
@@ -282,7 +282,7 @@ namespace Avalonia.Controls
             {
                 // If the item is its own container, then it will be removed from the logical tree
                 // when it is removed from the Items collection.
-                if (container?.ContainerControl != container?.Item)
+                if (container.ContainerControl != container.Item)
                 {
                     LogicalChildren.Remove(container.ContainerControl);
                 }
@@ -311,20 +311,20 @@ namespace Avalonia.Controls
                 var container = Presenter?.Panel as INavigableContainer;
 
                 if (container == null ||
-                    focus.Current == null ||
+                    focus?.Current == null ||
                     direction == null ||
                     direction.Value.IsTab())
                 {
                     return;
                 }
 
-                IVisual current = focus.Current;
+                IVisual? current = focus.Current;
 
                 while (current != null)
                 {
                     if (current.VisualParent == container && current is IInputElement inputElement)
                     {
-                        IInputElement next = GetNextControl(container, direction.Value, inputElement, false);
+                        var next = GetNextControl(container, direction.Value, inputElement, false);
 
                         if (next != null)
                         {
@@ -406,7 +406,7 @@ namespace Avalonia.Controls
         /// Given a collection of items, adds those that are controls to the logical children.
         /// </summary>
         /// <param name="items">The items.</param>
-        private void AddControlItemsToLogicalChildren(IEnumerable items)
+        private void AddControlItemsToLogicalChildren(IEnumerable? items)
         {
             var toAdd = new List<ILogical>();
 
@@ -430,7 +430,7 @@ namespace Avalonia.Controls
         /// Given a collection of items, removes those that are controls to from logical children.
         /// </summary>
         /// <param name="items">The items.</param>
-        private void RemoveControlItemsFromLogicalChildren(IEnumerable items)
+        private void RemoveControlItemsFromLogicalChildren(IEnumerable? items)
         {
             var toRemove = new List<ILogical>();
 
@@ -454,7 +454,7 @@ namespace Avalonia.Controls
         /// Subscribes to an <see cref="Items"/> collection.
         /// </summary>
         /// <param name="items">The items collection.</param>
-        private void SubscribeToItems(IEnumerable items)
+        private void SubscribeToItems(IEnumerable? items)
         {
             if (items is INotifyCollectionChanged incc)
             {
@@ -470,7 +470,7 @@ namespace Avalonia.Controls
         {
             if (_itemContainerGenerator != null)
             {
-                _itemContainerGenerator.ItemTemplate = (IDataTemplate)e.NewValue;
+                _itemContainerGenerator.ItemTemplate = (IDataTemplate?)e.NewValue;
                 // TODO: Rebuild the item containers.
             }
         }
@@ -497,13 +497,13 @@ namespace Avalonia.Controls
             PseudoClasses.Set(":singleitem", itemCount == 1);
         }
 
-        protected static IInputElement GetNextControl(
+        protected static IInputElement? GetNextControl(
             INavigableContainer container,
             NavigationDirection direction,
-            IInputElement from,
+            IInputElement? from,
             bool wrap)
         {
-            IInputElement result;
+            IInputElement? result;
             var c = from;
 
             do
@@ -525,7 +525,7 @@ namespace Avalonia.Controls
             return null;
         }
 
-        private void PresenterChildIndexChanged(object sender, ChildIndexChangedEventArgs e)
+        private void PresenterChildIndexChanged(object? sender, ChildIndexChangedEventArgs e)
         {
             _childIndexChanged?.Invoke(this, e);
         }
diff --git a/src/Avalonia.Controls/ItemsSourceView.cs b/src/Avalonia.Controls/ItemsSourceView.cs
index c2d20495ef..611f3424fd 100644
--- a/src/Avalonia.Controls/ItemsSourceView.cs
+++ b/src/Avalonia.Controls/ItemsSourceView.cs
@@ -7,7 +7,6 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Avalonia.Controls.Utils;
 
@@ -196,7 +195,7 @@ namespace Avalonia.Controls
             _collectionChanged?.Invoke(this, args);
         }
 
-        private void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             OnItemsSourceChanged(e);
         }
@@ -239,8 +238,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="index">The index.</param>
         /// <returns>The item.</returns>
-        [return: MaybeNull]
-        public new T GetAt(int index) => (T)Inner[index];
+        public new T GetAt(int index) => (T)Inner[index]!;
 
         public IEnumerator<T> GetEnumerator() => Inner.Cast<T>().GetEnumerator();
         IEnumerator IEnumerable.GetEnumerator() => Inner.GetEnumerator();
diff --git a/src/Avalonia.Controls/Label.cs b/src/Avalonia.Controls/Label.cs
index 9d5eda464f..5c8a6e0a5b 100644
--- a/src/Avalonia.Controls/Label.cs
+++ b/src/Avalonia.Controls/Label.cs
@@ -18,19 +18,19 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Target"/> Direct property
         /// </summary>
-        public static readonly DirectProperty<Label, IInputElement> TargetProperty =
-            AvaloniaProperty.RegisterDirect<Label, IInputElement>(nameof(Target), lbl => lbl.Target, (lbl, inp) => lbl.Target = inp);
+        public static readonly DirectProperty<Label, IInputElement?> TargetProperty =
+            AvaloniaProperty.RegisterDirect<Label, IInputElement?>(nameof(Target), lbl => lbl.Target, (lbl, inp) => lbl.Target = inp);
 
         /// <summary>
         /// Label focus target storage field
         /// </summary>
-        private IInputElement _target;
+        private IInputElement? _target;
 
         /// <summary>
         /// Label focus Target
         /// </summary>
         [ResolveByName]
-        public IInputElement Target
+        public IInputElement? Target
         {
             get => _target;
             set => SetAndRaise(TargetProperty, ref _target, value);
diff --git a/src/Avalonia.Controls/LayoutTransformControl.cs b/src/Avalonia.Controls/LayoutTransformControl.cs
index a8e15ee463..e896c13f25 100644
--- a/src/Avalonia.Controls/LayoutTransformControl.cs
+++ b/src/Avalonia.Controls/LayoutTransformControl.cs
@@ -14,8 +14,8 @@ namespace Avalonia.Controls
     /// </summary>
     public class LayoutTransformControl : Decorator
     {
-        public static readonly StyledProperty<ITransform> LayoutTransformProperty =
-            AvaloniaProperty.Register<LayoutTransformControl, ITransform>(nameof(LayoutTransform));
+        public static readonly StyledProperty<ITransform?> LayoutTransformProperty =
+            AvaloniaProperty.Register<LayoutTransformControl, ITransform?>(nameof(LayoutTransform));
 
         public static readonly StyledProperty<bool> UseRenderTransformProperty =
             AvaloniaProperty.Register<LayoutTransformControl, bool>(nameof(UseRenderTransform));
@@ -37,7 +37,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a graphics transformation that should apply to this element when layout is performed.
         /// </summary>
-        public ITransform LayoutTransform
+        public ITransform? LayoutTransform
         {
             get { return GetValue(LayoutTransformProperty); }
             set { SetValue(LayoutTransformProperty, value); }
@@ -52,7 +52,7 @@ namespace Avalonia.Controls
             set { SetValue(UseRenderTransformProperty, value); }
         }
 
-        public IControl TransformRoot => Child;
+        public IControl? TransformRoot => Child;
 
         /// <summary>
         /// Provides the behavior for the "Arrange" pass of layout.
@@ -146,7 +146,7 @@ namespace Avalonia.Controls
             return transformedDesiredSize;
         }
 
-        IDisposable _renderTransformChangedEvent;
+        IDisposable? _renderTransformChangedEvent;
 
         private void OnUseRenderTransformPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
@@ -158,7 +158,7 @@ namespace Avalonia.Controls
             //       workaround.
 
             var target = e.Sender as LayoutTransformControl;
-            var shouldUseRenderTransform = (bool)e.NewValue;
+            var shouldUseRenderTransform = (bool)e.NewValue!;
             if (target != null)
             {
                 if (shouldUseRenderTransform)
@@ -217,7 +217,7 @@ namespace Avalonia.Controls
         /// Transformation matrix corresponding to _matrixTransform.
         /// </summary>
         private Matrix _transformation;
-        private IDisposable _transformChangedEvent = null;
+        private IDisposable? _transformChangedEvent = null;
 
         /// <summary>
         /// Returns true if Size a is smaller than Size b in either dimension.
diff --git a/src/Avalonia.Controls/ListBox.cs b/src/Avalonia.Controls/ListBox.cs
index 43b4908482..9b7ae0d324 100644
--- a/src/Avalonia.Controls/ListBox.cs
+++ b/src/Avalonia.Controls/ListBox.cs
@@ -23,13 +23,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Scroll"/> property.
         /// </summary>
-        public static readonly DirectProperty<ListBox, IScrollable> ScrollProperty =
-            AvaloniaProperty.RegisterDirect<ListBox, IScrollable>(nameof(Scroll), o => o.Scroll);
+        public static readonly DirectProperty<ListBox, IScrollable?> ScrollProperty =
+            AvaloniaProperty.RegisterDirect<ListBox, IScrollable?>(nameof(Scroll), o => o.Scroll);
 
         /// <summary>
         /// Defines the <see cref="SelectedItems"/> property.
         /// </summary>
-        public static readonly new DirectProperty<SelectingItemsControl, IList> SelectedItemsProperty =
+        public static readonly new DirectProperty<SelectingItemsControl, IList?> SelectedItemsProperty =
             SelectingItemsControl.SelectedItemsProperty;
 
         /// <summary>
@@ -50,7 +50,7 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<ItemVirtualizationMode> VirtualizationModeProperty =
             ItemsPresenter.VirtualizationModeProperty.AddOwner<ListBox>();
 
-        private IScrollable _scroll;
+        private IScrollable? _scroll;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsControl"/> class.
@@ -64,14 +64,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the scroll information for the <see cref="ListBox"/>.
         /// </summary>
-        public IScrollable Scroll
+        public IScrollable? Scroll
         {
             get { return _scroll; }
             private set { SetAndRaise(ScrollProperty, ref _scroll, value); }
         }
 
         /// <inheritdoc/>
-        public new IList SelectedItems
+        public new IList? SelectedItems
         {
             get => base.SelectedItems;
             set => base.SelectedItems = value;
diff --git a/src/Avalonia.Controls/MaskedTextBox.cs b/src/Avalonia.Controls/MaskedTextBox.cs
index f59cef8223..866291f95b 100644
--- a/src/Avalonia.Controls/MaskedTextBox.cs
+++ b/src/Avalonia.Controls/MaskedTextBox.cs
@@ -241,7 +241,7 @@ namespace Avalonia.Controls
             switch (e.Key)
             {
                 case Key.Delete:
-                    if (CaretIndex < Text.Length)
+                    if (CaretIndex < Text?.Length)
                     {
                         if (MaskProvider.RemoveAt(CaretIndex))
                         {
@@ -286,7 +286,7 @@ namespace Avalonia.Controls
         {
             void UpdateMaskProvider()
             {
-                MaskProvider = new MaskedTextProvider(Mask, Culture, true, PromptChar, PasswordChar, AsciiOnly) { ResetOnSpace = ResetOnSpace, ResetOnPrompt = ResetOnPrompt };
+                MaskProvider = new MaskedTextProvider(Mask!, Culture, true, PromptChar, PasswordChar, AsciiOnly) { ResetOnSpace = ResetOnSpace, ResetOnPrompt = ResetOnPrompt };
                 if (Text != null)
                 {
                     MaskProvider.Set(Text);
@@ -383,11 +383,11 @@ namespace Avalonia.Controls
                     }
                 }
 
-                if (CaretIndex < Text.Length)
+                if (CaretIndex < Text?.Length)
                 {
                     CaretIndex = GetNextCharacterPosition(CaretIndex);
 
-                    if (MaskProvider.InsertAt(e.Text, CaretIndex))
+                    if (MaskProvider.InsertAt(e.Text!, CaretIndex))
                     {
                         CaretIndex++;
                     }
diff --git a/src/Avalonia.Controls/MenuBase.cs b/src/Avalonia.Controls/MenuBase.cs
index 0434928280..0c89a017e5 100644
--- a/src/Avalonia.Controls/MenuBase.cs
+++ b/src/Avalonia.Controls/MenuBase.cs
@@ -83,12 +83,13 @@ namespace Avalonia.Controls
             {
                 var index = SelectedIndex;
                 return (index != -1) ?
-                    (IMenuItem)ItemContainerGenerator.ContainerFromIndex(index) :
+                    (IMenuItem?)ItemContainerGenerator!.ContainerFromIndex(index) :
                     null;
             }
             set
             {
-                SelectedIndex = ItemContainerGenerator.IndexFromContainer(value);
+                SelectedIndex = value is not null ?
+                    ItemContainerGenerator!.IndexFromContainer(value) : -1;
             }
         }
 
@@ -97,7 +98,7 @@ namespace Avalonia.Controls
         {
             get
             {
-                return ItemContainerGenerator.Containers
+                return ItemContainerGenerator!.Containers
                     .Select(x => x.ContainerControl)
                     .OfType<IMenuItem>();
             }
@@ -111,7 +112,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when a <see cref="Menu"/> is opened.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> MenuOpened
+        public event EventHandler<RoutedEventArgs>? MenuOpened
         {
             add { AddHandler(MenuOpenedEvent, value); }
             remove { RemoveHandler(MenuOpenedEvent, value); }
@@ -120,7 +121,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when a <see cref="Menu"/> is closed.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> MenuClosed
+        public event EventHandler<RoutedEventArgs>? MenuClosed
         {
             add { AddHandler(MenuClosedEvent, value); }
             remove { RemoveHandler(MenuClosedEvent, value); }
diff --git a/src/Avalonia.Controls/MenuItem.cs b/src/Avalonia.Controls/MenuItem.cs
index b843992f04..79b3e8e7f3 100644
--- a/src/Avalonia.Controls/MenuItem.cs
+++ b/src/Avalonia.Controls/MenuItem.cs
@@ -42,20 +42,20 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="CommandParameter"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> CommandParameterProperty =
+        public static readonly StyledProperty<object?> CommandParameterProperty =
             Button.CommandParameterProperty.AddOwner<MenuItem>();
 
         /// <summary>
         /// Defines the <see cref="Icon"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> IconProperty =
-            AvaloniaProperty.Register<MenuItem, object>(nameof(Icon));
+        public static readonly StyledProperty<object?> IconProperty =
+            AvaloniaProperty.Register<MenuItem, object?>(nameof(Icon));
 
         /// <summary>
         /// Defines the <see cref="InputGesture"/> property.
         /// </summary>
-        public static readonly StyledProperty<KeyGesture> InputGestureProperty =
-            AvaloniaProperty.Register<MenuItem, KeyGesture>(nameof(InputGesture));
+        public static readonly StyledProperty<KeyGesture?> InputGestureProperty =
+            AvaloniaProperty.Register<MenuItem, KeyGesture?>(nameof(InputGesture));
 
         /// <summary>
         /// Defines the <see cref="IsSelected"/> property.
@@ -164,7 +164,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when a <see cref="MenuItem"/> without a submenu is clicked.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Click
+        public event EventHandler<RoutedEventArgs>? Click
         {
             add { AddHandler(ClickEvent, value); }
             remove { RemoveHandler(ClickEvent, value); }
@@ -176,7 +176,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// A bubbling version of the <see cref="InputElement.PointerEnter"/> event for menu items.
         /// </remarks>
-        public event EventHandler<PointerEventArgs> PointerEnterItem
+        public event EventHandler<PointerEventArgs>? PointerEnterItem
         {
             add { AddHandler(PointerEnterItemEvent, value); }
             remove { RemoveHandler(PointerEnterItemEvent, value); }
@@ -188,7 +188,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// A bubbling version of the <see cref="InputElement.PointerLeave"/> event for menu items.
         /// </remarks>
-        public event EventHandler<PointerEventArgs> PointerLeaveItem
+        public event EventHandler<PointerEventArgs>? PointerLeaveItem
         {
             add { AddHandler(PointerLeaveItemEvent, value); }
             remove { RemoveHandler(PointerLeaveItemEvent, value); }
@@ -197,7 +197,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when a <see cref="MenuItem"/>'s submenu is opened.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> SubmenuOpened
+        public event EventHandler<RoutedEventArgs>? SubmenuOpened
         {
             add { AddHandler(SubmenuOpenedEvent, value); }
             remove { RemoveHandler(SubmenuOpenedEvent, value); }
@@ -225,7 +225,7 @@ namespace Avalonia.Controls
         /// Gets or sets the parameter to pass to the <see cref="Command"/> property of a
         /// <see cref="MenuItem"/>.
         /// </summary>
-        public object CommandParameter
+        public object? CommandParameter
         {
             get { return GetValue(CommandParameterProperty); }
             set { SetValue(CommandParameterProperty, value); }
@@ -234,7 +234,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the icon that appears in a <see cref="MenuItem"/>.
         /// </summary>
-        public object Icon
+        public object? Icon
         {
             get { return GetValue(IconProperty); }
             set { SetValue(IconProperty, value); }
@@ -247,7 +247,7 @@ namespace Avalonia.Controls
         /// Setting this property does not cause the input gesture to be handled by the menu item,
         /// it simply displays the gesture text in the menu.
         /// </remarks>
-        public KeyGesture InputGesture
+        public KeyGesture? InputGesture
         {
             get { return GetValue(InputGestureProperty); }
             set { SetValue(InputGestureProperty, value); }
@@ -310,12 +310,12 @@ namespace Avalonia.Controls
             {
                 var index = SelectedIndex;
                 return (index != -1) ?
-                    (IMenuItem)ItemContainerGenerator.ContainerFromIndex(index) :
+                    (IMenuItem?)ItemContainerGenerator!.ContainerFromIndex(index) :
                     null;
             }
             set
             {
-                SelectedIndex = ItemContainerGenerator.IndexFromContainer(value);
+                SelectedIndex = value is not null ? ItemContainerGenerator!.IndexFromContainer(value) : -1;
             }
         }
 
@@ -324,7 +324,7 @@ namespace Avalonia.Controls
         {
             get
             {
-                return ItemContainerGenerator.Containers
+                return ItemContainerGenerator!.Containers
                     .Select(x => x.ContainerControl)
                     .OfType<IMenuItem>();
             }
@@ -390,7 +390,7 @@ namespace Avalonia.Controls
                 parent = parent.Parent;
             }
 
-            _isEmbeddedInMenu = parent.FindLogicalAncestorOfType<IMenu>(true) != null;
+            _isEmbeddedInMenu = parent?.FindLogicalAncestorOfType<IMenu>(true) != null;
         }
 
         protected override void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)
@@ -561,7 +561,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void CanExecuteChanged(object sender, EventArgs e)
+        private void CanExecuteChanged(object? sender, EventArgs e)
         {
             TryUpdateCanExecute();
         }
@@ -655,7 +655,7 @@ namespace Avalonia.Controls
 
             if (value)
             {
-                foreach (var item in Items.OfType<MenuItem>())
+                foreach (var item in Items!.OfType<MenuItem>())
                 {
                     item.TryUpdateCanExecute();
                 }
@@ -677,13 +677,13 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void PopupOpened(object sender, EventArgs e)
+        private void PopupOpened(object? sender, EventArgs e)
         {
             var selected = SelectedIndex;
 
             if (selected != -1)
             {
-                var container = ItemContainerGenerator.ContainerFromIndex(selected);
+                var container = ItemContainerGenerator?.ContainerFromIndex(selected);
                 container?.Focus();
             }
         }
@@ -693,7 +693,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void PopupClosed(object sender, EventArgs e)
+        private void PopupClosed(object? sender, EventArgs e)
         {
             SelectedItem = null;
         }
diff --git a/src/Avalonia.Controls/MenuItemAccessKeyHandler.cs b/src/Avalonia.Controls/MenuItemAccessKeyHandler.cs
index 2c3dac8482..4cc6a20082 100644
--- a/src/Avalonia.Controls/MenuItemAccessKeyHandler.cs
+++ b/src/Avalonia.Controls/MenuItemAccessKeyHandler.cs
@@ -19,7 +19,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// The window to which the handler belongs.
         /// </summary>
-        private IInputRoot _owner;
+        private IInputRoot? _owner;
 
         /// <summary>
         /// Gets or sets the window's main menu.
@@ -27,7 +27,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// This property is ignored as a menu item cannot have a main menu.
         /// </remarks>
-        public IMainMenu MainMenu { get; set; }
+        public IMainMenu? MainMenu { get; set; }
 
         /// <summary>
         /// Sets the owner of the access key handler.
@@ -38,7 +38,7 @@ namespace Avalonia.Controls
         /// </remarks>
         public void SetOwner(IInputRoot owner)
         {
-            Contract.Requires<ArgumentNullException>(owner != null);
+            _ = owner ?? throw new ArgumentNullException(nameof(owner));
 
             if (_owner != null)
             {
@@ -84,7 +84,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void OnTextInput(object sender, TextInputEventArgs e)
+        protected virtual void OnTextInput(object? sender, TextInputEventArgs e)
         {
             if (!string.IsNullOrWhiteSpace(e.Text))
             {
diff --git a/src/Avalonia.Controls/Mixins/SelectableMixin.cs b/src/Avalonia.Controls/Mixins/SelectableMixin.cs
index c9e2b684cb..fa964c0056 100644
--- a/src/Avalonia.Controls/Mixins/SelectableMixin.cs
+++ b/src/Avalonia.Controls/Mixins/SelectableMixin.cs
@@ -40,7 +40,7 @@ namespace Avalonia.Controls.Mixins
         public static void Attach<TControl>(AvaloniaProperty<bool> isSelected)
             where TControl : class, IControl
         {
-            Contract.Requires<ArgumentNullException>(isSelected != null);
+            _ = isSelected ?? throw new ArgumentNullException(nameof(isSelected));
 
             isSelected.Changed.Subscribe(x =>
             {
diff --git a/src/Avalonia.Controls/NativeControlHost.cs b/src/Avalonia.Controls/NativeControlHost.cs
index e18b7a5ba2..191b60f303 100644
--- a/src/Avalonia.Controls/NativeControlHost.cs
+++ b/src/Avalonia.Controls/NativeControlHost.cs
@@ -1,5 +1,6 @@
 using System;
 using System.Collections.Generic;
+using System.Diagnostics;
 using Avalonia.Controls.Platform;
 using Avalonia.Platform;
 using Avalonia.Threading;
@@ -9,10 +10,10 @@ namespace Avalonia.Controls
 {
     public class NativeControlHost : Control
     {
-        private TopLevel _currentRoot;
-        private INativeControlHostImpl _currentHost;
-        private INativeControlHostControlTopLevelAttachment _attachment;
-        private IPlatformHandle _nativeControlHandle;
+        private TopLevel? _currentRoot;
+        private INativeControlHostImpl? _currentHost;
+        private INativeControlHostControlTopLevelAttachment? _attachment;
+        private IPlatformHandle? _nativeControlHandle;
         private bool _queuedForDestruction;
         private bool _queuedForMoveResize;
         private readonly List<Visual> _propertyChangedSubscriptions = new List<Visual>();
@@ -35,7 +36,7 @@ namespace Avalonia.Controls
             UpdateHost();
         }
 
-        private void PropertyChangedHandler(object sender, AvaloniaPropertyChangedEventArgs e)
+        private void PropertyChangedHandler(object? sender, AvaloniaPropertyChangedEventArgs e)
         {
             if (e.IsEffectiveValueChange && (e.Property == BoundsProperty || e.Property == IsVisibleProperty))
                 EnqueueForMoveResize();
@@ -58,9 +59,8 @@ namespace Avalonia.Controls
         {
             _queuedForMoveResize = false;
             _currentHost = (_currentRoot?.PlatformImpl as ITopLevelImplWithNativeControlHost)?.NativeControlHost;
-            var needsAttachment = _currentHost != null;
             
-            if (needsAttachment)
+            if (_currentHost != null)
             {
                 // If there is an existing attachment, ensure that we are attached to the proper host or destroy the attachment
                 if (_attachment != null && _attachment.AttachedTo != _currentHost)
@@ -119,6 +119,8 @@ namespace Avalonia.Controls
         
         private Rect? GetAbsoluteBounds()
         {
+            Debug.Assert(_currentRoot is not null);
+
             var bounds = Bounds;
             var position = this.TranslatePoint(default, _currentRoot);
             if (position == null)
@@ -140,9 +142,8 @@ namespace Avalonia.Controls
                 return false;
             
             var bounds = GetAbsoluteBounds();
-            var needsShow = IsEffectivelyVisible && bounds.HasValue;
 
-            if (needsShow)
+            if (IsEffectivelyVisible && bounds.HasValue)
             {
                 if (bounds.Value.IsEmpty)
                     return false;
diff --git a/src/Avalonia.Controls/NativeMenu.Export.cs b/src/Avalonia.Controls/NativeMenu.Export.cs
index 6bfe5ebc82..fe650ab41e 100644
--- a/src/Avalonia.Controls/NativeMenu.Export.cs
+++ b/src/Avalonia.Controls/NativeMenu.Export.cs
@@ -18,7 +18,7 @@ namespace Avalonia.Controls
         class NativeMenuInfo
         {
             public bool ChangingIsExported { get; set; }
-            public ITopLevelNativeMenuExporter Exporter { get; }
+            public ITopLevelNativeMenuExporter? Exporter { get; }
 
             public NativeMenuInfo(TopLevel target)
             {
diff --git a/src/Avalonia.Controls/NativeMenu.cs b/src/Avalonia.Controls/NativeMenu.cs
index 58ee99722f..5ff4148e5a 100644
--- a/src/Avalonia.Controls/NativeMenu.cs
+++ b/src/Avalonia.Controls/NativeMenu.cs
@@ -11,7 +11,7 @@ namespace Avalonia.Controls
     {
         private readonly AvaloniaList<NativeMenuItemBase> _items =
             new AvaloniaList<NativeMenuItemBase> { ResetBehavior = ResetBehavior.Remove };
-        private NativeMenuItem _parent;
+        private NativeMenuItem? _parent;
 
         [Content]
         public IList<NativeMenuItemBase> Items => _items;
@@ -23,7 +23,7 @@ namespace Avalonia.Controls
         /// Use this event to add, remove or modify menu items before a menu is
         /// shown or a hotkey is pressed.
         /// </remarks>
-        public event EventHandler<EventArgs> NeedsUpdate;
+        public event EventHandler<EventArgs>? NeedsUpdate;
 
         /// <summary>
         /// Raised before the menu is opened.
@@ -31,7 +31,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// Do not update the menu in this event; use <see cref="NeedsUpdate"/>.
         /// </remarks>
-        public event EventHandler<EventArgs> Opening;
+        public event EventHandler<EventArgs>? Opening;
         
         /// <summary>
         /// Raised after the menu is closed.
@@ -39,7 +39,7 @@ namespace Avalonia.Controls
         /// <remarks>
         /// Do not update the menu in this event; use <see cref="NeedsUpdate"/>.
         /// </remarks>
-        public event EventHandler<EventArgs> Closed;
+        public event EventHandler<EventArgs>? Closed;
 
         public NativeMenu()
         {
@@ -68,7 +68,7 @@ namespace Avalonia.Controls
                 throw new InvalidOperationException("NativeMenuItem already has a parent");
         }
 
-        private void ItemsChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void ItemsChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (e.OldItems != null)
                 foreach (NativeMenuItemBase i in e.OldItems)
@@ -78,10 +78,10 @@ namespace Avalonia.Controls
                     i.Parent = this;
         }
 
-        public static readonly DirectProperty<NativeMenu, NativeMenuItem> ParentProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenu, NativeMenuItem>("Parent", o => o.Parent, (o, v) => o.Parent = v);
+        public static readonly DirectProperty<NativeMenu, NativeMenuItem?> ParentProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenu, NativeMenuItem?>("Parent", o => o.Parent, (o, v) => o.Parent = v);
 
-        public NativeMenuItem Parent
+        public NativeMenuItem? Parent
         {
             get => _parent;
             set => SetAndRaise(ParentProperty, ref _parent, value);
diff --git a/src/Avalonia.Controls/NativeMenuBar.cs b/src/Avalonia.Controls/NativeMenuBar.cs
index 5d2e1087a7..ae473f8500 100644
--- a/src/Avalonia.Controls/NativeMenuBar.cs
+++ b/src/Avalonia.Controls/NativeMenuBar.cs
@@ -27,9 +27,9 @@ namespace Avalonia.Controls
             menuItem.SetValue(EnableMenuItemClickForwardingProperty, enable);
         }
 
-        private static void OnMenuItemClick(object sender, RoutedEventArgs e)
+        private static void OnMenuItemClick(object? sender, RoutedEventArgs e)
         {
-            (((MenuItem)sender).DataContext as INativeMenuItemExporterEventsImplBridge)?.RaiseClicked();
+            (((MenuItem)sender!).DataContext as INativeMenuItemExporterEventsImplBridge)?.RaiseClicked();
         }
     }
 }
diff --git a/src/Avalonia.Controls/NativeMenuItem.cs b/src/Avalonia.Controls/NativeMenuItem.cs
index 4d048f0fb0..265e7119eb 100644
--- a/src/Avalonia.Controls/NativeMenuItem.cs
+++ b/src/Avalonia.Controls/NativeMenuItem.cs
@@ -9,23 +9,23 @@ namespace Avalonia.Controls
 {
     public class NativeMenuItem : NativeMenuItemBase, INativeMenuItemExporterEventsImplBridge
     {
-        private string _header;
-        private KeyGesture _gesture;
+        private string? _header;
+        private KeyGesture? _gesture;
         private bool _isEnabled = true;
-        private ICommand _command;
+        private ICommand? _command;
         private bool _isChecked = false;
         private NativeMenuItemToggleType _toggleType;
-        private IBitmap _icon;
+        private IBitmap? _icon;
         private readonly CanExecuteChangedSubscriber _canExecuteChangedSubscriber;
 
-        private NativeMenu _menu;
+        private NativeMenu? _menu;
 
         static NativeMenuItem()
         {
             MenuProperty.Changed.Subscribe(args =>
             {
                 var item = (NativeMenuItem)args.Sender;
-                var value = args.NewValue.GetValueOrDefault();
+                var value = args.NewValue.GetValueOrDefault()!;
                 if (value.Parent != null && value.Parent != item)
                     throw new InvalidOperationException("NativeMenu already has a parent");
                 value.Parent = item;
@@ -59,44 +59,44 @@ namespace Avalonia.Controls
             Header = header;
         }
 
-        public static readonly DirectProperty<NativeMenuItem, NativeMenu> MenuProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenuItem, NativeMenu>(nameof(Menu), o => o.Menu, (o, v) => o.Menu = v);
+        public static readonly DirectProperty<NativeMenuItem, NativeMenu?> MenuProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenuItem, NativeMenu?>(nameof(Menu), o => o.Menu, (o, v) => o.Menu = v);
 
         [Content]
-        public NativeMenu Menu
+        public NativeMenu? Menu
         {
             get => _menu;
             set
             {
-                if (value.Parent != null && value.Parent != this)
+                if (value != null && value.Parent != null && value.Parent != this)
                     throw new InvalidOperationException("NativeMenu already has a parent");
                 SetAndRaise(MenuProperty, ref _menu, value);
             }
         }
 
-        public static readonly DirectProperty<NativeMenuItem, IBitmap> IconProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenuItem, IBitmap>(nameof(Icon), o => o.Icon, (o, v) => o.Icon = v);
+        public static readonly DirectProperty<NativeMenuItem, IBitmap?> IconProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenuItem, IBitmap?>(nameof(Icon), o => o.Icon, (o, v) => o.Icon = v);
 
 
-        public IBitmap Icon
+        public IBitmap? Icon
         {
             get => _icon;
             set => SetAndRaise(IconProperty, ref _icon, value);
         }  
 
-        public static readonly DirectProperty<NativeMenuItem, string> HeaderProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenuItem, string>(nameof(Header), o => o.Header, (o, v) => o.Header = v);
+        public static readonly DirectProperty<NativeMenuItem, string?> HeaderProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenuItem, string?>(nameof(Header), o => o.Header, (o, v) => o.Header = v);
 
-        public string Header
+        public string? Header
         {
             get => _header;
             set => SetAndRaise(HeaderProperty, ref _header, value);
         }
 
-        public static readonly DirectProperty<NativeMenuItem, KeyGesture> GestureProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenuItem, KeyGesture>(nameof(Gesture), o => o.Gesture, (o, v) => o.Gesture = v);
+        public static readonly DirectProperty<NativeMenuItem, KeyGesture?> GestureProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenuItem, KeyGesture?>(nameof(Gesture), o => o.Gesture, (o, v) => o.Gesture = v);
 
-        public KeyGesture Gesture
+        public KeyGesture? Gesture
         {
             get => _gesture;
             set => SetAndRaise(GestureProperty, ref _gesture, value);
@@ -126,7 +126,7 @@ namespace Avalonia.Controls
             set => SetAndRaise(ToggleTypeProperty, ref _toggleType, value);
         }
 
-        public static readonly DirectProperty<NativeMenuItem, ICommand> CommandProperty =
+        public static readonly DirectProperty<NativeMenuItem, ICommand?> CommandProperty =
             Button.CommandProperty.AddOwner<NativeMenuItem>(
                 menuItem => menuItem.Command,
                 (menuItem, command) => menuItem.Command = command,
@@ -135,7 +135,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="CommandParameter"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> CommandParameterProperty =
+        public static readonly StyledProperty<object?> CommandParameterProperty =
             Button.CommandParameterProperty.AddOwner<MenuItem>();
 
         public static readonly DirectProperty<NativeMenuItem, bool> IsEnabledProperty =
@@ -154,7 +154,7 @@ namespace Avalonia.Controls
 
         public bool HasClickHandlers => Click != null;
 
-        public ICommand Command
+        public ICommand? Command
         {
             get => _command;
             set
@@ -175,7 +175,7 @@ namespace Avalonia.Controls
         /// Gets or sets the parameter to pass to the <see cref="Command"/> property of a
         /// <see cref="NativeMenuItem"/>.
         /// </summary>
-        public object CommandParameter
+        public object? CommandParameter
         {
             get { return GetValue(CommandParameterProperty); }
             set { SetValue(CommandParameterProperty, value); }
@@ -184,7 +184,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when a <see cref="NativeMenuItem"/> is clicked.
         /// </summary>
-        public event EventHandler Click;
+        public event EventHandler? Click;
 
         [Obsolete("Use Click event.")]
         public event EventHandler Clicked
diff --git a/src/Avalonia.Controls/NativeMenuItemBase.cs b/src/Avalonia.Controls/NativeMenuItemBase.cs
index 47eb86cdc3..a5e3308e3b 100644
--- a/src/Avalonia.Controls/NativeMenuItemBase.cs
+++ b/src/Avalonia.Controls/NativeMenuItemBase.cs
@@ -4,17 +4,17 @@ namespace Avalonia.Controls
 {
     public class NativeMenuItemBase : AvaloniaObject
     {
-        private NativeMenu _parent;
+        private NativeMenu? _parent;
 
         internal NativeMenuItemBase()
         {
 
         }
 
-        public static readonly DirectProperty<NativeMenuItem, NativeMenu> ParentProperty =
-            AvaloniaProperty.RegisterDirect<NativeMenuItem, NativeMenu>("Parent", o => o.Parent, (o, v) => o.Parent = v);
+        public static readonly DirectProperty<NativeMenuItem, NativeMenu?> ParentProperty =
+            AvaloniaProperty.RegisterDirect<NativeMenuItem, NativeMenu?>("Parent", o => o.Parent, (o, v) => o.Parent = v);
 
-        public NativeMenu Parent
+        public NativeMenu? Parent
         {
             get => _parent;
             set => SetAndRaise(ParentProperty, ref _parent, value);
diff --git a/src/Avalonia.Controls/Notifications/INotification.cs b/src/Avalonia.Controls/Notifications/INotification.cs
index dc69a12887..fa08233097 100644
--- a/src/Avalonia.Controls/Notifications/INotification.cs
+++ b/src/Avalonia.Controls/Notifications/INotification.cs
@@ -10,12 +10,12 @@ namespace Avalonia.Controls.Notifications
         /// <summary>
         /// Gets the Title of the notification.
         /// </summary>
-        string Title { get; }
+        string? Title { get; }
 
         /// <summary>
         /// Gets the notification message.
         /// </summary>
-        string Message { get; }
+        string? Message { get; }
 
         /// <summary>
         /// Gets the <see cref="NotificationType"/> of the notification.
@@ -31,11 +31,11 @@ namespace Avalonia.Controls.Notifications
         /// <summary>
         /// Gets an Action to be run when the notification is clicked.
         /// </summary>
-        Action OnClick { get; }
+        Action? OnClick { get; }
 
         /// <summary>
         /// Gets an Action to be run when the notification is closed.
         /// </summary>
-        Action OnClose { get; }
+        Action? OnClose { get; }
     }
 }
diff --git a/src/Avalonia.Controls/Notifications/Notification.cs b/src/Avalonia.Controls/Notifications/Notification.cs
index 9bb514971b..376df175f3 100644
--- a/src/Avalonia.Controls/Notifications/Notification.cs
+++ b/src/Avalonia.Controls/Notifications/Notification.cs
@@ -21,12 +21,12 @@ namespace Avalonia.Controls.Notifications
         /// Use <see cref="TimeSpan.Zero"/> for notifications that will remain open.</param>
         /// <param name="onClick">An Action to call when the notification is clicked.</param>
         /// <param name="onClose">An Action to call when the notification is closed.</param>
-        public Notification(string title,
-            string message,
+        public Notification(string? title,
+            string? message,
             NotificationType type = NotificationType.Information,
             TimeSpan? expiration = null,
-            Action onClick = null,
-            Action onClose = null)
+            Action? onClick = null,
+            Action? onClose = null)
         {
             Title = title;
             Message = message;
@@ -37,10 +37,10 @@ namespace Avalonia.Controls.Notifications
         }
 
         /// <inheritdoc/>
-        public string Title { get; private set; }
+        public string? Title { get; private set; }
 
         /// <inheritdoc/>
-        public string Message { get; private set; }
+        public string? Message { get; private set; }
 
         /// <inheritdoc/>
         public NotificationType Type { get; private set; }
@@ -49,9 +49,9 @@ namespace Avalonia.Controls.Notifications
         public TimeSpan Expiration { get; private set; }
 
         /// <inheritdoc/>
-        public Action OnClick { get; private set; }
+        public Action? OnClick { get; private set; }
 
         /// <inheritdoc/>
-        public Action OnClose { get; private set; }
+        public Action? OnClose { get; private set; }
     }
 }
diff --git a/src/Avalonia.Controls/Notifications/NotificationCard.cs b/src/Avalonia.Controls/Notifications/NotificationCard.cs
index ad883c8076..a103adf185 100644
--- a/src/Avalonia.Controls/Notifications/NotificationCard.cs
+++ b/src/Avalonia.Controls/Notifications/NotificationCard.cs
@@ -37,8 +37,11 @@ namespace Avalonia.Controls.Notifications
                     RaiseEvent(new RoutedEventArgs(NotificationClosedEvent));
                 });
 
+            // Disabling nullable checking because of https://github.com/dotnet/reactive/issues/1525
+#pragma warning disable CS8620
             this.GetObservable(ContentProperty)
                 .OfType<Notification>()
+#pragma warning restore CS8620
                 .Subscribe(x =>
                 {
                     switch (x.Type)
@@ -102,7 +105,7 @@ namespace Avalonia.Controls.Notifications
         /// <summary>
         /// Raised when the <see cref="NotificationCard"/> has closed.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> NotificationClosed
+        public event EventHandler<RoutedEventArgs>? NotificationClosed
         {
             add { AddHandler(NotificationClosedEvent, value); }
             remove { RemoveHandler(NotificationClosedEvent, value); }
@@ -110,13 +113,13 @@ namespace Avalonia.Controls.Notifications
 
         public static bool GetCloseOnClick(Button obj)
         {
-            Contract.Requires<ArgumentNullException>(obj != null);
+            _ = obj ?? throw new ArgumentNullException(nameof(obj));
             return (bool)obj.GetValue(CloseOnClickProperty);
         }
 
         public static void SetCloseOnClick(Button obj, bool value)
         {
-            Contract.Requires<ArgumentNullException>(obj != null);
+            _ = obj ?? throw new ArgumentNullException(nameof(obj));
             obj.SetValue(CloseOnClickProperty, value);
         }
 
@@ -129,7 +132,7 @@ namespace Avalonia.Controls.Notifications
         private static void OnCloseOnClickPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
             var button = (Button)d;
-            var value = (bool)e.NewValue;
+            var value = (bool)e.NewValue!;
             if (value)
             {
                 button.Click += Button_Click;
@@ -143,10 +146,10 @@ namespace Avalonia.Controls.Notifications
         /// <summary>
         /// Called when a button inside the Notification is clicked.
         /// </summary>
-        private static void Button_Click(object sender, RoutedEventArgs e)
+        private static void Button_Click(object? sender, RoutedEventArgs e)
         {
             var btn = sender as ILogical;
-            var notification = btn.GetLogicalAncestors().OfType<NotificationCard>().FirstOrDefault();
+            var notification = btn?.GetLogicalAncestors().OfType<NotificationCard>().FirstOrDefault();
             notification?.Close();
         }
 
diff --git a/src/Avalonia.Controls/Notifications/WindowNotificationManager.cs b/src/Avalonia.Controls/Notifications/WindowNotificationManager.cs
index 8f5c6faf40..9499995da3 100644
--- a/src/Avalonia.Controls/Notifications/WindowNotificationManager.cs
+++ b/src/Avalonia.Controls/Notifications/WindowNotificationManager.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Controls.Notifications
     [PseudoClasses(":topleft", ":topright", ":bottomleft", ":bottomright")]
     public class WindowNotificationManager : TemplatedControl, IManagedNotificationManager, ICustomSimpleHitTest
     {
-        private IList _items;
+        private IList? _items;
 
         /// <summary>
         /// Defines the <see cref="Position"/> property.
@@ -107,7 +107,7 @@ namespace Avalonia.Controls.Notifications
                 {
                     notification.OnClose?.Invoke();
 
-                    _items.Remove(sender);
+                    _items?.Remove(sender);
                 };
             }
 
@@ -121,9 +121,9 @@ namespace Avalonia.Controls.Notifications
                 (sender as NotificationCard)?.Close();
             };
 
-            _items.Add(notificationControl);
+            _items?.Add(notificationControl);
 
-            if (_items.OfType<NotificationCard>().Count(i => !i.IsClosing) > MaxItems)
+            if (_items?.OfType<NotificationCard>().Count(i => !i.IsClosing) > MaxItems)
             {
                 _items.OfType<NotificationCard>().First(i => !i.IsClosing).Close();
             }
diff --git a/src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs b/src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs
index eba381e5fa..f67377b310 100644
--- a/src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs
+++ b/src/Avalonia.Controls/NumericUpDown/NumericUpDown.cs
@@ -46,15 +46,15 @@ namespace Avalonia.Controls
         /// Defines the <see cref="CultureInfo"/> property.
         /// </summary>
         [Obsolete]
-        public static readonly DirectProperty<NumericUpDown, CultureInfo> CultureInfoProperty =
-            AvaloniaProperty.RegisterDirect<NumericUpDown, CultureInfo>(nameof(CultureInfo), o => o.CultureInfo,
+        public static readonly DirectProperty<NumericUpDown, CultureInfo?> CultureInfoProperty =
+            AvaloniaProperty.RegisterDirect<NumericUpDown, CultureInfo?>(nameof(CultureInfo), o => o.CultureInfo,
                 (o, v) => o.CultureInfo = v, CultureInfo.CurrentCulture);
 
         /// <summary>
         /// Defines the <see cref="NumberFormat"/> property.
         /// </summary>
-        public static readonly DirectProperty<NumericUpDown, NumberFormatInfo> NumberFormatProperty =
-            AvaloniaProperty.RegisterDirect<NumericUpDown, NumberFormatInfo>(nameof(NumberFormat), o => o.NumberFormat,
+        public static readonly DirectProperty<NumericUpDown, NumberFormatInfo?> NumberFormatProperty =
+            AvaloniaProperty.RegisterDirect<NumericUpDown, NumberFormatInfo?>(nameof(NumberFormat), o => o.NumberFormat,
                 (o, v) => o.NumberFormat = v, NumberFormatInfo.CurrentInfo);
 
         /// <summary>
@@ -97,8 +97,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Text"/> property.
         /// </summary>
-        public static readonly DirectProperty<NumericUpDown, string> TextProperty =
-            AvaloniaProperty.RegisterDirect<NumericUpDown, string>(nameof(Text), o => o.Text, (o, v) => o.Text = v,
+        public static readonly DirectProperty<NumericUpDown, string?> TextProperty =
+            AvaloniaProperty.RegisterDirect<NumericUpDown, string?>(nameof(Text), o => o.Text, (o, v) => o.Text = v,
                 defaultBindingMode: BindingMode.TwoWay, enableDataValidation: true);
 
         /// <summary>
@@ -111,8 +111,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Watermark"/> property.
         /// </summary>
-        public static readonly StyledProperty<string> WatermarkProperty =
-            AvaloniaProperty.Register<NumericUpDown, string>(nameof(Watermark));
+        public static readonly StyledProperty<string?> WatermarkProperty =
+            AvaloniaProperty.Register<NumericUpDown, string?>(nameof(Watermark));
 
         /// <summary>
         /// Defines the <see cref="HorizontalContentAlignment"/> property.
@@ -126,27 +126,27 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<VerticalAlignment> VerticalContentAlignmentProperty =
             ContentControl.VerticalContentAlignmentProperty.AddOwner<NumericUpDown>();
 
-        private IDisposable _textBoxTextChangedSubscription;
+        private IDisposable? _textBoxTextChangedSubscription;
 
         private decimal _value;
-        private string _text;
+        private string? _text;
         private bool _internalValueSet;
         private bool _clipValueToMinMax;
         private bool _isSyncingTextAndValueProperties;
         private bool _isTextChangedFromUI;
-        private CultureInfo _cultureInfo;
+        private CultureInfo? _cultureInfo;
         private NumberStyles _parsingNumberStyle = NumberStyles.Any;
-        private NumberFormatInfo _numberFormat;
+        private NumberFormatInfo? _numberFormat;
 
         /// <summary>
         /// Gets the Spinner template part.
         /// </summary>
-        private Spinner Spinner { get; set; }
+        private Spinner? Spinner { get; set; }
 
         /// <summary>
         /// Gets the TextBox template part.
         /// </summary>
-        private TextBox TextBox { get; set; }
+        private TextBox? TextBox { get; set; }
 
         /// <summary>
         /// Gets or sets the ability to perform increment/decrement operations via the keyboard, button spinners, or mouse wheel.
@@ -188,7 +188,7 @@ namespace Avalonia.Controls
         /// Gets or sets the current CultureInfo.
         /// </summary>
         [Obsolete("CultureInfo is obsolete, please use NumberFormat instead.")]
-        public CultureInfo CultureInfo
+        public CultureInfo? CultureInfo
         {
             get { return _cultureInfo; }
             set
@@ -202,7 +202,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the current NumberFormatInfo
         /// </summary>
-        public NumberFormatInfo NumberFormat
+        public NumberFormatInfo? NumberFormat
         {
             get { return _numberFormat; }
             set { SetAndRaise(NumberFormatProperty, ref _numberFormat, value); }
@@ -265,7 +265,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the formatted string representation of the value.
         /// </summary>
-        public string Text
+        public string? Text
         {
             get { return _text; }
             set { SetAndRaise(TextProperty, ref _text, value); }
@@ -287,7 +287,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the object to use as a watermark if the <see cref="Value"/> is null.
         /// </summary>
-        public string Watermark
+        public string? Watermark
         {
             get { return GetValue(WatermarkProperty); }
             set { SetValue(WatermarkProperty, value); }
@@ -414,7 +414,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="oldValue">The old value.</param>
         /// <param name="newValue">The new value.</param>
-        protected virtual void OnCultureInfoChanged(CultureInfo oldValue, CultureInfo newValue)
+        protected virtual void OnCultureInfoChanged(CultureInfo? oldValue, CultureInfo? newValue)
         {
             if (IsInitialized)
             {
@@ -427,7 +427,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="oldValue">The old value.</param>
         /// <param name="newValue">The new value.</param>
-        protected virtual void OnNumberFormatChanged(NumberFormatInfo oldValue, NumberFormatInfo newValue)
+        protected virtual void OnNumberFormatChanged(NumberFormatInfo? oldValue, NumberFormatInfo? newValue)
         {
             if (IsInitialized)
             {
@@ -440,7 +440,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="oldValue">The old value.</param>
         /// <param name="newValue">The new value.</param>
-        protected virtual void OnFormatStringChanged(string oldValue, string newValue)
+        protected virtual void OnFormatStringChanged(string? oldValue, string? newValue)
         {
             if (IsInitialized)
             {
@@ -510,7 +510,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="oldValue">The old value.</param>
         /// <param name="newValue">The new value.</param>
-        protected virtual void OnTextChanged(string oldValue, string newValue)
+        protected virtual void OnTextChanged(string? oldValue, string? newValue)
         {
             if (IsInitialized)
             {
@@ -706,8 +706,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (CultureInfo)e.OldValue;
-                var newValue = (CultureInfo)e.NewValue;
+                var oldValue = (CultureInfo?)e.OldValue;
+                var newValue = (CultureInfo?)e.NewValue;
                 upDown.OnCultureInfoChanged(oldValue, newValue);
             }
         }
@@ -720,8 +720,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (NumberFormatInfo)e.OldValue;
-                var newValue = (NumberFormatInfo)e.NewValue;
+                var oldValue = (NumberFormatInfo?)e.OldValue;
+                var newValue = (NumberFormatInfo?)e.NewValue;
                 upDown.OnNumberFormatChanged(oldValue, newValue);
             }
         }
@@ -734,8 +734,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (decimal)e.OldValue;
-                var newValue = (decimal)e.NewValue;
+                var oldValue = (decimal)e.OldValue!;
+                var newValue = (decimal)e.NewValue!;
                 upDown.OnIncrementChanged(oldValue, newValue);
             }
         }
@@ -748,8 +748,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (string)e.OldValue;
-                var newValue = (string)e.NewValue;
+                var oldValue = (string?)e.OldValue;
+                var newValue = (string?)e.NewValue;
                 upDown.OnFormatStringChanged(oldValue, newValue);
             }
         }
@@ -762,8 +762,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (bool)e.OldValue;
-                var newValue = (bool)e.NewValue;
+                var oldValue = (bool)e.OldValue!;
+                var newValue = (bool)e.NewValue!;
                 upDown.OnIsReadOnlyChanged(oldValue, newValue);
             }
         }
@@ -776,8 +776,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (decimal)e.OldValue;
-                var newValue = (decimal)e.NewValue;
+                var oldValue = (decimal)e.OldValue!;
+                var newValue = (decimal)e.NewValue!;
                 upDown.OnMaximumChanged(oldValue, newValue);
             }
         }
@@ -790,8 +790,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (decimal)e.OldValue;
-                var newValue = (decimal)e.NewValue;
+                var oldValue = (decimal)e.OldValue!;
+                var newValue = (decimal)e.NewValue!;
                 upDown.OnMinimumChanged(oldValue, newValue);
             }
         }
@@ -804,8 +804,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (string)e.OldValue;
-                var newValue = (string)e.NewValue;
+                var oldValue = (string?)e.OldValue;
+                var newValue = (string?)e.NewValue;
                 upDown.OnTextChanged(oldValue, newValue);
             }
         }
@@ -818,8 +818,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is NumericUpDown upDown)
             {
-                var oldValue = (decimal)e.OldValue;
-                var newValue = (decimal)e.NewValue;
+                var oldValue = (decimal)e.OldValue!;
+                var newValue = (decimal)e.NewValue!;
                 upDown.OnValueChanged(oldValue, newValue);
             }
         }
@@ -883,7 +883,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnSpinnerSpin(object sender, SpinEventArgs e)
+        private void OnSpinnerSpin(object? sender, SpinEventArgs e)
         {
             if (AllowSpin && !IsReadOnly)
             {
@@ -914,9 +914,9 @@ namespace Avalonia.Controls
             }
         }
 
-        public event EventHandler<SpinEventArgs> Spinned;
+        public event EventHandler<SpinEventArgs>? Spinned;
 
-        private void TextBoxOnPointerPressed(object sender, PointerPressedEventArgs e)
+        private void TextBoxOnPointerPressed(object? sender, PointerPressedEventArgs e)
         {
             if (e.Pointer.Captured != Spinner)
             {
@@ -933,7 +933,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Raised when the <see cref="Value"/> changes.
         /// </summary>
-        public event EventHandler<NumericUpDownValueChangedEventArgs> ValueChanged
+        public event EventHandler<NumericUpDownValueChangedEventArgs>? ValueChanged
         {
             add { AddHandler(ValueChangedEvent, value); }
             remove { RemoveHandler(ValueChangedEvent, value); }
@@ -949,7 +949,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="updateValueFromText">If value should be updated from text.</param>
         /// <param name="text">The text.</param>
-        private bool SyncTextAndValueProperties(bool updateValueFromText, string text)
+        private bool SyncTextAndValueProperties(bool updateValueFromText, string? text)
         {
             return SyncTextAndValueProperties(updateValueFromText, text, false);
         }
@@ -960,7 +960,7 @@ namespace Avalonia.Controls
         /// <param name="updateValueFromText">If value should be updated from text.</param>
         /// <param name="text">The text.</param>
         /// <param name="forceTextUpdate">Force text update.</param>
-        private bool SyncTextAndValueProperties(bool updateValueFromText, string text, bool forceTextUpdate)
+        private bool SyncTextAndValueProperties(bool updateValueFromText, string? text, bool forceTextUpdate)
         {
             if (_isSyncingTextAndValueProperties)
                 return true;
@@ -1092,7 +1092,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="text">Text to parse.</param>
         /// <param name="cultureInfo">The culture info.</param>
-        private static decimal ParsePercent(string text, IFormatProvider cultureInfo)
+        private static decimal ParsePercent(string text, IFormatProvider? cultureInfo)
         {
             var info = NumberFormatInfo.GetInstance(cultureInfo);
             text = text.Replace(info.PercentSymbol, null);
diff --git a/src/Avalonia.Controls/Panel.cs b/src/Avalonia.Controls/Panel.cs
index b182f9d261..482a7fab84 100644
--- a/src/Avalonia.Controls/Panel.cs
+++ b/src/Avalonia.Controls/Panel.cs
@@ -21,7 +21,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BackgroundProperty =
+        public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<Panel>();
 
         /// <summary>
@@ -32,7 +32,7 @@ namespace Avalonia.Controls
             AffectsRender<Panel>(BackgroundProperty);
         }
 
-        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="Panel"/> class.
@@ -51,13 +51,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or Sets Panel background brush.
         /// </summary>
-        public IBrush Background
+        public IBrush? Background
         {
             get { return GetValue(BackgroundProperty); }
             set { SetValue(BackgroundProperty, value); }
         }
 
-        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
             remove => _childIndexChanged -= value;
@@ -110,34 +110,34 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        protected virtual void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected virtual void ChildrenChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             List<Control> controls;
 
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    controls = e.NewItems.OfType<Control>().ToList();
+                    controls = e.NewItems!.OfType<Control>().ToList();
                     LogicalChildren.InsertRange(e.NewStartingIndex, controls);
-                    VisualChildren.InsertRange(e.NewStartingIndex, e.NewItems.OfType<Visual>());
+                    VisualChildren.InsertRange(e.NewStartingIndex, e.NewItems!.OfType<Visual>());
                     break;
 
                 case NotifyCollectionChangedAction.Move:
-                    LogicalChildren.MoveRange(e.OldStartingIndex, e.OldItems.Count, e.NewStartingIndex);
+                    LogicalChildren.MoveRange(e.OldStartingIndex, e.OldItems!.Count, e.NewStartingIndex);
                     VisualChildren.MoveRange(e.OldStartingIndex, e.OldItems.Count, e.NewStartingIndex);
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    controls = e.OldItems.OfType<Control>().ToList();
+                    controls = e.OldItems!.OfType<Control>().ToList();
                     LogicalChildren.RemoveAll(controls);
-                    VisualChildren.RemoveAll(e.OldItems.OfType<Visual>());
+                    VisualChildren.RemoveAll(e.OldItems!.OfType<Visual>());
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    for (var i = 0; i < e.OldItems.Count; ++i)
+                    for (var i = 0; i < e.OldItems!.Count; ++i)
                     {
                         var index = i + e.OldStartingIndex;
-                        var child = (IControl)e.NewItems[i];
+                        var child = (IControl)e.NewItems![i]!;
                         LogicalChildren[index] = child;
                         VisualChildren[index] = child;
                     }
diff --git a/src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs b/src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs
index 18348571bf..ecbce8851b 100644
--- a/src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs
+++ b/src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs
@@ -4,6 +4,7 @@ using Avalonia.Input;
 using Avalonia.Input.Raw;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
+using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
@@ -68,8 +69,8 @@ namespace Avalonia.Controls.Platform
                 window.Deactivated += WindowDeactivated;
             }
 
-            if (_root is TopLevel tl)
-                tl.PlatformImpl.LostFocus += TopLevelLostPlatformFocus;
+            if (_root is TopLevel tl && tl.PlatformImpl is ITopLevelImpl pimpl)
+                pimpl.LostFocus += TopLevelLostPlatformFocus;
 
             _inputManagerSubscription = InputManager?.Process.Subscribe(RawInput);
         }
@@ -118,7 +119,7 @@ namespace Avalonia.Controls.Platform
 
         protected static TimeSpan MenuShowDelay { get; } = TimeSpan.FromMilliseconds(400);
 
-        protected internal virtual void GotFocus(object sender, GotFocusEventArgs e)
+        protected internal virtual void GotFocus(object? sender, GotFocusEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -128,7 +129,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void LostFocus(object sender, RoutedEventArgs e)
+        protected internal virtual void LostFocus(object? sender, RoutedEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -138,7 +139,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void KeyDown(object sender, KeyEventArgs e)
+        protected internal virtual void KeyDown(object? sender, KeyEventArgs e)
         {
             KeyDown(GetMenuItem(e.Source as IControl), e);
         }
@@ -267,7 +268,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void AccessKeyPressed(object sender, RoutedEventArgs e)
+        protected internal virtual void AccessKeyPressed(object? sender, RoutedEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -288,7 +289,7 @@ namespace Avalonia.Controls.Platform
             e.Handled = true;
         }
 
-        protected internal virtual void PointerEnter(object sender, PointerEventArgs e)
+        protected internal virtual void PointerEnter(object? sender, PointerEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -333,7 +334,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void PointerLeave(object sender, PointerEventArgs e)
+        protected internal virtual void PointerLeave(object? sender, PointerEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -368,12 +369,12 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void PointerPressed(object sender, PointerPressedEventArgs e)
+        protected internal virtual void PointerPressed(object? sender, PointerPressedEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
-            var visual = (IVisual)sender;
 
-            if (e.GetCurrentPoint(visual).Properties.IsLeftButtonPressed && item?.HasSubMenu == true)
+            if (sender is IVisual visual &&
+                e.GetCurrentPoint(visual).Properties.IsLeftButtonPressed && item?.HasSubMenu == true)
             {
                 if (item.IsSubMenuOpen)
                 {
@@ -399,7 +400,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void PointerReleased(object sender, PointerReleasedEventArgs e)
+        protected internal virtual void PointerReleased(object? sender, PointerReleasedEventArgs e)
         {
             var item = GetMenuItem(e.Source as IControl);
 
@@ -410,7 +411,7 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void MenuOpened(object sender, RoutedEventArgs e)
+        protected internal virtual void MenuOpened(object? sender, RoutedEventArgs e)
         {
             if (e.Source == Menu)
             {
@@ -428,20 +429,18 @@ namespace Avalonia.Controls.Platform
             }
         }
 
-        protected internal virtual void RootPointerPressed(object sender, PointerPressedEventArgs e)
+        protected internal virtual void RootPointerPressed(object? sender, PointerPressedEventArgs e)
         {
             if (Menu?.IsOpen == true)
             {
-                var control = e.Source as ILogical;
-
-                if (!Menu.IsLogicalAncestorOf(control))
+                if (e.Source is ILogical control && !Menu.IsLogicalAncestorOf(control))
                 {
                     Menu.Close();
                 }
             }
         }
 
-        protected internal virtual void WindowDeactivated(object sender, EventArgs e)
+        protected internal virtual void WindowDeactivated(object? sender, EventArgs e)
         {
             Menu?.Close();
         }
diff --git a/src/Avalonia.Controls/Platform/ExportWindowingSubsystemAttribute.cs b/src/Avalonia.Controls/Platform/ExportWindowingSubsystemAttribute.cs
index e958b7aa15..2a401ad912 100644
--- a/src/Avalonia.Controls/Platform/ExportWindowingSubsystemAttribute.cs
+++ b/src/Avalonia.Controls/Platform/ExportWindowingSubsystemAttribute.cs
@@ -6,7 +6,7 @@ namespace Avalonia.Platform
     public class ExportWindowingSubsystemAttribute : Attribute
     {
         public ExportWindowingSubsystemAttribute(OperatingSystemType requiredRuntimePlatform, int priority, string name, Type initializationType,
-            string initializationMethod, Type environmentChecker = null)
+            string initializationMethod, Type? environmentChecker = null)
         {
             Name = name;
             InitializationType = initializationType;
@@ -17,7 +17,7 @@ namespace Avalonia.Platform
         }
 
         public string InitializationMethod { get; private set; }
-        public Type EnvironmentChecker { get; }
+        public Type? EnvironmentChecker { get; }
         public Type InitializationType { get; private set; }
         public string Name { get; private set; }
         public int Priority { get; private set; }
diff --git a/src/Avalonia.Controls/Platform/INativeControlHostImpl.cs b/src/Avalonia.Controls/Platform/INativeControlHostImpl.cs
index c6b1d09849..df13613848 100644
--- a/src/Avalonia.Controls/Platform/INativeControlHostImpl.cs
+++ b/src/Avalonia.Controls/Platform/INativeControlHostImpl.cs
@@ -1,4 +1,5 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Platform;
 using Avalonia.VisualTree;
 
@@ -19,7 +20,8 @@ namespace Avalonia.Controls.Platform
 
     public interface INativeControlHostControlTopLevelAttachment : IDisposable
     {
-        INativeControlHostImpl AttachedTo { get; set; }
+        INativeControlHostImpl? AttachedTo { get; set; }
+
         bool IsCompatibleWith(INativeControlHostImpl host);
         void HideWithSize(Size size);
         void ShowInBounds(Rect rect);
@@ -27,6 +29,6 @@ namespace Avalonia.Controls.Platform
 
     public interface ITopLevelImplWithNativeControlHost
     {
-        INativeControlHostImpl NativeControlHost { get; }
+        INativeControlHostImpl? NativeControlHost { get; }
     }
 }
diff --git a/src/Avalonia.Controls/Platform/ITopLevelImpl.cs b/src/Avalonia.Controls/Platform/ITopLevelImpl.cs
index f4f4d29168..80434882f7 100644
--- a/src/Avalonia.Controls/Platform/ITopLevelImpl.cs
+++ b/src/Avalonia.Controls/Platform/ITopLevelImpl.cs
@@ -82,27 +82,27 @@ namespace Avalonia.Platform
         /// <summary>
         /// Gets or sets a method called when the toplevel receives input.
         /// </summary>
-        Action<RawInputEventArgs> Input { get; set; }
+        Action<RawInputEventArgs>? Input { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the toplevel requires painting.
         /// </summary>
-        Action<Rect> Paint { get; set; }
+        Action<Rect>? Paint { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the toplevel is resized.
         /// </summary>
-        Action<Size, PlatformResizeReason> Resized { get; set; }
+        Action<Size, PlatformResizeReason>? Resized { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the toplevel's scaling changes.
         /// </summary>
-        Action<double> ScalingChanged { get; set; }
+        Action<double>? ScalingChanged { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the toplevel's TransparencyLevel changes.
         /// </summary>
-        Action<WindowTransparencyLevel> TransparencyLevelChanged { get; set; }
+        Action<WindowTransparencyLevel>? TransparencyLevelChanged { get; set; }
 
         /// <summary>
         /// Creates a new renderer for the toplevel.
@@ -138,17 +138,17 @@ namespace Avalonia.Platform
         /// Sets the cursor associated with the toplevel.
         /// </summary>
         /// <param name="cursor">The cursor. Use null for default cursor</param>
-        void SetCursor(ICursorImpl cursor);
+        void SetCursor(ICursorImpl? cursor);
 
         /// <summary>
         /// Gets or sets a method called when the underlying implementation is destroyed.
         /// </summary>
-        Action Closed { get; set; }
+        Action? Closed { get; set; }
         
         /// <summary>
         /// Gets or sets a method called when the input focus is lost.
         /// </summary>
-        Action LostFocus { get; set; }
+        Action? LostFocus { get; set; }
 
         /// <summary>
         /// Gets a mouse device associated with toplevel
@@ -156,7 +156,7 @@ namespace Avalonia.Platform
         [CanBeNull]
         IMouseDevice MouseDevice { get; }
 
-        IPopupImpl CreatePopup();
+        IPopupImpl? CreatePopup();
 
         /// <summary>
         /// Sets the <see cref="WindowTransparencyLevel"/> hint of the TopLevel.
diff --git a/src/Avalonia.Controls/Platform/ITopLevelImplWithTextInputMethod.cs b/src/Avalonia.Controls/Platform/ITopLevelImplWithTextInputMethod.cs
index 9c29415a6a..bafb973765 100644
--- a/src/Avalonia.Controls/Platform/ITopLevelImplWithTextInputMethod.cs
+++ b/src/Avalonia.Controls/Platform/ITopLevelImplWithTextInputMethod.cs
@@ -6,6 +6,6 @@ namespace Avalonia.Controls.Platform
 {
     public interface ITopLevelImplWithTextInputMethod : ITopLevelImpl
     {
-        public ITextInputMethodImpl TextInputMethod { get; }
+        public ITextInputMethodImpl? TextInputMethod { get; }
     }
 }
diff --git a/src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs b/src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs
index 9b779054f3..3358e83663 100644
--- a/src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs
+++ b/src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs
@@ -24,6 +24,6 @@ namespace Avalonia.Controls.Platform
     
     public interface ITopLevelImplWithNativeMenuExporter : ITopLevelImpl
     {
-        ITopLevelNativeMenuExporter NativeMenuExporter { get; }
+        ITopLevelNativeMenuExporter? NativeMenuExporter { get; }
     }
 }
diff --git a/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs b/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
index 5172569726..ff83e007b4 100644
--- a/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
+++ b/src/Avalonia.Controls/Platform/IWindowBaseImpl.cs
@@ -29,7 +29,7 @@ namespace Avalonia.Platform
         /// <summary>
         /// Gets or sets a method called when the window's position changes.
         /// </summary>
-        Action<PixelPoint> PositionChanged { get; set; }
+        Action<PixelPoint>? PositionChanged { get; set; }
 
         /// <summary>
         /// Activates the window.
@@ -39,12 +39,12 @@ namespace Avalonia.Platform
         /// <summary>
         /// Gets or sets a method called when the window is deactivated (loses focus).
         /// </summary>
-        Action Deactivated { get; set; }
+        Action? Deactivated { get; set; }
 
         /// <summary>
         /// Gets or sets a method called when the window is activated (receives focus).
         /// </summary>
-        Action Activated { get; set; }
+        Action? Activated { get; set; }
 
         /// <summary>
         /// Gets the platform window handle.
diff --git a/src/Avalonia.Controls/Platform/IWindowImpl.cs b/src/Avalonia.Controls/Platform/IWindowImpl.cs
index 17a90eddfe..d4be4f9f45 100644
--- a/src/Avalonia.Controls/Platform/IWindowImpl.cs
+++ b/src/Avalonia.Controls/Platform/IWindowImpl.cs
@@ -23,7 +23,7 @@ namespace Avalonia.Platform
         /// Sets the title of the window.
         /// </summary>
         /// <param name="title">The title.</param>
-        void SetTitle(string title);
+        void SetTitle(string? title);
 
         /// <summary>
         /// Sets the parent of the window.
@@ -50,7 +50,7 @@ namespace Avalonia.Platform
         /// <summary>
         /// Sets the icon of this window.
         /// </summary>
-        void SetIcon(IWindowIconImpl icon);
+        void SetIcon(IWindowIconImpl? icon);
 
         /// <summary>
         /// Enables or disables the taskbar icon
diff --git a/src/Avalonia.Controls/Platform/InProcessDragSource.cs b/src/Avalonia.Controls/Platform/InProcessDragSource.cs
index 414c523e0d..209d5f03dc 100644
--- a/src/Avalonia.Controls/Platform/InProcessDragSource.cs
+++ b/src/Avalonia.Controls/Platform/InProcessDragSource.cs
@@ -20,17 +20,17 @@ namespace Avalonia.Platform
         private readonly Subject<DragDropEffects> _result = new Subject<DragDropEffects>();
 
         private DragDropEffects _allowedEffects;
-        private IDataObject _draggedData;
-        private IInputRoot _lastRoot;
+        private IDataObject? _draggedData;
+        private IInputRoot? _lastRoot;
         private Point _lastPosition;
         private StandardCursorType _lastCursorType;
-        private object _originalCursor;
+        private object? _originalCursor;
         private RawInputModifiers? _initialInputModifiers;
 
         public InProcessDragSource()
         {
-            _inputManager = AvaloniaLocator.Current.GetService<IInputManager>();
-            _dragDrop = AvaloniaLocator.Current.GetService<IDragDropDevice>();
+            _inputManager = AvaloniaLocator.Current.GetRequiredService<IInputManager>();
+            _dragDrop = AvaloniaLocator.Current.GetRequiredService<IDragDropDevice>();
         }
 
         public async Task<DragDropEffects> DoDragDrop(PointerEventArgs triggerEvent, IDataObject data, DragDropEffects allowedEffects)
@@ -60,9 +60,9 @@ namespace Avalonia.Platform
         {
             _lastPosition = pt;
 
-            RawDragEvent rawEvent = new RawDragEvent(_dragDrop, type, root, pt, _draggedData, _allowedEffects, modifiers);
+            RawDragEvent rawEvent = new RawDragEvent(_dragDrop, type, root, pt, _draggedData!, _allowedEffects, modifiers);
             var tl = root.GetSelfAndVisualAncestors().OfType<TopLevel>().FirstOrDefault();
-            tl.PlatformImpl?.Input(rawEvent);
+            tl?.PlatformImpl?.Input?.Invoke(rawEvent);
 
             var effect = GetPreferredEffect(rawEvent.Effects & _allowedEffects, modifiers);
             UpdateCursor(root, effect);
@@ -91,7 +91,7 @@ namespace Avalonia.Platform
             return StandardCursorType.No;
         }
         
-        private void UpdateCursor(IInputRoot root, DragDropEffects effect)
+        private void UpdateCursor(IInputRoot? root, DragDropEffects effect)
         {
             if (_lastRoot != root)
             {
diff --git a/src/Avalonia.Controls/Platform/InternalPlatformThreadingInterface.cs b/src/Avalonia.Controls/Platform/InternalPlatformThreadingInterface.cs
index 6a95c2e047..4e5908456e 100644
--- a/src/Avalonia.Controls/Platform/InternalPlatformThreadingInterface.cs
+++ b/src/Avalonia.Controls/Platform/InternalPlatformThreadingInterface.cs
@@ -36,7 +36,7 @@ namespace Avalonia.Controls.Platform
             private readonly DispatcherPriority _priority;
             private readonly TimeSpan _interval;
             private readonly Action _tick;
-            private Timer _timer;
+            private Timer? _timer;
             private GCHandle _handle;
 
             public TimerImpl(DispatcherPriority priority, TimeSpan interval, Action tick)
@@ -48,7 +48,7 @@ namespace Avalonia.Controls.Platform
                 _handle = GCHandle.Alloc(_timer);
             }
 
-            private void OnTimer(object state)
+            private void OnTimer(object? state)
             {
                 if (_timer == null)
                     return;
@@ -66,7 +66,7 @@ namespace Avalonia.Controls.Platform
             public void Dispose()
             {
                 _handle.Free();
-                _timer.Dispose();
+                _timer?.Dispose();
                 _timer = null;
             }
         }
@@ -84,9 +84,9 @@ namespace Avalonia.Controls.Platform
         [ThreadStatic] private static bool TlsCurrentThreadIsLoopThread;
 
         public bool CurrentThreadIsLoopThread => TlsCurrentThreadIsLoopThread;
-        public event Action<DispatcherPriority?> Signaled;
+        public event Action<DispatcherPriority?>? Signaled;
 #pragma warning disable CS0067
-        public event Action<TimeSpan> Tick;
+        public event Action<TimeSpan>? Tick;
 #pragma warning restore CS0067
 
     }
diff --git a/src/Avalonia.Controls/Platform/MountedDriveInfo.cs b/src/Avalonia.Controls/Platform/MountedDriveInfo.cs
index 9448f16c38..f3104e4360 100644
--- a/src/Avalonia.Controls/Platform/MountedDriveInfo.cs
+++ b/src/Avalonia.Controls/Platform/MountedDriveInfo.cs
@@ -7,14 +7,14 @@ namespace Avalonia.Controls.Platform
     /// </summary>
     public class MountedVolumeInfo : IEquatable<MountedVolumeInfo>
     {
-        public string VolumeLabel { get; set; }
-        public string VolumePath { get; set; }
+        public string? VolumeLabel { get; set; }
+        public string? VolumePath { get; set; }
         public ulong VolumeSizeBytes { get; set; }
 
-        public bool Equals(MountedVolumeInfo other)
+        public bool Equals(MountedVolumeInfo? other)
         {
-            return this.VolumeSizeBytes.Equals(other.VolumeSizeBytes) &&
-                   this.VolumePath.Equals(other.VolumePath) &&
+            return this.VolumeSizeBytes.Equals(other?.VolumeSizeBytes) &&
+                   Equals(this.VolumePath, other.VolumePath) &&
                    (this.VolumeLabel ?? string.Empty).Equals(other.VolumeLabel ?? string.Empty);
         }
     }
diff --git a/src/Avalonia.Controls/Presenters/CarouselPresenter.cs b/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
index 81f43865a7..8883c9e8ce 100644
--- a/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/CarouselPresenter.cs
@@ -35,7 +35,7 @@ namespace Avalonia.Controls.Presenters
             Carousel.PageTransitionProperty.AddOwner<CarouselPresenter>();
 
         private int _selectedIndex = -1;
-        private Task _currentTransition;
+        private Task? _currentTransition;
         private int _queuedTransitionIndex = -1;
 
         /// <summary>
@@ -126,7 +126,7 @@ namespace Avalonia.Controls.Presenters
                         break;
                     case NotifyCollectionChangedAction.Replace:
                         if (e.OldStartingIndex > SelectedIndex ||
-                            e.OldStartingIndex + e.OldItems.Count - 1 < SelectedIndex)
+                            e.OldStartingIndex + e.OldItems!.Count - 1 < SelectedIndex)
                         {
                             return;
                         }
@@ -170,13 +170,13 @@ namespace Avalonia.Controls.Presenters
         {
             if (fromIndex != toIndex)
             {
-                var generator = ItemContainerGenerator;
-                IControl from = null;
-                IControl to = null;
+                var generator = ItemContainerGenerator!;
+                IControl? from = null;
+                IControl? to = null;
 
                 if (fromIndex != -1)
                 {
-                    from = ItemContainerGenerator.ContainerFromIndex(fromIndex);
+                    from = generator.ContainerFromIndex(fromIndex);
                 }
 
                 if (toIndex != -1)
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Presenters
 
                 if (PageTransition != null && (from != null || to != null))
                 {
-                    await PageTransition.Start((Visual)from, (Visual)to, fromIndex < toIndex, default);
+                    await PageTransition.Start((Visual?)from, (Visual?)to, fromIndex < toIndex, default);
                 }
                 else if (to != null)
                 {
@@ -197,7 +197,7 @@ namespace Avalonia.Controls.Presenters
                 {
                     if (IsVirtualized)
                     {
-                        Panel.Children.Remove(from);
+                        Panel!.Children.Remove(from);
                         generator.Dematerialize(fromIndex, 1);
                     }
                     else
@@ -208,15 +208,15 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        private IControl GetOrCreateContainer(int index)
+        private IControl? GetOrCreateContainer(int index)
         {
             var container = ItemContainerGenerator.ContainerFromIndex(index);
 
             if (container == null && IsVirtualized)
             {
-                var item = Items.Cast<object>().ElementAt(index);
+                var item = Items!.Cast<object>().ElementAt(index);
                 var materialized = ItemContainerGenerator.Materialize(index, item);
-                Panel.Children.Add(materialized.ContainerControl);
+                Panel!.Children.Add(materialized.ContainerControl);
                 container = materialized.ContainerControl;
             }
 
@@ -245,8 +245,8 @@ namespace Avalonia.Controls.Presenters
             {
                 if (_currentTransition == null)
                 {
-                    int fromIndex = (int)e.OldValue;
-                    int toIndex = (int)e.NewValue;
+                    int fromIndex = (int)e.OldValue!;
+                    int toIndex = (int)e.NewValue!;
 
                     for (;;)
                     {
@@ -268,7 +268,7 @@ namespace Avalonia.Controls.Presenters
                 }
                 else
                 {
-                    _queuedTransitionIndex = (int)e.NewValue;
+                    _queuedTransitionIndex = (int)e.NewValue!;
                 }
             }
         }
diff --git a/src/Avalonia.Controls/Presenters/ContentPresenter.cs b/src/Avalonia.Controls/Presenters/ContentPresenter.cs
index 1a46d84558..9886dd913a 100644
--- a/src/Avalonia.Controls/Presenters/ContentPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ContentPresenter.cs
@@ -20,13 +20,13 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BackgroundProperty =
+        public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<ContentPresenter>();
 
         /// <summary>
         /// Defines the <see cref="BorderBrush"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BorderBrushProperty =
+        public static readonly StyledProperty<IBrush?> BorderBrushProperty =
             Border.BorderBrushProperty.AddOwner<ContentPresenter>();
 
         /// <summary>
@@ -50,21 +50,21 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="Child"/> property.
         /// </summary>
-        public static readonly DirectProperty<ContentPresenter, IControl> ChildProperty =
-            AvaloniaProperty.RegisterDirect<ContentPresenter, IControl>(
+        public static readonly DirectProperty<ContentPresenter, IControl?> ChildProperty =
+            AvaloniaProperty.RegisterDirect<ContentPresenter, IControl?>(
                 nameof(Child),
                 o => o.Child);
 
         /// <summary>
         /// Defines the <see cref="Content"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> ContentProperty =
+        public static readonly StyledProperty<object?> ContentProperty =
             ContentControl.ContentProperty.AddOwner<ContentPresenter>();
 
         /// <summary>
         /// Defines the <see cref="ContentTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ContentTemplateProperty =
+        public static readonly StyledProperty<IDataTemplate?> ContentTemplateProperty =
             ContentControl.ContentTemplateProperty.AddOwner<ContentPresenter>();
 
         /// <summary>
@@ -93,9 +93,9 @@ namespace Avalonia.Controls.Presenters
                 nameof(RecognizesAccessKey),
                 cp => cp.RecognizesAccessKey, (cp, value) => cp.RecognizesAccessKey = value);
 
-        private IControl _child;
+        private IControl? _child;
         private bool _createdChild;
-        private IRecyclingDataTemplate _recyclingDataTemplate;
+        private IRecyclingDataTemplate? _recyclingDataTemplate;
         private readonly BorderRenderHelper _borderRenderer = new BorderRenderHelper();
         private bool _recognizesAccessKey;
 
@@ -120,7 +120,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets a brush with which to paint the background.
         /// </summary>
-        public IBrush Background
+        public IBrush? Background
         {
             get { return GetValue(BackgroundProperty); }
             set { SetValue(BackgroundProperty, value); }
@@ -129,7 +129,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets a brush with which to paint the border.
         /// </summary>
-        public IBrush BorderBrush
+        public IBrush? BorderBrush
         {
             get { return GetValue(BorderBrushProperty); }
             set { SetValue(BorderBrushProperty, value); }
@@ -165,7 +165,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets the control displayed by the presenter.
         /// </summary>
-        public IControl Child
+        public IControl? Child
         {
             get { return _child; }
             private set { SetAndRaise(ChildProperty, ref _child, value); }
@@ -175,7 +175,7 @@ namespace Avalonia.Controls.Presenters
         /// Gets or sets the content to be displayed by the presenter.
         /// </summary>
         [DependsOn(nameof(ContentTemplate))]
-        public object Content
+        public object? Content
         {
             get { return GetValue(ContentProperty); }
             set { SetValue(ContentProperty, value); }
@@ -184,7 +184,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets the data template used to display the content of the control.
         /// </summary>
-        public IDataTemplate ContentTemplate
+        public IDataTemplate? ContentTemplate
         {
             get { return GetValue(ContentTemplateProperty); }
             set { SetValue(ContentTemplateProperty, value); }
@@ -229,7 +229,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets the host content control.
         /// </summary>
-        internal IContentPresenterHost Host { get; private set; }
+        internal IContentPresenterHost? Host { get; private set; }
 
         /// <inheritdoc/>
         public sealed override void ApplyTemplate()
@@ -321,7 +321,7 @@ namespace Avalonia.Controls.Presenters
         /// Creates the child control.
         /// </summary>
         /// <returns>The child control or null.</returns>
-        protected virtual IControl CreateChild()
+        protected virtual IControl? CreateChild()
         {
             var content = Content;
             var oldChild = Child;
diff --git a/src/Avalonia.Controls/Presenters/IContentPresenter.cs b/src/Avalonia.Controls/Presenters/IContentPresenter.cs
index b1e64c2f25..ab4d61e3bd 100644
--- a/src/Avalonia.Controls/Presenters/IContentPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/IContentPresenter.cs
@@ -11,11 +11,11 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets the control displayed by the presenter.
         /// </summary>
-        IControl Child { get; }
+        IControl? Child { get; }
 
         /// <summary>
         /// Gets or sets the content to be displayed by the presenter.
         /// </summary>
-        object Content { get; set; }
+        object? Content { get; set; }
     }
 }
diff --git a/src/Avalonia.Controls/Presenters/IItemsPresenter.cs b/src/Avalonia.Controls/Presenters/IItemsPresenter.cs
index 873f0d756f..e7da3d4618 100644
--- a/src/Avalonia.Controls/Presenters/IItemsPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/IItemsPresenter.cs
@@ -5,9 +5,9 @@ namespace Avalonia.Controls.Presenters
 {
     public interface IItemsPresenter : IPresenter
     {
-        IEnumerable Items { get; set; }
+        IEnumerable? Items { get; set; }
 
-        IPanel Panel { get; }
+        IPanel? Panel { get; }
 
         void ItemsChanged(NotifyCollectionChangedEventArgs e);
 
diff --git a/src/Avalonia.Controls/Presenters/ItemContainerSync.cs b/src/Avalonia.Controls/Presenters/ItemContainerSync.cs
index 6e72908e6b..81184f719e 100644
--- a/src/Avalonia.Controls/Presenters/ItemContainerSync.cs
+++ b/src/Avalonia.Controls/Presenters/ItemContainerSync.cs
@@ -11,7 +11,7 @@ namespace Avalonia.Controls.Presenters
     {
         public static void ItemsChanged(
             ItemsPresenterBase owner,
-            IEnumerable items,
+            IEnumerable? items,
             NotifyCollectionChangedEventArgs e)
         {
             var generator = owner.ItemContainerGenerator;
@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Presenters
 
             void Add()
             {
-                if (e.NewStartingIndex + e.NewItems.Count < items.Count())
+                if (e.NewStartingIndex + e.NewItems!.Count < items!.Count())
                 {
                     generator.InsertSpace(e.NewStartingIndex, e.NewItems.Count);
                 }
@@ -34,7 +34,7 @@ namespace Avalonia.Controls.Presenters
 
             void Remove()
             {
-                RemoveContainers(panel, generator.RemoveRange(e.OldStartingIndex, e.OldItems.Count));
+                RemoveContainers(panel, generator.RemoveRange(e.OldStartingIndex, e.OldItems!.Count));
             }
 
             switch (e.Action)
@@ -48,8 +48,8 @@ namespace Avalonia.Controls.Presenters
                     break;
 
                 case NotifyCollectionChangedAction.Replace:
-                    RemoveContainers(panel, generator.Dematerialize(e.OldStartingIndex, e.OldItems.Count));
-                    var containers = AddContainers(owner, e.NewStartingIndex, e.NewItems);
+                    RemoveContainers(panel, generator.Dematerialize(e.OldStartingIndex, e.OldItems!.Count));
+                    var containers = AddContainers(owner, e.NewStartingIndex, e.NewItems!);
 
                     var i = e.NewStartingIndex;
 
@@ -92,7 +92,7 @@ namespace Avalonia.Controls.Presenters
 
                 if (i.ContainerControl != null)
                 {
-                    if (i.Index < panel.Children.Count)
+                    if (i.Index < panel!.Children.Count)
                     {
                         // TODO: This will insert at the wrong place when there are null items.
                         panel.Children.Insert(i.Index, i.ContainerControl);
diff --git a/src/Avalonia.Controls/Presenters/ItemVirtualizer.cs b/src/Avalonia.Controls/Presenters/ItemVirtualizer.cs
index be2b1e86f0..343c72f742 100644
--- a/src/Avalonia.Controls/Presenters/ItemVirtualizer.cs
+++ b/src/Avalonia.Controls/Presenters/ItemVirtualizer.cs
@@ -16,7 +16,7 @@ namespace Avalonia.Controls.Presenters
     internal abstract class ItemVirtualizer : IVirtualizingController, IDisposable
     {
         private double _crossAxisOffset;
-        private IDisposable _subscriptions;
+        private IDisposable? _subscriptions;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemVirtualizer"/> class.
@@ -46,12 +46,12 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets the <see cref="IVirtualizingPanel"/> which will host the items.
         /// </summary>
-        public IVirtualizingPanel VirtualizingPanel => Owner.Panel as IVirtualizingPanel;
+        public IVirtualizingPanel? VirtualizingPanel => Owner.Panel as IVirtualizingPanel;
 
         /// <summary>
         /// Gets the items to display.
         /// </summary>
-        public IEnumerable Items { get; private set; }
+        public IEnumerable? Items { get; private set; }
 
         /// <summary>
         /// Gets the number of items in <see cref="Items"/>.
@@ -100,11 +100,11 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                if (IsLogicalScrollEnabled)
+                if (IsLogicalScrollEnabled && Owner.Panel is Panel panel)
                 {
                     return Vertical ?
-                        new Size(Owner.Panel.DesiredSize.Width, ExtentValue) :
-                        new Size(ExtentValue, Owner.Panel.DesiredSize.Height);
+                        new Size(panel.DesiredSize.Width, ExtentValue) :
+                        new Size(ExtentValue, panel.DesiredSize.Height);
                 }
 
                 return default;
@@ -118,11 +118,11 @@ namespace Avalonia.Controls.Presenters
         {
             get
             {
-                if (IsLogicalScrollEnabled)
+                if (IsLogicalScrollEnabled && Owner.Panel is Panel panel)
                 {
                     return Vertical ?
-                        new Size(Owner.Panel.Bounds.Width, ViewportValue) :
-                        new Size(ViewportValue, Owner.Panel.Bounds.Height);
+                        new Size(panel.Bounds.Width, ViewportValue) :
+                        new Size(ViewportValue, panel.Bounds.Height);
                 }
 
                 return default;
@@ -177,7 +177,7 @@ namespace Avalonia.Controls.Presenters
         /// </summary>
         /// <param name="owner">The items presenter.</param>
         /// <returns>An <see cref="ItemVirtualizer"/>.</returns>
-        public static ItemVirtualizer Create(ItemsPresenter owner)
+        public static ItemVirtualizer? Create(ItemsPresenter owner)
         {
             if (owner.Panel == null)
             {
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Presenters
 
             var virtualizingPanel = owner.Panel as IVirtualizingPanel;
             var scrollContentPresenter = owner.Parent as IScrollable;
-            ItemVirtualizer result = null;
+            ItemVirtualizer? result = null;
 
             if (virtualizingPanel != null && scrollContentPresenter is object)
             {
@@ -218,7 +218,7 @@ namespace Avalonia.Controls.Presenters
         /// <returns>The desired size for the control.</returns>
         public virtual Size MeasureOverride(Size availableSize)
         {
-            Owner.Panel.Measure(availableSize);
+            Owner.Panel!.Measure(availableSize);
             return Owner.Panel.DesiredSize;
         }
 
@@ -232,12 +232,12 @@ namespace Avalonia.Controls.Presenters
             if (VirtualizingPanel != null)
             {
                 VirtualizingPanel.CrossAxisOffset = _crossAxisOffset;
-                Owner.Panel.Arrange(new Rect(finalSize));
+                Owner.Panel!.Arrange(new Rect(finalSize));
             }
             else
             {
                 var origin = Vertical ? new Point(-_crossAxisOffset, 0) : new Point(0, _crossAxisOffset);
-                Owner.Panel.Arrange(new Rect(origin, finalSize));
+                Owner.Panel!.Arrange(new Rect(origin, finalSize));
             }
 
             return finalSize;
@@ -254,7 +254,7 @@ namespace Avalonia.Controls.Presenters
         /// <param name="direction">The movement direction.</param>
         /// <param name="from">The control from which movement begins.</param>
         /// <returns>The control.</returns>
-        public virtual IControl GetControlInDirection(NavigationDirection direction, IControl from)
+        public virtual IControl? GetControlInDirection(NavigationDirection direction, IControl from)
         {
             return null;
         }
@@ -266,10 +266,10 @@ namespace Avalonia.Controls.Presenters
         /// </summary>
         /// <param name="items">The items.</param>
         /// <param name="e">A description of the change.</param>
-        public virtual void ItemsChanged(IEnumerable items, NotifyCollectionChangedEventArgs e)
+        public virtual void ItemsChanged(IEnumerable? items, NotifyCollectionChangedEventArgs e)
         {
             Items = items;
-            ItemCount = items.Count();
+            ItemCount = items?.Count() ?? 0;
         }
 
         /// <summary>
diff --git a/src/Avalonia.Controls/Presenters/ItemVirtualizerNone.cs b/src/Avalonia.Controls/Presenters/ItemVirtualizerNone.cs
index 275f4d418e..c1b6842b88 100644
--- a/src/Avalonia.Controls/Presenters/ItemVirtualizerNone.cs
+++ b/src/Avalonia.Controls/Presenters/ItemVirtualizerNone.cs
@@ -54,7 +54,7 @@ namespace Avalonia.Controls.Presenters
         }
 
         /// <inheritdoc/>
-        public override void ItemsChanged(IEnumerable items, NotifyCollectionChangedEventArgs e)
+        public override void ItemsChanged(IEnumerable? items, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsChanged(items, e);
             ItemContainerSync.ItemsChanged(Owner, items, e);
@@ -86,7 +86,7 @@ namespace Avalonia.Controls.Presenters
 
                 if (i.ContainerControl != null)
                 {
-                    if (i.Index < panel.Children.Count)
+                    if (i.Index < panel!.Children.Count)
                     {
                         // TODO: This will insert at the wrong place when there are null items.
                         panel.Children.Insert(i.Index, i.ContainerControl);
@@ -102,18 +102,5 @@ namespace Avalonia.Controls.Presenters
 
             return result;
         }
-
-        private void RemoveContainers(IEnumerable<ItemContainerInfo> items)
-        {
-            var panel = Owner.Panel;
-
-            foreach (var i in items)
-            {
-                if (i.ContainerControl != null)
-                {
-                    panel.Children.Remove(i.ContainerControl);
-                }
-            }
-        }
     }
 }
diff --git a/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs b/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
index 7d50ef7d33..a34e5d6438 100644
--- a/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
+++ b/src/Avalonia.Controls/Presenters/ItemVirtualizerSimple.cs
@@ -108,6 +108,8 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
+        public new IVirtualizingPanel VirtualizingPanel => base.VirtualizingPanel!;
+
         /// <inheritdoc/>
         public override Size MeasureOverride(Size availableSize)
         {
@@ -151,7 +153,7 @@ namespace Avalonia.Controls.Presenters
                 }
             }
 
-            Owner.Panel.Measure(availableSize);
+            Owner.Panel!.Measure(availableSize);
             return Owner.Panel.DesiredSize;
         }
 
@@ -163,7 +165,7 @@ namespace Avalonia.Controls.Presenters
         }
 
         /// <inheritdoc/>
-        public override void ItemsChanged(IEnumerable items, NotifyCollectionChangedEventArgs e)
+        public override void ItemsChanged(IEnumerable? items, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsChanged(items, e);
 
@@ -224,7 +226,7 @@ namespace Avalonia.Controls.Presenters
             InvalidateScroll();
         }
 
-        public override IControl GetControlInDirection(NavigationDirection direction, IControl from)
+        public override IControl? GetControlInDirection(NavigationDirection direction, IControl from)
         {
             var generator = Owner.ItemContainerGenerator;
             var panel = VirtualizingPanel;
@@ -331,7 +333,7 @@ namespace Avalonia.Controls.Presenters
                         }
                     }
 
-                    var materialized = generator.Materialize(index, Items.ElementAt(index));
+                    var materialized = generator.Materialize(index, Items.ElementAt(index)!);
 
                     if (step == 1)
                     {
@@ -382,7 +384,7 @@ namespace Avalonia.Controls.Presenters
 
             foreach (var container in containers)
             {
-                var item = Items.ElementAt(itemIndex);
+                var item = Items!.ElementAt(itemIndex)!;
 
                 if (!object.Equals(container.Item, item))
                 {
@@ -427,7 +429,7 @@ namespace Avalonia.Controls.Presenters
                 var oldItemIndex = FirstIndex + first + i;
                 var newItemIndex = oldItemIndex + delta + ((panel.Children.Count - count) * sign);
 
-                var item = Items.ElementAt(newItemIndex);
+                var item = Items!.ElementAt(newItemIndex)!;
 
                 if (!generator.TryRecycle(oldItemIndex, newItemIndex, item))
                 {
@@ -506,7 +508,7 @@ namespace Avalonia.Controls.Presenters
         /// </summary>
         /// <param name="index">The item index.</param>
         /// <returns>The container that was brought into view.</returns>
-        private IControl ScrollIntoViewCore(int index)
+        private IControl? ScrollIntoViewCore(int index)
         {
             var panel = VirtualizingPanel;
             var generator = Owner.ItemContainerGenerator;
diff --git a/src/Avalonia.Controls/Presenters/ItemsPresenter.cs b/src/Avalonia.Controls/Presenters/ItemsPresenter.cs
index 80c9e972d5..2c04b03faf 100644
--- a/src/Avalonia.Controls/Presenters/ItemsPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ItemsPresenter.cs
@@ -21,7 +21,7 @@ namespace Avalonia.Controls.Presenters
 
         private bool _canHorizontallyScroll;
         private bool _canVerticallyScroll;
-        private EventHandler _scrollInvalidated;
+        private EventHandler? _scrollInvalidated;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsPresenter"/> class.
@@ -96,7 +96,7 @@ namespace Avalonia.Controls.Presenters
         Size IScrollable.Viewport => Virtualizer?.Viewport ?? Bounds.Size;
 
         /// <inheritdoc/>
-        event EventHandler ILogicalScrollable.ScrollInvalidated
+        event EventHandler? ILogicalScrollable.ScrollInvalidated
         {
             add => _scrollInvalidated += value;
             remove => _scrollInvalidated -= value;
@@ -108,7 +108,7 @@ namespace Avalonia.Controls.Presenters
         /// <inheritdoc/>
         Size ILogicalScrollable.PageScrollSize => Virtualizer?.Viewport ?? new Size(16, 16);
 
-        internal ItemVirtualizer Virtualizer { get; private set; }
+        internal ItemVirtualizer? Virtualizer { get; private set; }
 
         /// <inheritdoc/>
         bool ILogicalScrollable.BringIntoView(IControl target, Rect targetRect)
@@ -117,7 +117,7 @@ namespace Avalonia.Controls.Presenters
         }
 
         /// <inheritdoc/>
-        IControl ILogicalScrollable.GetControlInDirection(NavigationDirection direction, IControl from)
+        IControl? ILogicalScrollable.GetControlInDirection(NavigationDirection direction, IControl from)
         {
             return Virtualizer?.GetControlInDirection(direction, from);
         }
@@ -152,7 +152,7 @@ namespace Avalonia.Controls.Presenters
             _scrollInvalidated?.Invoke(this, EventArgs.Empty);
 
             KeyboardNavigation.SetTabNavigation(
-                (InputElement)Panel,
+                (InputElement)panel,
                 KeyboardNavigation.GetTabNavigation(this));
         }
 
diff --git a/src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs b/src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs
index aeead7bfd0..f938c8d437 100644
--- a/src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs
+++ b/src/Avalonia.Controls/Presenters/ItemsPresenterBase.cs
@@ -18,7 +18,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="Items"/> property.
         /// </summary>
-        public static readonly DirectProperty<ItemsPresenterBase, IEnumerable> ItemsProperty =
+        public static readonly DirectProperty<ItemsPresenterBase, IEnumerable?> ItemsProperty =
             ItemsControl.ItemsProperty.AddOwner<ItemsPresenterBase>(o => o.Items, (o, v) => o.Items = v);
 
         /// <summary>
@@ -30,14 +30,14 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="ItemTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ItemTemplateProperty =
+        public static readonly StyledProperty<IDataTemplate?> ItemTemplateProperty =
             ItemsControl.ItemTemplateProperty.AddOwner<ItemsPresenterBase>();
 
-        private IEnumerable _items;
-        private IDisposable _itemsSubscription;
+        private IEnumerable? _items;
+        private IDisposable? _itemsSubscription;
         private bool _createdPanel;
-        private IItemContainerGenerator _generator;
-        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+        private IItemContainerGenerator? _generator;
+        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
 
         /// <summary>
         /// Initializes static members of the <see cref="ItemsPresenter"/> class.
@@ -50,7 +50,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets the items to be displayed.
         /// </summary>
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get
             {
@@ -114,7 +114,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets the data template used to display the items in the control.
         /// </summary>
-        public IDataTemplate ItemTemplate
+        public IDataTemplate? ItemTemplate
         {
             get { return GetValue(ItemTemplateProperty); }
             set { SetValue(ItemTemplateProperty, value); }
@@ -123,7 +123,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets the panel used to display the items.
         /// </summary>
-        public IPanel Panel
+        public IPanel? Panel
         {
             get;
             private set;
@@ -131,7 +131,7 @@ namespace Avalonia.Controls.Presenters
 
         protected bool IsHosted => TemplatedParent is IItemsPresenterHost;
 
-        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
             remove => _childIndexChanged -= value;
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Presenters
             return result;
         }
 
-        private void ContainerActionHandler(object sender, ItemContainerEventArgs e)
+        private void ContainerActionHandler(object? sender, ItemContainerEventArgs e)
         {
             for (var i = 0; i < e.Containers.Count; i++)
             {
@@ -197,14 +197,14 @@ namespace Avalonia.Controls.Presenters
         /// <inheritdoc/>
         protected override Size MeasureOverride(Size availableSize)
         {
-            Panel.Measure(availableSize);
+            Panel!.Measure(availableSize);
             return Panel.DesiredSize;
         }
 
         /// <inheritdoc/>
         protected override Size ArrangeOverride(Size finalSize)
         {
-            Panel.Arrange(new Rect(finalSize));
+            Panel!.Arrange(new Rect(finalSize));
             return finalSize;
         }
 
@@ -258,7 +258,7 @@ namespace Avalonia.Controls.Presenters
         /// </summary>
         /// <param name="sender">The sender.</param>
         /// <param name="e">The event args.</param>
-        private void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void ItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (_createdPanel)
             {
diff --git a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
index 8c5e644851..61a7e7ae1d 100644
--- a/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs
@@ -288,8 +288,8 @@ namespace Avalonia.Controls.Presenters
         private Size ArrangeWithAnchoring(Size finalSize)
         {
             var size = new Size(
-                CanHorizontallyScroll ? Math.Max(Child.DesiredSize.Width, finalSize.Width) : finalSize.Width,
-                CanVerticallyScroll ? Math.Max(Child.DesiredSize.Height, finalSize.Height) : finalSize.Height);
+                CanHorizontallyScroll ? Math.Max(Child!.DesiredSize.Width, finalSize.Width) : finalSize.Width,
+                CanVerticallyScroll ? Math.Max(Child!.DesiredSize.Height, finalSize.Height) : finalSize.Height);
 
             Vector TrackAnchor()
             {
@@ -297,7 +297,7 @@ namespace Avalonia.Controls.Presenters
                 // arrange then that change wasn't just due to scrolling (as scrolling doesn't adjust
                 // relative positions within Child).
                 if (_anchorElement != null &&
-                    TranslateBounds(_anchorElement, Child, out var updatedBounds) &&
+                    TranslateBounds(_anchorElement, Child!, out var updatedBounds) &&
                     updatedBounds.Position != _anchorElementBounds.Position)
                 {
                     var offset = updatedBounds.Position - _anchorElementBounds.Position;
@@ -357,13 +357,13 @@ namespace Avalonia.Controls.Presenters
             }
 
             Viewport = finalSize;
-            Extent = Child.Bounds.Size.Inflate(Child.Margin);
+            Extent = Child!.Bounds.Size.Inflate(Child.Margin);
             _isAnchorElementDirty = true;
 
             return finalSize;
         }
 
-        private void OnScrollGesture(object sender, ScrollGestureEventArgs e)
+        private void OnScrollGesture(object? sender, ScrollGestureEventArgs e)
         {
             if (Extent.Height > Viewport.Height || Extent.Width > Viewport.Width)
             {
@@ -433,7 +433,7 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        private void OnScrollGestureEnded(object sender, ScrollGestureEndedEventArgs e)
+        private void OnScrollGestureEnded(object? sender, ScrollGestureEndedEventArgs e)
             => _activeLogicalGestureScrolls?.Remove(e.Id);
 
         /// <inheritdoc/>
@@ -481,9 +481,10 @@ namespace Avalonia.Controls.Presenters
             base.OnPropertyChanged(change);
         }
 
-        private void BringIntoViewRequested(object sender, RequestBringIntoViewEventArgs e)
+        private void BringIntoViewRequested(object? sender, RequestBringIntoViewEventArgs e)
         {
-            e.Handled = BringDescendantIntoView(e.TargetObject, e.TargetRect);
+            if (e.TargetObject is not null)
+                e.Handled = BringDescendantIntoView(e.TargetObject, e.TargetRect);
         }
 
         private void ChildChanged(AvaloniaPropertyChangedEventArgs e)
@@ -522,9 +523,9 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        private void ScrollInvalidated(object sender, EventArgs e)
+        private void ScrollInvalidated(object? sender, EventArgs e)
         {
-            UpdateFromScrollable((ILogicalScrollable)sender);
+            UpdateFromScrollable((ILogicalScrollable)sender!);
         }
 
         private void UpdateFromScrollable(ILogicalScrollable scrollable)
@@ -581,7 +582,7 @@ namespace Avalonia.Controls.Presenters
                 // We have a candidate, calculate its bounds relative to Child. Because these
                 // bounds aren't relative to the ScrollContentPresenter itself, if they change
                 // then we know it wasn't just due to scrolling.
-                var unscrolledBounds = TranslateBounds(bestCandidate, Child);
+                var unscrolledBounds = TranslateBounds(bestCandidate, Child!);
                 _anchorElement = bestCandidate;
                 _anchorElementBounds = unscrolledBounds;
             }
@@ -589,7 +590,7 @@ namespace Avalonia.Controls.Presenters
 
         private bool GetViewportBounds(IControl element, out Rect bounds)
         {
-            if (TranslateBounds(element, Child, out var childBounds))
+            if (TranslateBounds(element, Child!, out var childBounds))
             {
                 // We want the bounds relative to the new Offset, regardless of whether the child
                 // control has actually been arranged to this offset yet, so translate first to the
diff --git a/src/Avalonia.Controls/Presenters/TextPresenter.cs b/src/Avalonia.Controls/Presenters/TextPresenter.cs
index ff63e5644f..1484e4f2a1 100644
--- a/src/Avalonia.Controls/Presenters/TextPresenter.cs
+++ b/src/Avalonia.Controls/Presenters/TextPresenter.cs
@@ -66,7 +66,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BackgroundProperty =
+        public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextPresenter>();
 
         private readonly DispatcherTimer _caretTimer;
@@ -75,7 +75,7 @@ namespace Avalonia.Controls.Presenters
         private int _selectionEnd;
         private bool _caretBlink;
         private string _text;
-        private FormattedText _formattedText;
+        private FormattedText? _formattedText;
         private Size _constraint;
 
         static TextPresenter()
@@ -96,7 +96,7 @@ namespace Avalonia.Controls.Presenters
                 SelectionForegroundBrushProperty.Changed, PasswordCharProperty.Changed, RevealPasswordProperty.Changed
             ).AddClassHandler<TextPresenter>((x, _) => x.InvalidateFormattedText());
 
-            CaretIndexProperty.Changed.AddClassHandler<TextPresenter>((x, e) => x.CaretIndexChanged((int)e.NewValue));
+            CaretIndexProperty.Changed.AddClassHandler<TextPresenter>((x, e) => x.CaretIndexChanged((int)e.NewValue!));
         }
 
         public TextPresenter()
@@ -109,7 +109,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets a brush used to paint the control's background.
         /// </summary>
-        public IBrush Background
+        public IBrush? Background
         {
             get => GetValue(BackgroundProperty);
             set => SetValue(BackgroundProperty, value);
@@ -164,7 +164,7 @@ namespace Avalonia.Controls.Presenters
         /// <summary>
         /// Gets or sets a brush used to paint the text.
         /// </summary>
-        public IBrush Foreground
+        public IBrush? Foreground
         {
             get => TextBlock.GetForeground(this);
             set => TextBlock.SetForeground(this, value);
@@ -334,7 +334,8 @@ namespace Avalonia.Controls.Presenters
                 }
             }
 
-            context.DrawText(Foreground, new Point(0, top), FormattedText);
+            if (Foreground is IBrush foreground)
+                context.DrawText(foreground, new Point(0, top), FormattedText);
         }
 
         public override void Render(DrawingContext context)
@@ -457,7 +458,7 @@ namespace Avalonia.Controls.Presenters
         /// <returns>A <see cref="FormattedText"/> object.</returns>
         protected virtual FormattedText CreateFormattedText()
         {
-            FormattedText result = null;
+            FormattedText? result = null;
 
             var text = Text;
 
@@ -540,7 +541,7 @@ namespace Avalonia.Controls.Presenters
             return Math.Max(0, Math.Min(length, value));
         }
 
-        private void CaretTimerTick(object sender, EventArgs e)
+        private void CaretTimerTick(object? sender, EventArgs e)
         {
             _caretBlink = !_caretBlink;
             InvalidateVisual();
diff --git a/src/Avalonia.Controls/Primitives/AccessText.cs b/src/Avalonia.Controls/Primitives/AccessText.cs
index 3c82386991..5114e13386 100644
--- a/src/Avalonia.Controls/Primitives/AccessText.cs
+++ b/src/Avalonia.Controls/Primitives/AccessText.cs
@@ -19,7 +19,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// The access key handler for the current window.
         /// </summary>
-        private IAccessKeyHandler _accessKeys;
+        private IAccessKeyHandler? _accessKeys;
 
         /// <summary>
         /// Initializes static members of the <see cref="AccessText"/> class.
@@ -67,7 +67,7 @@ namespace Avalonia.Controls.Primitives
 
             if (underscore != -1 && ShowAccessKey)
             {
-                var rect = TextLayout.HitTestTextPosition(underscore);
+                var rect = TextLayout!.HitTestTextPosition(underscore);
                 var offset = new Vector(0, -1.5);
                 context.DrawLine(
                     new Pen(Foreground, 1),
@@ -77,7 +77,7 @@ namespace Avalonia.Controls.Primitives
         }
 
         /// <inheritdoc/>
-        protected override TextLayout CreateTextLayout(Size constraint, string text)
+        protected override TextLayout? CreateTextLayout(Size constraint, string? text)
         {
             return base.CreateTextLayout(constraint, StripAccessKey(text));
         }
@@ -111,8 +111,13 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="text">The text.</param>
         /// <returns>The text with the first underscore stripped.</returns>
-        private string StripAccessKey(string text)
+        private string? StripAccessKey(string? text)
         {
+            if (text is null)
+            {
+                return null;
+            }
+
             var position = text.IndexOf('_');
 
             if (position == -1)
@@ -129,7 +134,7 @@ namespace Avalonia.Controls.Primitives
         /// Called when the <see cref="TextBlock.Text"/> property changes.
         /// </summary>
         /// <param name="text">The new text.</param>
-        private void TextChanged(string text)
+        private void TextChanged(string? text)
         {
             var key = (char)0;
 
diff --git a/src/Avalonia.Controls/Primitives/AdornerLayer.cs b/src/Avalonia.Controls/Primitives/AdornerLayer.cs
index 2ead82f95d..c53a16f6ff 100644
--- a/src/Avalonia.Controls/Primitives/AdornerLayer.cs
+++ b/src/Avalonia.Controls/Primitives/AdornerLayer.cs
@@ -142,12 +142,12 @@ namespace Avalonia.Controls.Primitives
             clip.Rect = clipBounds;
         }
 
-        private void ChildrenCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void ChildrenCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    foreach (Visual i in e.NewItems)
+                    foreach (Visual i in e.NewItems!)
                     {
                         UpdateAdornedElement(i, i.GetValue(AdornedElementProperty));
                     }
diff --git a/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs b/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
index be3c100ecf..2f051c4fda 100644
--- a/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
+++ b/src/Avalonia.Controls/Primitives/HeaderedItemsControl.cs
@@ -1,5 +1,4 @@
 using Avalonia.Collections;
-using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Presenters;
 using Avalonia.LogicalTree;
 
@@ -13,7 +12,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Defines the <see cref="Header"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> HeaderProperty =
+        public static readonly StyledProperty<object?> HeaderProperty =
             HeaderedContentControl.HeaderProperty.AddOwner<HeaderedItemsControl>();
 
         /// <summary>
@@ -27,7 +26,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the content of the control's header.
         /// </summary>
-        public object Header
+        public object? Header
         {
             get { return GetValue(HeaderProperty); }
             set { SetValue(HeaderProperty, value); }
@@ -36,7 +35,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the header presenter from the control's template.
         /// </summary>
-        public IContentPresenter HeaderPresenter
+        public IContentPresenter? HeaderPresenter
         {
             get;
             private set;
diff --git a/src/Avalonia.Controls/Primitives/HeaderedSelectingItemsControl.cs b/src/Avalonia.Controls/Primitives/HeaderedSelectingItemsControl.cs
index 78b5771b80..49fc58c8f5 100644
--- a/src/Avalonia.Controls/Primitives/HeaderedSelectingItemsControl.cs
+++ b/src/Avalonia.Controls/Primitives/HeaderedSelectingItemsControl.cs
@@ -1,5 +1,4 @@
 using Avalonia.Collections;
-using Avalonia.Controls.Mixins;
 using Avalonia.Controls.Presenters;
 using Avalonia.LogicalTree;
 
@@ -13,7 +12,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Defines the <see cref="Header"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> HeaderProperty =
+        public static readonly StyledProperty<object?> HeaderProperty =
             HeaderedContentControl.HeaderProperty.AddOwner<HeaderedSelectingItemsControl>();
 
         /// <summary>
@@ -27,7 +26,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the content of the control's header.
         /// </summary>
-        public object Header
+        public object? Header
         {
             get { return GetValue(HeaderProperty); }
             set { SetValue(HeaderProperty, value); }
@@ -36,7 +35,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the header presenter from the control's template.
         /// </summary>
-        public IContentPresenter HeaderPresenter
+        public IContentPresenter? HeaderPresenter
         {
             get;
             private set;
diff --git a/src/Avalonia.Controls/Primitives/ILogicalScrollable.cs b/src/Avalonia.Controls/Primitives/ILogicalScrollable.cs
index 5c29945735..2be91c046e 100644
--- a/src/Avalonia.Controls/Primitives/ILogicalScrollable.cs
+++ b/src/Avalonia.Controls/Primitives/ILogicalScrollable.cs
@@ -48,7 +48,7 @@ namespace Avalonia.Controls.Primitives
         /// This event notifies an attached <see cref="ScrollViewer"/> of a change in 
         /// one of the scroll properties.
         /// </remarks>
-        event EventHandler ScrollInvalidated;
+        event EventHandler? ScrollInvalidated;
 
         /// <summary>
         /// Attempts to bring a portion of the target visual into view by scrolling the content.
@@ -64,7 +64,7 @@ namespace Avalonia.Controls.Primitives
         /// <param name="direction">The movement direction.</param>
         /// <param name="from">The control from which movement begins.</param>
         /// <returns>The control.</returns>
-        IControl GetControlInDirection(NavigationDirection direction, IControl from);
+        IControl? GetControlInDirection(NavigationDirection direction, IControl from);
 
         /// <summary>
         /// Raises the <see cref="ScrollInvalidated"/> event.
diff --git a/src/Avalonia.Controls/Primitives/IPopupHost.cs b/src/Avalonia.Controls/Primitives/IPopupHost.cs
index ab81fe869e..36d2ae9230 100644
--- a/src/Avalonia.Controls/Primitives/IPopupHost.cs
+++ b/src/Avalonia.Controls/Primitives/IPopupHost.cs
@@ -20,23 +20,23 @@ namespace Avalonia.Controls.Primitives
         /// Sets the control to display in the popup.
         /// </summary>
         /// <param name="control"></param>
-        void SetChild(IControl control);
+        void SetChild(IControl? control);
 
         /// <summary>
         /// Gets the presenter from the control's template.
         /// </summary>
-        IContentPresenter Presenter { get; }
+        IContentPresenter? Presenter { get; }
 
         /// <summary>
         /// Gets the root of the visual tree in the case where the popup is presented using a
         /// separate visual tree.
         /// </summary>
-        IVisual HostedVisualTreeRoot { get; }
+        IVisual? HostedVisualTreeRoot { get; }
 
         /// <summary>
         /// Raised when the control's template is applied.
         /// </summary>
-        event EventHandler<TemplateAppliedEventArgs> TemplateApplied;
+        event EventHandler<TemplateAppliedEventArgs>? TemplateApplied;
 
         /// <summary>
         /// Configures the position of the popup according to a target control and a set of
diff --git a/src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs b/src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs
index 74d804f2bf..3d62297abb 100644
--- a/src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs
+++ b/src/Avalonia.Controls/Primitives/LightDismissOverlayLayer.cs
@@ -52,7 +52,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (InputPassThroughElement is object)
             {
-                var hit = VisualRoot.GetVisualAt(point, x => x != this);
+                var hit = VisualRoot?.GetVisualAt(point, x => x != this);
 
                 if (hit is object)
                 {
diff --git a/src/Avalonia.Controls/Primitives/OverlayLayer.cs b/src/Avalonia.Controls/Primitives/OverlayLayer.cs
index 5150033a53..6a0408d6d1 100644
--- a/src/Avalonia.Controls/Primitives/OverlayLayer.cs
+++ b/src/Avalonia.Controls/Primitives/OverlayLayer.cs
@@ -7,7 +7,7 @@ namespace Avalonia.Controls.Primitives
     public class OverlayLayer : Canvas, ICustomSimpleHitTest
     {
         public Size AvailableSize { get; private set; }
-        public static OverlayLayer GetOverlayLayer(IVisual visual)
+        public static OverlayLayer? GetOverlayLayer(IVisual visual)
         {
             foreach(var v in visual.GetVisualAncestors())
                 if(v is VisualLayerManager vlm)
diff --git a/src/Avalonia.Controls/Primitives/OverlayPopupHost.cs b/src/Avalonia.Controls/Primitives/OverlayPopupHost.cs
index 403902f676..6251d5cda7 100644
--- a/src/Avalonia.Controls/Primitives/OverlayPopupHost.cs
+++ b/src/Avalonia.Controls/Primitives/OverlayPopupHost.cs
@@ -23,15 +23,15 @@ namespace Avalonia.Controls.Primitives
             _positioner = new ManagedPopupPositioner(this);
         }
 
-        public void SetChild(IControl control)
+        public void SetChild(IControl? control)
         {
             Content = control;
         }
 
-        public IVisual HostedVisualTreeRoot => null;
+        public IVisual? HostedVisualTreeRoot => null;
         
         /// <inheritdoc/>
-        IInteractive IInteractive.InteractiveParent => Parent;
+        IInteractive? IInteractive.InteractiveParent => Parent;
 
         public void Dispose() => Hide();
 
@@ -75,7 +75,7 @@ namespace Avalonia.Controls.Primitives
             PopupPositionerConstraintAdjustment constraintAdjustment = PopupPositionerConstraintAdjustment.All,
             Rect? rect = null)
         {
-            _positionerParameters.ConfigurePosition((TopLevel)_overlayLayer.GetVisualRoot(), target, placement, offset, anchor,
+            _positionerParameters.ConfigurePosition((TopLevel)_overlayLayer.GetVisualRoot()!, target, placement, offset, anchor,
                 gravity, constraintAdjustment, rect);
             UpdatePosition();
         }
@@ -126,11 +126,11 @@ namespace Avalonia.Controls.Primitives
 
         double IManagedPopupPositionerPopup.Scaling => 1;
        
-        public static IPopupHost CreatePopupHost(IVisual target, IAvaloniaDependencyResolver dependencyResolver)
+        public static IPopupHost CreatePopupHost(IVisual target, IAvaloniaDependencyResolver? dependencyResolver)
         {
             var platform = (target.GetVisualRoot() as TopLevel)?.PlatformImpl?.CreatePopup();
             if (platform != null)
-                return new PopupRoot((TopLevel)target.GetVisualRoot(), platform, dependencyResolver);
+                return new PopupRoot((TopLevel)target.GetVisualRoot()!, platform, dependencyResolver);
             
             var overlayLayer = OverlayLayer.GetOverlayLayer(target);
             if (overlayLayer == null)
diff --git a/src/Avalonia.Controls/Primitives/Popup.cs b/src/Avalonia.Controls/Primitives/Popup.cs
index a47149a9e0..8d005b2fdc 100644
--- a/src/Avalonia.Controls/Primitives/Popup.cs
+++ b/src/Avalonia.Controls/Primitives/Popup.cs
@@ -419,7 +419,7 @@ namespace Avalonia.Controls.Primitives
                 (x, handler) => x.TemplateApplied += handler,
                 (x, handler) => x.TemplateApplied -= handler).DisposeWith(handlerCleanup);
 
-            if (topLevel is Window window)
+            if (topLevel is Window window && window.PlatformImpl != null)
             {
                 SubscribeToEventHandler<Window, EventHandler>(window, WindowDeactivated,
                     (x, handler) => x.Deactivated += handler,
@@ -571,7 +571,7 @@ namespace Avalonia.Controls.Primitives
 
         private void WindowManagerAddShadowHintChanged(IPopupHost host, bool hint)
         {
-            if(host is PopupRoot pr)
+            if(host is PopupRoot pr && pr.PlatformImpl is not null)
             {
                 pr.PlatformImpl.SetWindowManagerAddShadowHint(hint);
             }
@@ -676,7 +676,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void PointerPressedDismissOverlay(object sender, PointerPressedEventArgs e)
+        private void PointerPressedDismissOverlay(object? sender, PointerPressedEventArgs e)
         {
             if (IsLightDismissEnabled && e.Source is IVisual v && !IsChildOrThis(v))
             {
@@ -706,7 +706,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void RootTemplateApplied(object sender, TemplateAppliedEventArgs e)
+        private void RootTemplateApplied(object? sender, TemplateAppliedEventArgs e)
         {
             if (_openState is null)
             {
@@ -733,7 +733,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void SetTemplatedParentAndApplyChildTemplates(IControl control)
+        private void SetTemplatedParentAndApplyChildTemplates(IControl? control)
         {
             if (control != null)
             {
@@ -794,7 +794,7 @@ namespace Avalonia.Controls.Primitives
 
         public bool IsPointerOverPopup => ((IInputElement?)_openState?.PopupHost)?.IsPointerOver ?? false;
 
-        private void WindowDeactivated(object sender, EventArgs e)
+        private void WindowDeactivated(object? sender, EventArgs e)
         {
             if (IsLightDismissEnabled)
             {
@@ -802,7 +802,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void ParentClosed(object sender, EventArgs e)
+        private void ParentClosed(object? sender, EventArgs e)
         {
             if (IsLightDismissEnabled)
             {
@@ -818,9 +818,9 @@ namespace Avalonia.Controls.Primitives
 
         private void WindowPositionChanged(PixelPoint pp) => HandlePositionChange();
 
-        private void PlacementTargetLayoutUpdated(object src, EventArgs e) => HandlePositionChange();
+        private void PlacementTargetLayoutUpdated(object? src, EventArgs e) => HandlePositionChange();
 
-        private void ParentPopupPositionChanged(object src, PixelPointEventArgs e) => HandlePositionChange();
+        private void ParentPopupPositionChanged(object? src, PixelPointEventArgs e) => HandlePositionChange();
 
         private IgnoreIsOpenScope BeginIgnoringIsOpen()
         {
diff --git a/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs b/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs
index 844069965b..340076a407 100644
--- a/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs
+++ b/src/Avalonia.Controls/Primitives/PopupPositioning/IPopupPositioner.cs
@@ -448,7 +448,7 @@ namespace Avalonia.Controls.Primitives.PopupPositioning
         {
             // We need a better way for tracking the last pointer position
 #pragma warning disable CS0618 // Type or member is obsolete
-            var pointer = topLevel.PointToClient(topLevel.PlatformImpl.MouseDevice.Position);
+            var pointer = topLevel.PointToClient(topLevel.PlatformImpl?.MouseDevice.Position ?? default);
 #pragma warning restore CS0618 // Type or member is obsolete
 
             positionerParameters.Offset = offset;
diff --git a/src/Avalonia.Controls/Primitives/PopupRoot.cs b/src/Avalonia.Controls/Primitives/PopupRoot.cs
index c59c7c11d9..b18e436532 100644
--- a/src/Avalonia.Controls/Primitives/PopupRoot.cs
+++ b/src/Avalonia.Controls/Primitives/PopupRoot.cs
@@ -43,7 +43,7 @@ namespace Avalonia.Controls.Primitives
         /// <param name="dependencyResolver">
         /// The dependency resolver to use. If null the default dependency resolver will be used.
         /// </param>
-        public PopupRoot(TopLevel parent, IPopupImpl impl, IAvaloniaDependencyResolver dependencyResolver)
+        public PopupRoot(TopLevel parent, IPopupImpl impl, IAvaloniaDependencyResolver? dependencyResolver)
             : base(ValidatingPopupImpl.Wrap(impl), dependencyResolver)
         {
             _parent = parent;
@@ -52,8 +52,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
-        [CanBeNull]
-        public new IPopupImpl PlatformImpl => (IPopupImpl)base.PlatformImpl;               
+        public new IPopupImpl? PlatformImpl => (IPopupImpl?)base.PlatformImpl;               
 
         /// <summary>
         /// Gets the parent control in the event route.
@@ -61,17 +60,17 @@ namespace Avalonia.Controls.Primitives
         /// <remarks>
         /// Popup events are passed to their parent window. This facilitates this.
         /// </remarks>
-        IInteractive IInteractive.InteractiveParent => Parent;
+        IInteractive? IInteractive.InteractiveParent => Parent;
 
         /// <summary>
         /// Gets the control that is hosting the popup root.
         /// </summary>
-        IVisual IHostedVisualTreeRoot.Host => Parent;
+        IVisual? IHostedVisualTreeRoot.Host => Parent;
 
         /// <summary>
         /// Gets the styling parent of the popup root.
         /// </summary>
-        IStyleHost IStyleHost.StylingParent => Parent;
+        IStyleHost? IStyleHost.StylingParent => Parent;
 
         /// <inheritdoc/>
         public void Dispose() => PlatformImpl?.Dispose();
@@ -94,7 +93,7 @@ namespace Avalonia.Controls.Primitives
                 UpdatePosition();
         }
 
-        public void SetChild(IControl control) => Content = control;
+        public void SetChild(IControl? control) => Content = control;
 
         IVisual IPopupHost.HostedVisualTreeRoot => this;
         
diff --git a/src/Avalonia.Controls/Primitives/ScrollBar.cs b/src/Avalonia.Controls/Primitives/ScrollBar.cs
index d5d6af8bfa..8460fe3017 100644
--- a/src/Avalonia.Controls/Primitives/ScrollBar.cs
+++ b/src/Avalonia.Controls/Primitives/ScrollBar.cs
@@ -69,11 +69,11 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<TimeSpan> ShowDelayProperty =
             AvaloniaProperty.Register<ScrollBar, TimeSpan>(nameof(ShowDelay), TimeSpan.FromSeconds(0.5));
 
-        private Button _lineUpButton;
-        private Button _lineDownButton;
-        private Button _pageUpButton;
-        private Button _pageDownButton;
-        private DispatcherTimer _timer;
+        private Button? _lineUpButton;
+        private Button? _lineDownButton;
+        private Button? _pageUpButton;
+        private Button? _pageDownButton;
+        private DispatcherTimer? _timer;
         private bool _isExpanded;
 
         /// <summary>
@@ -157,7 +157,7 @@ namespace Avalonia.Controls.Primitives
             set => SetValue(ShowDelayProperty, value);
         }
 
-        public event EventHandler<ScrollEventArgs> Scroll;
+        public event EventHandler<ScrollEventArgs>? Scroll;
 
         /// <summary>
         /// Calculates and updates whether the scrollbar should be visible.
@@ -293,7 +293,7 @@ namespace Avalonia.Controls.Primitives
                 _timer = new DispatcherTimer(DispatcherPriority.Normal);
                 _timer.Tick += (sender, args) =>
                 {
-                    var senderTimer = (DispatcherTimer)sender;
+                    var senderTimer = (DispatcherTimer)sender!;
 
                     if (senderTimer.Tag is Action action)
                     {
@@ -344,22 +344,22 @@ namespace Avalonia.Controls.Primitives
             IsExpanded = true;
         }
 
-        private void LineUpClick(object sender, RoutedEventArgs e)
+        private void LineUpClick(object? sender, RoutedEventArgs e)
         {
             SmallDecrement();
         }
 
-        private void LineDownClick(object sender, RoutedEventArgs e)
+        private void LineDownClick(object? sender, RoutedEventArgs e)
         {
             SmallIncrement();
         }
 
-        private void PageUpClick(object sender, RoutedEventArgs e)
+        private void PageUpClick(object? sender, RoutedEventArgs e)
         {
             LargeDecrement();
         }
 
-        private void PageDownClick(object sender, RoutedEventArgs e)
+        private void PageDownClick(object? sender, RoutedEventArgs e)
         {
             LargeIncrement();
         }
diff --git a/src/Avalonia.Controls/Primitives/SelectingItemsControl.cs b/src/Avalonia.Controls/Primitives/SelectingItemsControl.cs
index 80f3d345a6..8c0ae609e4 100644
--- a/src/Avalonia.Controls/Primitives/SelectingItemsControl.cs
+++ b/src/Avalonia.Controls/Primitives/SelectingItemsControl.cs
@@ -138,7 +138,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Occurs when the control's selection changes.
         /// </summary>
-        public event EventHandler<SelectionChangedEventArgs> SelectionChanged
+        public event EventHandler<SelectionChangedEventArgs>? SelectionChanged
         {
             add { AddHandler(SelectionChangedEvent, value); }
             remove { RemoveHandler(SelectionChangedEvent, value); }
@@ -386,9 +386,9 @@ namespace Avalonia.Controls.Primitives
             return null;
         }
 
-        protected override void ItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected override void ItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            base.ItemsCollectionChanged(sender, e);
+            base.ItemsCollectionChanged(sender!, e);
 
             if (AlwaysSelected && SelectedIndex == -1 && ItemCount > 0)
             {
@@ -406,7 +406,7 @@ namespace Avalonia.Controls.Primitives
         {
             base.OnApplyTemplate(e);
 
-            void ExecuteScrollWhenLayoutUpdated(object sender, EventArgs e)
+            void ExecuteScrollWhenLayoutUpdated(object? sender, EventArgs e)
             {
                 LayoutUpdated -= ExecuteScrollWhenLayoutUpdated;
                 AutoScrollToSelectedItemIfNecessary();
@@ -443,9 +443,7 @@ namespace Avalonia.Controls.Primitives
         {
             base.OnContainersDematerialized(e);
 
-            var panel = (InputElement)Presenter.Panel;
-
-            if (panel != null)
+            if (Presenter?.Panel is InputElement panel)
             {
                 foreach (var container in e.Containers)
                 {
@@ -523,7 +521,7 @@ namespace Avalonia.Controls.Primitives
                     info.ContainerControl is IContentControl control &&
                     control.Content?.ToString()?.StartsWith(_textSearchTerm, StringComparison.OrdinalIgnoreCase) == true;
 
-                var info = ItemContainerGenerator.Containers.FirstOrDefault(match);
+                var info = ItemContainerGenerator?.Containers.FirstOrDefault(match);
 
                 if (info != null)
                 {
@@ -594,7 +592,7 @@ namespace Avalonia.Controls.Primitives
         /// <returns>True if the selection was moved; otherwise false.</returns>
         protected bool MoveSelection(NavigationDirection direction, bool wrap)
         {
-            var from = SelectedIndex != -1 ? ItemContainerGenerator.ContainerFromIndex(SelectedIndex) : null;
+            var from = SelectedIndex != -1 ? ItemContainerGenerator?.ContainerFromIndex(SelectedIndex) : null;
             return MoveSelection(from, direction, wrap);
         }
 
@@ -610,7 +608,7 @@ namespace Avalonia.Controls.Primitives
             if (Presenter?.Panel is INavigableContainer container &&
                 GetNextControl(container, direction, from, wrap) is IControl next)
             {
-                var index = ItemContainerGenerator.IndexFromContainer(next);
+                var index = ItemContainerGenerator?.IndexFromContainer(next) ?? -1;
 
                 if (index != -1)
                 {
@@ -688,7 +686,7 @@ namespace Avalonia.Controls.Primitives
 
             if (Presenter?.Panel != null)
             {
-                var container = ItemContainerGenerator.ContainerFromIndex(index);
+                var container = ItemContainerGenerator?.ContainerFromIndex(index);
                 KeyboardNavigation.SetTabOnceActiveElement(
                     (InputElement)Presenter.Panel,
                     container);
@@ -755,7 +753,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="sender">The sender.</param>
         /// <param name="e">The event args.</param>
-        private void OnSelectionModelPropertyChanged(object sender, PropertyChangedEventArgs e)
+        private void OnSelectionModelPropertyChanged(object? sender, PropertyChangedEventArgs e)
         {
             if (e.PropertyName == nameof(ISelectionModel.AnchorIndex))
             {
@@ -789,11 +787,11 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="sender">The sender.</param>
         /// <param name="e">The event args.</param>
-        private void OnSelectionModelSelectionChanged(object sender, SelectionModelSelectionChangedEventArgs e)
+        private void OnSelectionModelSelectionChanged(object? sender, SelectionModelSelectionChangedEventArgs e)
         {
             void Mark(int index, bool selected)
             {
-                var container = ItemContainerGenerator.ContainerFromIndex(index);
+                var container = ItemContainerGenerator?.ContainerFromIndex(index);
 
                 if (container != null)
                 {
@@ -829,7 +827,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="sender">The sender.</param>
         /// <param name="e">The event args.</param>
-        private void OnSelectionModelLostSelection(object sender, EventArgs e)
+        private void OnSelectionModelLostSelection(object? sender, EventArgs e)
         {
             if (AlwaysSelected && Items is object)
             {
@@ -912,7 +910,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     MarkContainerSelected(
                         container,
-                        Selection.IsSelected(ItemContainerGenerator.IndexFromContainer(container)));
+                        Selection.IsSelected(ItemContainerGenerator!.IndexFromContainer(container)));
                 }
             }
         }
@@ -1034,7 +1032,7 @@ namespace Avalonia.Controls.Primitives
             _textSearchTimer = null;
         }
 
-        private void TextSearchTimer_Tick(object sender, EventArgs e)
+        private void TextSearchTimer_Tick(object? sender, EventArgs e)
         {
             _textSearchTerm = string.Empty;
             StopTextSearchTimer();
diff --git a/src/Avalonia.Controls/Primitives/TemplatedControl.cs b/src/Avalonia.Controls/Primitives/TemplatedControl.cs
index 59975b072d..a777b522ad 100644
--- a/src/Avalonia.Controls/Primitives/TemplatedControl.cs
+++ b/src/Avalonia.Controls/Primitives/TemplatedControl.cs
@@ -109,7 +109,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Raised when the control's template is applied.
         /// </summary>
-        public event EventHandler<TemplateAppliedEventArgs> TemplateApplied
+        public event EventHandler<TemplateAppliedEventArgs>? TemplateApplied
         {
             add { AddHandler(TemplateAppliedEvent, value); }
             remove { RemoveHandler(TemplateAppliedEvent, value); }
diff --git a/src/Avalonia.Controls/Primitives/Thumb.cs b/src/Avalonia.Controls/Primitives/Thumb.cs
index 91141058e4..cb3195cf52 100644
--- a/src/Avalonia.Controls/Primitives/Thumb.cs
+++ b/src/Avalonia.Controls/Primitives/Thumb.cs
@@ -26,19 +26,19 @@ namespace Avalonia.Controls.Primitives
             DragCompletedEvent.AddClassHandler<Thumb>((x, e) => x.OnDragCompleted(e), RoutingStrategies.Bubble);
         }
 
-        public event EventHandler<VectorEventArgs> DragStarted
+        public event EventHandler<VectorEventArgs>? DragStarted
         {
             add { AddHandler(DragStartedEvent, value); }
             remove { RemoveHandler(DragStartedEvent, value); }
         }
 
-        public event EventHandler<VectorEventArgs> DragDelta
+        public event EventHandler<VectorEventArgs>? DragDelta
         {
             add { AddHandler(DragDeltaEvent, value); }
             remove { RemoveHandler(DragDeltaEvent, value); }
         }
 
-        public event EventHandler<VectorEventArgs> DragCompleted
+        public event EventHandler<VectorEventArgs>? DragCompleted
         {
             add { AddHandler(DragCompletedEvent, value); }
             remove { RemoveHandler(DragCompletedEvent, value); }
diff --git a/src/Avalonia.Controls/Primitives/ToggleButton.cs b/src/Avalonia.Controls/Primitives/ToggleButton.cs
index 6b2c566422..4bdf6db2fc 100644
--- a/src/Avalonia.Controls/Primitives/ToggleButton.cs
+++ b/src/Avalonia.Controls/Primitives/ToggleButton.cs
@@ -61,7 +61,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Raised when a <see cref="ToggleButton"/> is checked.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Checked
+        public event EventHandler<RoutedEventArgs>? Checked
         {
             add => AddHandler(CheckedEvent, value);
             remove => RemoveHandler(CheckedEvent, value);
@@ -70,7 +70,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Raised when a <see cref="ToggleButton"/> is unchecked.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Unchecked
+        public event EventHandler<RoutedEventArgs>? Unchecked
         {
             add => AddHandler(UncheckedEvent, value);
             remove => RemoveHandler(UncheckedEvent, value);
@@ -79,7 +79,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Raised when a <see cref="ToggleButton"/> is neither checked nor unchecked.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Indeterminate
+        public event EventHandler<RoutedEventArgs>? Indeterminate
         {
             add => AddHandler(IndeterminateEvent, value);
             remove => RemoveHandler(IndeterminateEvent, value);
diff --git a/src/Avalonia.Controls/Primitives/Track.cs b/src/Avalonia.Controls/Primitives/Track.cs
index 9399f5fb31..f8d6046101 100644
--- a/src/Avalonia.Controls/Primitives/Track.cs
+++ b/src/Avalonia.Controls/Primitives/Track.cs
@@ -403,8 +403,8 @@ namespace Avalonia.Controls.Primitives
 
         private void ThumbChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldThumb = (Thumb)e.OldValue;
-            var newThumb = (Thumb)e.NewValue;
+            var oldThumb = (Thumb?)e.OldValue;
+            var newThumb = (Thumb?)e.NewValue;
 
             if (oldThumb != null)
             {
@@ -424,8 +424,8 @@ namespace Avalonia.Controls.Primitives
 
         private void ButtonChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldButton = (Button)e.OldValue;
-            var newButton = (Button)e.NewValue;
+            var oldButton = (Button?)e.OldValue;
+            var newButton = (Button?)e.NewValue;
 
             if (oldButton != null)
             {
@@ -440,7 +440,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void ThumbDragged(object sender, VectorEventArgs e)
+        private void ThumbDragged(object? sender, VectorEventArgs e)
         {
             if (IsThumbDragHandled)
                 return;
diff --git a/src/Avalonia.Controls/Primitives/VisualLayerManager.cs b/src/Avalonia.Controls/Primitives/VisualLayerManager.cs
index 03a84e26f2..df4edbd252 100644
--- a/src/Avalonia.Controls/Primitives/VisualLayerManager.cs
+++ b/src/Avalonia.Controls/Primitives/VisualLayerManager.cs
@@ -11,7 +11,7 @@ namespace Avalonia.Controls.Primitives
         private const int LightDismissOverlayZIndex = int.MaxValue - 98;
         private const int OverlayZIndex = int.MaxValue - 97;
 
-        private ILogicalRoot _logicalRoot;
+        private ILogicalRoot? _logicalRoot;
         private readonly List<Control> _layers = new List<Control>();
 
         public static readonly StyledProperty<ChromeOverlayLayer> ChromeOverlayLayerProperty =
@@ -50,7 +50,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        public OverlayLayer OverlayLayer
+        public OverlayLayer? OverlayLayer
         {
             get
             {
@@ -81,7 +81,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        T FindLayer<T>() where T : class
+        T? FindLayer<T>() where T : class
         {
             foreach (var layer in _layers)
                 if (layer is T match)
@@ -97,7 +97,7 @@ namespace Avalonia.Controls.Primitives
             VisualChildren.Add(layer);
             if (((ILogical)this).IsAttachedToLogicalTree)
                 ((ILogical)layer).NotifyAttachedToLogicalTree(
-                    new LogicalTreeAttachmentEventArgs(_logicalRoot, layer, this));
+                    new LogicalTreeAttachmentEventArgs(_logicalRoot!, layer, this));
             InvalidateArrange();
         }
 
diff --git a/src/Avalonia.Controls/ProgressBar.cs b/src/Avalonia.Controls/ProgressBar.cs
index a848901bc3..017a053c48 100644
--- a/src/Avalonia.Controls/ProgressBar.cs
+++ b/src/Avalonia.Controls/ProgressBar.cs
@@ -96,7 +96,7 @@ namespace Avalonia.Controls
 
         private double _indeterminateStartingOffset;
         private double _indeterminateEndingOffset;
-        private Border _indicator;
+        private Border? _indicator;
 
         public static readonly StyledProperty<bool> IsIndeterminateProperty =
             AvaloniaProperty.Register<ProgressBar, bool>(nameof(IsIndeterminate));
diff --git a/src/Avalonia.Controls/RadioButton.cs b/src/Avalonia.Controls/RadioButton.cs
index 681278d247..9dbc5f040e 100644
--- a/src/Avalonia.Controls/RadioButton.cs
+++ b/src/Avalonia.Controls/RadioButton.cs
@@ -22,7 +22,7 @@ namespace Avalonia.Controls
             readonly Dictionary<string, List<WeakReference<RadioButton>>> s_registeredGroups
                 = new Dictionary<string, List<WeakReference<RadioButton>>>();
 
-            public static RadioButtonGroupManager GetOrCreateForRoot(IRenderRoot root)
+            public static RadioButtonGroupManager GetOrCreateForRoot(IRenderRoot? root)
             {
                 if (root == null)
                     return Default;
@@ -33,7 +33,7 @@ namespace Avalonia.Controls
             {
                 lock (s_registeredGroups)
                 {
-                    string groupName = radioButton.GroupName;
+                    string groupName = radioButton.GroupName!;
                     if (!s_registeredGroups.TryGetValue(groupName, out var group))
                     {
                         group = new List<WeakReference<RadioButton>>();
@@ -71,7 +71,7 @@ namespace Avalonia.Controls
             {
                 lock (s_registeredGroups)
                 {
-                    string groupName = radioButton.GroupName;
+                    string groupName = radioButton.GroupName!;
                     if (s_registeredGroups.TryGetValue(groupName, out var group))
                     {
                         int i = 0;
@@ -95,21 +95,21 @@ namespace Avalonia.Controls
             }
         }
 
-        public static readonly DirectProperty<RadioButton, string> GroupNameProperty =
-            AvaloniaProperty.RegisterDirect<RadioButton, string>(
+        public static readonly DirectProperty<RadioButton, string?> GroupNameProperty =
+            AvaloniaProperty.RegisterDirect<RadioButton, string?>(
                 nameof(GroupName),
                 o => o.GroupName,
                 (o, v) => o.GroupName = v);
 
-        private string _groupName;
-        private RadioButtonGroupManager _groupManager;
+        private string? _groupName;
+        private RadioButtonGroupManager? _groupManager;
 
         public RadioButton()
         {
             this.GetObservable(IsCheckedProperty).Subscribe(IsCheckedChanged);
         }
 
-        public string GroupName
+        public string? GroupName
         {
             get { return _groupName; }
             set { SetGroupName(value); }
@@ -127,7 +127,7 @@ namespace Avalonia.Controls
         {
             if (!string.IsNullOrEmpty(GroupName))
             {
-                _groupManager?.Remove(this, _groupName);
+                _groupManager?.Remove(this, GroupName);
 
                 _groupManager = RadioButtonGroupManager.GetOrCreateForRoot(e.Root);
 
@@ -142,13 +142,13 @@ namespace Avalonia.Controls
 
             if (!string.IsNullOrEmpty(GroupName))
             {
-                _groupManager?.Remove(this, _groupName);
+                _groupManager?.Remove(this, GroupName);
             }
         }
 
-        private void SetGroupName(string newGroupName)
+        private void SetGroupName(string? newGroupName)
         {
-            string oldGroupName = GroupName;
+            var oldGroupName = GroupName;
             if (newGroupName != oldGroupName)
             {
                 if (!string.IsNullOrEmpty(oldGroupName))
@@ -169,7 +169,7 @@ namespace Avalonia.Controls
 
         private void IsCheckedChanged(bool? value)
         {
-            string groupName = GroupName;
+            var groupName = GroupName;
             if (string.IsNullOrEmpty(groupName))
             {
                 var parent = this.GetVisualParent();
diff --git a/src/Avalonia.Controls/Remote/RemoteServer.cs b/src/Avalonia.Controls/Remote/RemoteServer.cs
index 419792f004..f4cc91a0e6 100644
--- a/src/Avalonia.Controls/Remote/RemoteServer.cs
+++ b/src/Avalonia.Controls/Remote/RemoteServer.cs
@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Remote
             //TODO: Somehow react on closed connection?
         }
 
-        public object Content
+        public object? Content
         {
             get => _topLevel.Content;
             set => _topLevel.Content = value;
diff --git a/src/Avalonia.Controls/Remote/RemoteWidget.cs b/src/Avalonia.Controls/Remote/RemoteWidget.cs
index b839a8769a..a88e0fd3d8 100644
--- a/src/Avalonia.Controls/Remote/RemoteWidget.cs
+++ b/src/Avalonia.Controls/Remote/RemoteWidget.cs
@@ -18,8 +18,8 @@ namespace Avalonia.Controls.Remote
         }
 
         private readonly IAvaloniaRemoteTransportConnection _connection;
-        private FrameMessage _lastFrame;
-        private WriteableBitmap _bitmap;
+        private FrameMessage? _lastFrame;
+        private WriteableBitmap? _bitmap;
         public RemoteWidget(IAvaloniaRemoteTransportConnection connection)
         {
             Mode = SizingMode.Local;
diff --git a/src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs b/src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs
index 931c27c575..c9fd1dc3b8 100644
--- a/src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs
+++ b/src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs
@@ -21,22 +21,22 @@ namespace Avalonia.Controls.Remote.Server
     public class RemoteServerTopLevelImpl : OffscreenTopLevelImplBase, IFramebufferPlatformSurface
     {
         private readonly IAvaloniaRemoteTransportConnection _transport;
-        private LockedFramebuffer _framebuffer;
+        private LockedFramebuffer? _framebuffer;
         private object _lock = new object();
         private long _lastSentFrame = -1;
         private long _lastReceivedFrame = -1;
         private long _nextFrameNumber = 1;
-        private ClientViewportAllocatedMessage _pendingAllocation;
+        private ClientViewportAllocatedMessage? _pendingAllocation;
         private bool _invalidated;
         private Vector _dpi = new Vector(96, 96);
-        private ProtocolPixelFormat[] _supportedFormats;
+        private ProtocolPixelFormat[]? _supportedFormats;
 
         public RemoteServerTopLevelImpl(IAvaloniaRemoteTransportConnection transport)
         {
             _transport = transport;
             _transport.OnMessage += OnMessage;
 
-            KeyboardDevice = AvaloniaLocator.Current.GetService<IKeyboardDevice>();
+            KeyboardDevice = AvaloniaLocator.Current.GetRequiredService<IKeyboardDevice>();
         }
 
         private static RawPointerEventType GetAvaloniaEventType (Avalonia.Remote.Protocol.Input.MouseButton button, bool pressed)
@@ -155,7 +155,7 @@ namespace Avalonia.Controls.Remote.Server
                                 ClientViewportAllocatedMessage allocation;
                                 lock (_lock)
                                 {
-                                    allocation = _pendingAllocation;
+                                    allocation = _pendingAllocation!;
                                     _pendingAllocation = null;
                                 }
                                 _dpi = new Vector(allocation.DpiX, allocation.DpiY);
@@ -173,7 +173,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawPointerEventArgs(
                             MouseDevice, 
                             0, 
-                            InputRoot, 
+                            InputRoot!, 
                             RawPointerEventType.Move, 
                             new Point(pointer.X, pointer.Y), 
                             GetAvaloniaInputModifiers(pointer.Modifiers)));
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawPointerEventArgs(
                             MouseDevice,
                             0,
-                            InputRoot,
+                            InputRoot!,
                             GetAvaloniaEventType(pressed.Button, true),
                             new Point(pressed.X, pressed.Y),
                             GetAvaloniaInputModifiers(pressed.Modifiers)));
@@ -199,7 +199,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawPointerEventArgs(
                             MouseDevice,
                             0,
-                            InputRoot,
+                            InputRoot!,
                             GetAvaloniaEventType(released.Button, false),
                             new Point(released.X, released.Y),
                             GetAvaloniaInputModifiers(released.Modifiers)));
@@ -212,7 +212,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawMouseWheelEventArgs(
                             MouseDevice,
                             0,
-                            InputRoot,
+                            InputRoot!,
                             new Point(scroll.X, scroll.Y),
                             new Vector(scroll.DeltaX, scroll.DeltaY),
                             GetAvaloniaInputModifiers(scroll.Modifiers)));
@@ -227,7 +227,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawKeyEventArgs(
                             KeyboardDevice,
                             0,
-                            InputRoot,
+                            InputRoot!,
                             key.IsDown ? RawKeyEventType.KeyDown : RawKeyEventType.KeyUp,
                             (Key)key.Key,
                             GetAvaloniaRawInputModifiers(key.Modifiers)));
@@ -242,7 +242,7 @@ namespace Avalonia.Controls.Remote.Server
                         Input?.Invoke(new RawTextInputEventArgs(
                             KeyboardDevice,
                             0,
-                            InputRoot,
+                            InputRoot!,
                             text.Text));
                     }, DispatcherPriority.Input);
                 }
@@ -257,7 +257,7 @@ namespace Avalonia.Controls.Remote.Server
 
         protected virtual Size Measure(Size constraint)
         {
-            var l = (ILayoutable) InputRoot;
+            var l = (ILayoutable) InputRoot!;
             l.Measure(constraint);
             return l.DesiredSize;
         }
@@ -294,7 +294,7 @@ namespace Avalonia.Controls.Remote.Server
             return new FrameMessage
             {
                 Data = data,
-                Format = (ProtocolPixelFormat) format,
+                Format = fmt,
                 Width = width,
                 Height = height,
                 Stride = width * bpp,
diff --git a/src/Avalonia.Controls/RepeatButton.cs b/src/Avalonia.Controls/RepeatButton.cs
index a21725cadf..0415a78721 100644
--- a/src/Avalonia.Controls/RepeatButton.cs
+++ b/src/Avalonia.Controls/RepeatButton.cs
@@ -21,7 +21,7 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<int> DelayProperty =
             AvaloniaProperty.Register<Button, int>(nameof(Delay), 300);
 
-        private DispatcherTimer _repeatTimer;
+        private DispatcherTimer? _repeatTimer;
 
         /// <summary>
         /// Gets or sets the amount of time, in milliseconds, of repeating clicks.
@@ -55,10 +55,10 @@ namespace Avalonia.Controls
             _repeatTimer.Start();
         }
 
-        private void RepeatTimerOnTick(object sender, EventArgs e)
+        private void RepeatTimerOnTick(object? sender, EventArgs e)
         {
             var interval = TimeSpan.FromMilliseconds(Interval);
-            if (_repeatTimer.Interval != interval)
+            if (_repeatTimer!.Interval != interval)
             {
                 _repeatTimer.Interval = interval;
             }
diff --git a/src/Avalonia.Controls/Repeater/ElementFactory.cs b/src/Avalonia.Controls/Repeater/ElementFactory.cs
index 644e077221..6b776803d7 100644
--- a/src/Avalonia.Controls/Repeater/ElementFactory.cs
+++ b/src/Avalonia.Controls/Repeater/ElementFactory.cs
@@ -4,7 +4,7 @@ namespace Avalonia.Controls
 {
     public abstract class ElementFactory : IElementFactory
     {
-        public IControl Build(object data)
+        public IControl Build(object? data)
         {
             return GetElementCore(new ElementFactoryGetArgs { Data = data });
         }
@@ -14,7 +14,7 @@ namespace Avalonia.Controls
             return GetElementCore(args);
         }
 
-        public bool Match(object data) => true;
+        public bool Match(object? data) => true;
 
         public void RecycleElement(ElementFactoryRecycleArgs args)
         {
diff --git a/src/Avalonia.Controls/Repeater/IElementFactory.cs b/src/Avalonia.Controls/Repeater/IElementFactory.cs
index f424ae29b7..daab7e855f 100644
--- a/src/Avalonia.Controls/Repeater/IElementFactory.cs
+++ b/src/Avalonia.Controls/Repeater/IElementFactory.cs
@@ -12,13 +12,13 @@ namespace Avalonia.Controls
         /// Gets or sets the data item for which an appropriate element tree should be realized
         /// when calling <see cref="IElementFactory.GetElement"/>.
         /// </summary>
-        public object Data { get; set; }
+        public object? Data { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="IControl"/> that is expected to be the parent of the
         /// realized element from <see cref="IElementFactory.GetElement"/>.
         /// </summary>
-        public IControl Parent { get; set; }
+        public IControl? Parent { get; set; }
 
         /// <summary>
         /// Gets or sets the index of the item that should be realized.
@@ -36,13 +36,13 @@ namespace Avalonia.Controls
         /// Gets or sets the <see cref="IControl"/> to recycle when calling 
         /// <see cref="IElementFactory.RecycleElement"/>.
         /// </summary>
-        public IControl Element { get; set; }
+        public IControl? Element { get; set; }
 
         /// <summary>
         /// Gets or sets the <see cref="IControl"/> that is expected to be the parent of the
         /// realized element from <see cref="IElementFactory.GetElement"/>.
         /// </summary>
-        public IControl Parent { get; set; }
+        public IControl? Parent { get; set; }
     }
 
     /// <summary>
diff --git a/src/Avalonia.Controls/Repeater/ItemTemplateWrapper.cs b/src/Avalonia.Controls/Repeater/ItemTemplateWrapper.cs
index dd97cde218..eb3b2ea787 100644
--- a/src/Avalonia.Controls/Repeater/ItemTemplateWrapper.cs
+++ b/src/Avalonia.Controls/Repeater/ItemTemplateWrapper.cs
@@ -13,8 +13,8 @@ namespace Avalonia.Controls
 
         public ItemTemplateWrapper(IDataTemplate dataTemplate) => _dataTemplate = dataTemplate;
 
-        public IControl Build(object param) => GetElement(null, param);
-        public bool Match(object data) => _dataTemplate.Match(data);
+        public IControl Build(object? param) => GetElement(null, param);
+        public bool Match(object? data) => _dataTemplate.Match(data);
 
         public IControl GetElement(ElementFactoryGetArgs args)
         {
@@ -23,14 +23,14 @@ namespace Avalonia.Controls
 
         public void RecycleElement(ElementFactoryRecycleArgs args)
         {
-            RecycleElement(args.Parent, args.Element);
+            RecycleElement(args.Parent, args.Element!);
         }
 
-        private IControl GetElement(IControl parent, object data)
+        private IControl GetElement(IControl? parent, object? data)
         {
             var selectedTemplate = _dataTemplate;
             var recyclePool = RecyclePool.GetPoolInstance(selectedTemplate);
-            IControl element = null;
+            IControl? element = null;
 
             if (recyclePool != null)
             {
@@ -41,7 +41,7 @@ namespace Avalonia.Controls
             if (element == null)
             {
                 // no element was found in recycle pool, create a new element
-                element = selectedTemplate.Build(data);
+                element = selectedTemplate.Build(data)!;
 
                 // Associate template with element
                 element.SetValue(RecyclePool.OriginTemplateProperty, selectedTemplate);
@@ -50,7 +50,7 @@ namespace Avalonia.Controls
             return element;
         }
 
-        private void RecycleElement(IControl parent, IControl element)
+        private void RecycleElement(IControl? parent, IControl element)
         {
             var selectedTemplate = _dataTemplate;
             var recyclePool = RecyclePool.GetPoolInstance(selectedTemplate);
diff --git a/src/Avalonia.Controls/Repeater/ItemsRepeater.cs b/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
index b40cf26df5..09c0e58332 100644
--- a/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
+++ b/src/Avalonia.Controls/Repeater/ItemsRepeater.cs
@@ -31,13 +31,13 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="ItemTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ItemTemplateProperty =
+        public static readonly StyledProperty<IDataTemplate?> ItemTemplateProperty =
             ItemsControl.ItemTemplateProperty.AddOwner<ItemsRepeater>();
 
         /// <summary>
         /// Defines the <see cref="Items"/> property.
         /// </summary>
-        public static readonly DirectProperty<ItemsRepeater, IEnumerable> ItemsProperty =
+        public static readonly DirectProperty<ItemsRepeater, IEnumerable?> ItemsProperty =
             ItemsControl.ItemsProperty.AddOwner<ItemsRepeater>(o => o.Items, (o, v) => o.Items = v);
 
         /// <summary>
@@ -60,14 +60,14 @@ namespace Avalonia.Controls
 
         private readonly ViewManager _viewManager;
         private readonly ViewportManager _viewportManager;
-        private IEnumerable _items;
-        private VirtualizingLayoutContext _layoutContext;
-        private EventHandler<ChildIndexChangedEventArgs> _childIndexChanged;
+        private IEnumerable? _items;
+        private VirtualizingLayoutContext? _layoutContext;
+        private EventHandler<ChildIndexChangedEventArgs>? _childIndexChanged;
         private bool _isLayoutInProgress;
-        private NotifyCollectionChangedEventArgs _processingItemsSourceChange;
-        private ItemsRepeaterElementPreparedEventArgs _elementPreparedArgs;
-        private ItemsRepeaterElementClearingEventArgs _elementClearingArgs;
-        private ItemsRepeaterElementIndexChangedEventArgs _elementIndexChangedArgs;
+        private NotifyCollectionChangedEventArgs? _processingItemsSourceChange;
+        private ItemsRepeaterElementPreparedEventArgs? _elementPreparedArgs;
+        private ItemsRepeaterElementClearingEventArgs? _elementClearingArgs;
+        private ItemsRepeaterElementIndexChangedEventArgs? _elementIndexChangedArgs;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemsRepeater"/> class.
@@ -102,7 +102,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets an object source used to generate the content of the ItemsRepeater.
         /// </summary>
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get => _items;
             set => SetAndRaise(ItemsProperty, ref _items, value);
@@ -111,7 +111,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the template used to display each item.
         /// </summary>
-        public IDataTemplate ItemTemplate
+        public IDataTemplate? ItemTemplate
         {
             get => GetValue(ItemTemplateProperty);
             set => SetValue(ItemTemplateProperty, value);
@@ -141,14 +141,14 @@ namespace Avalonia.Controls
         /// Gets a standardized view of the supported interactions between a given Items object and
         /// the ItemsRepeater control and its associated components.
         /// </summary>
-        public ItemsSourceView ItemsSourceView { get; private set; }
+        public ItemsSourceView? ItemsSourceView { get; private set; }
 
-        internal IElementFactory ItemTemplateShim { get; set; }
+        internal IElementFactory? ItemTemplateShim { get; set; }
         internal Point LayoutOrigin { get; set; }
-        internal object LayoutState { get; set; }
-        internal IControl MadeAnchor => _viewportManager.MadeAnchor;
+        internal object? LayoutState { get; set; }
+        internal IControl? MadeAnchor => _viewportManager.MadeAnchor;
         internal Rect RealizationWindow => _viewportManager.GetLayoutRealizationWindow();
-        internal IControl SuggestedAnchor => _viewportManager.SuggestedAnchor;
+        internal IControl? SuggestedAnchor => _viewportManager.SuggestedAnchor;
 
         private bool IsProcessingCollectionChange => _processingItemsSourceChange != null;
 
@@ -165,7 +165,7 @@ namespace Avalonia.Controls
             }
         }
 
-        event EventHandler<ChildIndexChangedEventArgs> IChildIndexProvider.ChildIndexChanged
+        event EventHandler<ChildIndexChangedEventArgs>? IChildIndexProvider.ChildIndexChanged
         {
             add => _childIndexChanged += value;
             remove => _childIndexChanged -= value;
@@ -180,7 +180,7 @@ namespace Avalonia.Controls
 
         bool IChildIndexProvider.TryGetTotalCount(out int count)
         {
-            count = ItemsSourceView.Count;
+            count = ItemsSourceView?.Count ?? 0;
             return true;
         }
 
@@ -192,7 +192,7 @@ namespace Avalonia.Controls
         /// outside the range of realized items. Elements are cleared when they become available
         /// for re-use.
         /// </remarks>
-        public event EventHandler<ItemsRepeaterElementClearingEventArgs> ElementClearing;
+        public event EventHandler<ItemsRepeaterElementClearingEventArgs>? ElementClearing;
 
         /// <summary>
         /// Occurs for each realized <see cref="IControl"/> when the index for the item it
@@ -207,7 +207,7 @@ namespace Avalonia.Controls
         /// represents has changed. For example, when another item is added or removed in the data
         /// source, the index for items that come after in the ordering will be impacted.
         /// </remarks>
-        public event EventHandler<ItemsRepeaterElementIndexChangedEventArgs> ElementIndexChanged;
+        public event EventHandler<ItemsRepeaterElementIndexChangedEventArgs>? ElementIndexChanged;
 
         /// <summary>
         /// Occurs each time an element is prepared for use.
@@ -216,7 +216,7 @@ namespace Avalonia.Controls
         /// The prepared element might be newly created or an existing element that is being re-
         /// used.
         /// </remarks>
-        public event EventHandler<ItemsRepeaterElementPreparedEventArgs> ElementPrepared;
+        public event EventHandler<ItemsRepeaterElementPreparedEventArgs>? ElementPrepared;
 
         /// <summary>
         /// Retrieves the index of the item from the data source that corresponds to the specified
@@ -240,7 +240,7 @@ namespace Avalonia.Controls
         /// he UIElement that corresponds to the item at the specified index if the item is
         /// realized, or null if the item is not realized.
         /// </returns>
-        public IControl TryGetElement(int index) => GetElementFromIndexImpl(index);
+        public IControl? TryGetElement(int index) => GetElementFromIndexImpl(index);
 
         /// <summary>
         /// Retrieves the UIElement that corresponds to the item at the specified index in the
@@ -497,9 +497,9 @@ namespace Avalonia.Controls
             return -1;
         }
 
-        private IControl GetElementFromIndexImpl(int index)
+        private IControl? GetElementFromIndexImpl(int index)
         {
-            IControl result = null;
+            IControl? result = null;
 
             var children = Children;
             for (var i = 0; i < children.Count && result == null; ++i)
@@ -517,7 +517,7 @@ namespace Avalonia.Controls
 
         private IControl GetOrCreateElementImpl(int index)
         {
-            if (index >= 0 && index >= ItemsSourceView.Count)
+            if (index >= 0 && index >= (ItemsSourceView?.Count ?? 0))
             {
                 throw new ArgumentException("Argument index is invalid.", "index");
             }
@@ -544,7 +544,7 @@ namespace Avalonia.Controls
             _viewportManager.OnMakeAnchor(element, isAnchorOutsideRealizedRange);
             InvalidateMeasure();
 
-            return element;
+            return element!;
         }
 
         internal void OnElementPrepared(IControl element, VirtualizationInfo virtInfo)
@@ -608,7 +608,7 @@ namespace Avalonia.Controls
             _childIndexChanged?.Invoke(this, new ChildIndexChangedEventArgs(element));
         }
 
-        private void OnDataSourcePropertyChanged(ItemsSourceView oldValue, ItemsSourceView newValue)
+        private void OnDataSourcePropertyChanged(ItemsSourceView? oldValue, ItemsSourceView? newValue)
         {
             if (_isLayoutInProgress)
             {
@@ -664,7 +664,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnItemTemplateChanged(IDataTemplate oldValue, IDataTemplate newValue)
+        private void OnItemTemplateChanged(IDataTemplate? oldValue, IDataTemplate? newValue)
         {
             if (_isLayoutInProgress && oldValue != null)
             {
@@ -705,12 +705,17 @@ namespace Avalonia.Controls
                 }
             }
 
-            ItemTemplateShim = newValue as IElementFactory ?? new ItemTemplateWrapper(newValue);
+            ItemTemplateShim = newValue switch
+            {
+                IElementFactory factory => factory,
+                null => null,
+                _ => new ItemTemplateWrapper(newValue)
+            };
 
             InvalidateMeasure();
         }
 
-        private void OnLayoutChanged(AttachedLayout oldValue, AttachedLayout newValue)
+        private void OnLayoutChanged(AttachedLayout? oldValue, AttachedLayout? newValue)
         {
             if (_isLayoutInProgress)
             {
@@ -751,7 +756,7 @@ namespace Avalonia.Controls
             InvalidateMeasure();
         }
 
-        private void OnItemsSourceViewChanged(object sender, NotifyCollectionChangedEventArgs args)
+        private void OnItemsSourceViewChanged(object? sender, NotifyCollectionChangedEventArgs args)
         {
             if (_isLayoutInProgress)
             {
diff --git a/src/Avalonia.Controls/Repeater/RecyclePool.cs b/src/Avalonia.Controls/Repeater/RecyclePool.cs
index 28f299043c..9e2da81f99 100644
--- a/src/Avalonia.Controls/Repeater/RecyclePool.cs
+++ b/src/Avalonia.Controls/Repeater/RecyclePool.cs
@@ -22,7 +22,7 @@ namespace Avalonia.Controls
         private static ConditionalWeakTable<IDataTemplate, RecyclePool> s_pools = new ConditionalWeakTable<IDataTemplate, RecyclePool>();
         private readonly Dictionary<string, List<ElementInfo>> _elements = new Dictionary<string, List<ElementInfo>>();
 
-        public static RecyclePool GetPoolInstance(IDataTemplate dataTemplate)
+        public static RecyclePool? GetPoolInstance(IDataTemplate dataTemplate)
         {
             s_pools.TryGetValue(dataTemplate, out var result);
             return result;
@@ -30,7 +30,7 @@ namespace Avalonia.Controls
 
         public static void SetPoolInstance(IDataTemplate dataTemplate, RecyclePool value) => s_pools.Add(dataTemplate, value);
 
-        public void PutElement(IControl element, string key, IControl owner)
+        public void PutElement(IControl element, string key, IControl? owner)
         {
             var ownerAsPanel = EnsureOwnerIsPanelOrNull(owner);
             var elementInfo = new ElementInfo(element, ownerAsPanel);
@@ -44,7 +44,7 @@ namespace Avalonia.Controls
             pool.Add(elementInfo);
         }
 
-        public IControl TryGetElement(string key, IControl owner)
+        public IControl? TryGetElement(string key, IControl? owner)
         {
             if (_elements.TryGetValue(key, out var elements))
             {
@@ -54,10 +54,10 @@ namespace Avalonia.Controls
                     // the enter/leave cost during recycling.
                     // TODO: prioritize elements with the same owner to those without an owner.
                     var elementInfo = elements.FirstOrDefault(x => x.Owner == owner) ?? elements.LastOrDefault();
-                    elements.Remove(elementInfo);
+                    elements.Remove(elementInfo!);
 
                     var ownerAsPanel = EnsureOwnerIsPanelOrNull(owner);
-                    if (elementInfo.Owner != null && elementInfo.Owner != ownerAsPanel)
+                    if (elementInfo!.Owner != null && elementInfo.Owner != ownerAsPanel)
                     {
                         // Element is still under its parent. remove it from its parent.
                         var panel = elementInfo.Owner;
@@ -83,7 +83,7 @@ namespace Avalonia.Controls
         internal string GetReuseKey(IControl element) => element.GetValue(ReuseKeyProperty);
         internal void SetReuseKey(IControl element, string value) => element.SetValue(ReuseKeyProperty, value);
 
-        private IPanel EnsureOwnerIsPanelOrNull(IControl owner)
+        private IPanel? EnsureOwnerIsPanelOrNull(IControl? owner)
         {
             if (owner is IPanel panel)
             {
@@ -99,14 +99,14 @@ namespace Avalonia.Controls
 
         private class ElementInfo
         {
-            public ElementInfo(IControl element, IPanel owner)
+            public ElementInfo(IControl element, IPanel? owner)
             {
                 Element = element;
                 Owner = owner;
             }
 
             public IControl Element { get; }
-            public IPanel Owner { get;}
+            public IPanel? Owner { get;}
         }
     }
 }
diff --git a/src/Avalonia.Controls/Repeater/RecyclingElementFactory.cs b/src/Avalonia.Controls/Repeater/RecyclingElementFactory.cs
index 9503239e34..877df3741e 100644
--- a/src/Avalonia.Controls/Repeater/RecyclingElementFactory.cs
+++ b/src/Avalonia.Controls/Repeater/RecyclingElementFactory.cs
@@ -71,7 +71,7 @@ namespace Avalonia.Controls
                 }
 
                 var dataTemplate = Templates[templateKey];
-                element = dataTemplate.Build(args.Data);
+                element = dataTemplate.Build(args.Data)!;
 
                 // Associate ReuseKey with element
                 RecyclePool.SetReuseKey(element, templateKey);
@@ -82,12 +82,12 @@ namespace Avalonia.Controls
 
         protected override void RecycleElementCore(ElementFactoryRecycleArgs args)
         {
-            var element = args.Element;
+            var element = args.Element!;
             var key = RecyclePool.GetReuseKey(element);
             RecyclePool.PutElement(element, key, args.Parent);
         }
 
-        protected virtual string OnSelectTemplateKeyCore(object dataContext, IControl owner)
+        protected virtual string OnSelectTemplateKeyCore(object? dataContext, IControl? owner)
         {
             if (SelectTemplateKey is object)
             {
diff --git a/src/Avalonia.Controls/Repeater/RepeaterLayoutContext.cs b/src/Avalonia.Controls/Repeater/RepeaterLayoutContext.cs
index 733b045e1b..3370b36801 100644
--- a/src/Avalonia.Controls/Repeater/RepeaterLayoutContext.cs
+++ b/src/Avalonia.Controls/Repeater/RepeaterLayoutContext.cs
@@ -26,7 +26,7 @@ namespace Avalonia.Controls
             set => _owner.LayoutOrigin = value;
         }
 
-        protected override object LayoutStateCore
+        protected override object? LayoutStateCore
         {
             get => _owner.LayoutState;
             set => _owner.LayoutState = value;
@@ -57,7 +57,7 @@ namespace Avalonia.Controls
                 options.HasAllFlags(ElementRealizationOptions.SuppressAutoRecycle));
         }
 
-        protected override object GetItemAtCore(int index) => _owner.ItemsSourceView.GetAt(index);
+        protected override object GetItemAtCore(int index) => _owner.ItemsSourceView!.GetAt(index)!;
 
         protected override void RecycleElementCore(ILayoutable element)
         {
diff --git a/src/Avalonia.Controls/Repeater/UniqueIdElementPool.cs b/src/Avalonia.Controls/Repeater/UniqueIdElementPool.cs
index 775aa3f113..778b8bebcc 100644
--- a/src/Avalonia.Controls/Repeater/UniqueIdElementPool.cs
+++ b/src/Avalonia.Controls/Repeater/UniqueIdElementPool.cs
@@ -20,7 +20,7 @@ namespace Avalonia.Controls
         public void Add(IControl element)
         {
             var virtInfo = ItemsRepeater.GetVirtualizationInfo(element);
-            var key = virtInfo.UniqueId;
+            var key = virtInfo.UniqueId!;
 
             if (_elementMap.ContainsKey(key))
             {
@@ -30,10 +30,10 @@ namespace Avalonia.Controls
             _elementMap.Add(key, element);
         }
 
-        public IControl Remove(int index)
+        public IControl? Remove(int index)
         {
             // Check if there is already a element in the mapping and if so, use it.
-            string key = _owner.ItemsSourceView.KeyFromIndex(index);
+            string key = _owner.ItemsSourceView!.KeyFromIndex(index);
 
             if (_elementMap.TryGetValue(key, out var element))
             {
diff --git a/src/Avalonia.Controls/Repeater/ViewManager.cs b/src/Avalonia.Controls/Repeater/ViewManager.cs
index a7b6cf7f18..b28753b518 100644
--- a/src/Avalonia.Controls/Repeater/ViewManager.cs
+++ b/src/Avalonia.Controls/Repeater/ViewManager.cs
@@ -22,10 +22,10 @@ namespace Avalonia.Controls
         private readonly ItemsRepeater _owner;
         private readonly List<PinnedElementInfo> _pinnedPool = new List<PinnedElementInfo>();
         private readonly UniqueIdElementPool _resetPool;
-        private IControl _lastFocusedElement;
+        private IControl? _lastFocusedElement;
         private bool _isDataSourceStableResetPending;
-        private ElementFactoryGetArgs _elementFactoryGetArgs;
-        private ElementFactoryRecycleArgs _elementFactoryRecycleArgs;
+        private ElementFactoryGetArgs? _elementFactoryGetArgs;
+        private ElementFactoryRecycleArgs? _elementFactoryRecycleArgs;
         private int _firstRealizedElementIndexHeldByLayout = FirstRealizedElementIndexDefault;
         private int _lastRealizedElementIndexHeldByLayout = LastRealizedElementIndexDefault;
         private bool _eventsSubscribed;
@@ -170,7 +170,7 @@ namespace Avalonia.Controls
                 _lastFocusedElement = focusedChild;
 
                 // Add pin to hold the focused element.
-                UpdatePin(focusedChild, true /* addPin */);
+                UpdatePin(focusedChild!, true /* addPin */);
             }
             else
             {
@@ -179,14 +179,14 @@ namespace Avalonia.Controls
             }
         }
 
-        IControl FindFocusCandidate(int clearedIndex, out IControl focusedChild)
+        IControl? FindFocusCandidate(int clearedIndex, out IControl? focusedChild)
         {
             // Walk through all the children and find elements with index before and after the cleared index.
             // Note that during a delete the next element would now have the same index.
             int previousIndex = int.MinValue;
             int nextIndex = int.MaxValue;
-            IControl nextElement = null;
-            IControl previousElement = null;
+            IControl? nextElement = null;
+            IControl? previousElement = null;
 
             foreach (var child in _owner.Children)
             {
@@ -287,7 +287,7 @@ namespace Avalonia.Controls
             }
         }
 
-        public void OnItemsSourceChanged(object sender, NotifyCollectionChangedEventArgs args)
+        public void OnItemsSourceChanged(object? sender, NotifyCollectionChangedEventArgs args)
         {
             // Note: For items that have been removed, the index will not be touched. It will hold
             // the old index before it was removed. It is not valid anymore.
@@ -296,7 +296,7 @@ namespace Avalonia.Controls
                 case NotifyCollectionChangedAction.Add:
                     {
                         var newIndex = args.NewStartingIndex;
-                        var newCount = args.NewItems.Count;
+                        var newCount = args.NewItems!.Count;
                         EnsureFirstLastRealizedIndices();
                         if (newIndex <= _lastRealizedElementIndexHeldByLayout)
                         {
@@ -343,8 +343,8 @@ namespace Avalonia.Controls
                         //         depending on the counts.
                         var oldStartIndex = args.OldStartingIndex;
                         var newStartingIndex = args.NewStartingIndex;
-                        var oldCount = args.OldItems.Count;
-                        var newCount = args.NewItems.Count;
+                        var oldCount = args.OldItems!.Count;
+                        var newCount = args.NewItems!.Count;
                         if (oldStartIndex != newStartingIndex)
                         {
                             throw new NotSupportedException("Replace is only allowed with OldStartingIndex equals to NewStartingIndex.");
@@ -388,7 +388,7 @@ namespace Avalonia.Controls
                 case NotifyCollectionChangedAction.Remove:
                     {
                         var oldStartIndex = args.OldStartingIndex;
-                        var oldCount = args.OldItems.Count;
+                        var oldCount = args.OldItems!.Count;
                         foreach (var element in _owner.Children)
                         {
                             var virtInfo = ItemsRepeater.GetVirtualizationInfo(element);
@@ -417,7 +417,7 @@ namespace Avalonia.Controls
                     // running layout, we dont have to clear all the elements again.
                     if (!_isDataSourceStableResetPending)
                     {
-                        if (_owner.ItemsSourceView.HasKeyIndexMapping)
+                        if (_owner.ItemsSourceView!.HasKeyIndexMapping)
                         {
                             _isDataSourceStableResetPending = true;
                         }
@@ -482,9 +482,9 @@ namespace Avalonia.Controls
         // If an index that is realized is requested by the layout, we unfortunately have to walk the
         // children. Not ideal, but a reasonable default to provide consistent behavior between virtualizing
         // and non-virtualizing hosts.
-        private IControl GetElementIfAlreadyHeldByLayout(int index)
+        private IControl? GetElementIfAlreadyHeldByLayout(int index)
         {
-            IControl element = null;
+            IControl? element = null;
 
             bool cachedFirstLastIndicesInvalid = _firstRealizedElementIndexHeldByLayout == FirstRealizedElementIndexDefault;
             bool isRequestedIndexInRealizedRange = (_firstRealizedElementIndexHeldByLayout <= index && index <= _lastRealizedElementIndexHeldByLayout);
@@ -518,9 +518,9 @@ namespace Avalonia.Controls
             return element;
         }
 
-        private IControl GetElementFromUniqueIdResetPool(int index)
+        private IControl? GetElementFromUniqueIdResetPool(int index)
         {
-            IControl element = null;
+            IControl? element = null;
             // See if you can get it from the reset pool.
             if (_isDataSourceStableResetPending)
             {
@@ -541,9 +541,9 @@ namespace Avalonia.Controls
             return element;
         }
 
-        private IControl GetElementFromPinnedElements(int index)
+        private IControl? GetElementFromPinnedElements(int index)
         {
-            IControl element = null;
+            IControl? element = null;
 
             // See if you can find something among the pinned elements.
             for (var i = 0; i < _pinnedPool.Count; ++i)
@@ -583,7 +583,7 @@ namespace Avalonia.Controls
         private IControl GetElementFromElementFactory(int index)
         {
             // The view generator is the provider of last resort.
-            var data = _owner.ItemsSourceView.GetAt(index);
+            var data = _owner.ItemsSourceView!.GetAt(index);
             var providedElementFactory = _owner.ItemTemplateShim;
 
             IElementFactory GetElementFactory()
@@ -592,7 +592,7 @@ namespace Avalonia.Controls
                 {
                     var factory = FuncDataTemplate.Default;
                     _owner.ItemTemplate = factory;
-                    return _owner.ItemTemplateShim;
+                    return _owner.ItemTemplateShim!;
                 }
 
                 return providedElementFactory;
@@ -697,9 +697,9 @@ namespace Avalonia.Controls
 
         private void UpdateFocusedElement()
         {
-            IControl focusedElement = null;
+            IControl? focusedElement = null;
 
-            var child = FocusManager.Instance.Current;
+            var child = FocusManager.Instance?.Current;
 
             if (child != null)
             {
@@ -713,7 +713,9 @@ namespace Avalonia.Controls
                     if (parent is ItemsRepeater repeater)
                     {
                         var element = child as IControl;
-                        if (repeater == owner && ItemsRepeater.GetVirtualizationInfo(element).IsRealized)
+                        if (repeater == owner &&
+                            element is not null &&
+                            ItemsRepeater.GetVirtualizationInfo(element).IsRealized)
                         {
                             focusedElement = element;
                         }
@@ -722,7 +724,7 @@ namespace Avalonia.Controls
                     }
 
                     child = parent as IInputElement;
-                    parent = child.VisualParent;
+                    parent = child?.VisualParent;
                 }
             }
 
@@ -744,7 +746,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnFocusChanged(object sender, RoutedEventArgs e) => UpdateFocusedElement();
+        private void OnFocusChanged(object? sender, RoutedEventArgs e) => UpdateFocusedElement();
 
         private void EnsureEventSubscriptions()
         {
diff --git a/src/Avalonia.Controls/Repeater/ViewportManager.cs b/src/Avalonia.Controls/Repeater/ViewportManager.cs
index 1a90da5830..ec25fcb265 100644
--- a/src/Avalonia.Controls/Repeater/ViewportManager.cs
+++ b/src/Avalonia.Controls/Repeater/ViewportManager.cs
@@ -24,8 +24,8 @@ namespace Avalonia.Controls
         private const double CacheBufferPerSideInflationPixelDelta = 40.0;
         private readonly ItemsRepeater _owner;
         private bool _ensuredScroller;
-        private IScrollAnchorProvider _scroller;
-        private IControl _makeAnchorElement;
+        private IScrollAnchorProvider? _scroller;
+        private IControl? _makeAnchorElement;
         private bool _isAnchorOutsideRealizedRange;
         private Rect _visibleWindow;
         private Rect _layoutExtent;
@@ -57,7 +57,7 @@ namespace Avalonia.Controls
             _owner = owner;
         }
 
-        public IControl SuggestedAnchor
+        public IControl? SuggestedAnchor
         {
             get
             {
@@ -97,7 +97,7 @@ namespace Avalonia.Controls
 
         public bool HasScroller => _scroller != null;
 
-        public IControl MadeAnchor => _makeAnchorElement;
+        public IControl? MadeAnchor => _makeAnchorElement;
 
         public double HorizontalCacheLength
         {
@@ -213,7 +213,7 @@ namespace Avalonia.Controls
 
             // We just finished a measure pass and have a new extent.
             // Let's make sure the scrollers will run its arrange so that they track the anchor.
-            ((IControl)_scroller)?.InvalidateArrange();
+            ((IControl?)_scroller)?.InvalidateArrange();
         }
 
         public Point GetOrigin() => _layoutExtent.TopLeft;
@@ -294,7 +294,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnLayoutUpdated(object sender, EventArgs args)
+        private void OnLayoutUpdated(object? sender, EventArgs args)
         {
             _owner.LayoutUpdated -= OnLayoutUpdated;
             _layoutUpdatedSubscribed = false;
@@ -324,7 +324,7 @@ namespace Avalonia.Controls
             }
         }
 
-        public void OnMakeAnchor(IControl anchor, bool isAnchorOutsideRealizedRange)
+        public void OnMakeAnchor(IControl? anchor, bool isAnchorOutsideRealizedRange)
         {
             if (_makeAnchorElement != anchor)
             {
@@ -346,7 +346,7 @@ namespace Avalonia.Controls
 
                 // get the targetChild - i.e the immediate child of this repeater that is being brought into view.
                 // Note that the element being brought into view could be a descendant.
-                var targetChild = GetImmediateChildOfRepeater((IControl)args.TargetObject);
+                var targetChild = GetImmediateChildOfRepeater((IControl)args.TargetObject!);
 
                 if (targetChild is null)
                 {
@@ -382,14 +382,14 @@ namespace Avalonia.Controls
             _scroller?.RegisterAnchorCandidate(element);
         }
 
-        private IControl GetImmediateChildOfRepeater(IControl descendant)
+        private IControl? GetImmediateChildOfRepeater(IControl descendant)
         {
             var targetChild = descendant;
-            var parent = (IControl)descendant.VisualParent;
+            var parent = (IControl?)descendant.VisualParent;
             while (parent != null && parent != _owner)
             {
                 targetChild = parent;
-                parent = (IControl)parent.VisualParent;
+                parent = (IControl?)parent.VisualParent;
             }
 
             if (parent == null)
@@ -441,7 +441,7 @@ namespace Avalonia.Controls
             _ensuredScroller = false;
         }
 
-        private void OnEffectiveViewportChanged(object sender, EffectiveViewportChangedEventArgs e)
+        private void OnEffectiveViewportChanged(object? sender, EffectiveViewportChangedEventArgs e)
         {
             Logger.TryGet(LogEventLevel.Verbose, "Repeater")?.Log(this, "{LayoutId}: EffectiveViewportChanged event callback", _owner.Layout.LayoutId);
             UpdateViewport(e.EffectiveViewport);
diff --git a/src/Avalonia.Controls/Repeater/VirtualizationInfo.cs b/src/Avalonia.Controls/Repeater/VirtualizationInfo.cs
index 7e6b24f1b5..7378dbcfa1 100644
--- a/src/Avalonia.Controls/Repeater/VirtualizationInfo.cs
+++ b/src/Avalonia.Controls/Repeater/VirtualizationInfo.cs
@@ -39,7 +39,7 @@ namespace Avalonia.Controls
         public bool KeepAlive { get; set; }
         public bool IsRegisteredAsAnchorCandidate { get; set; }
         public ElementOwner Owner { get; private set; } = ElementOwner.ElementFactory;
-        public string UniqueId { get; private set; }
+        public string? UniqueId { get; private set; }
 
         public void MoveOwnershipToLayoutFromElementFactory(int index, string uniqueId)
         {
diff --git a/src/Avalonia.Controls/RequestBringIntoViewEventArgs.cs b/src/Avalonia.Controls/RequestBringIntoViewEventArgs.cs
index dd55de85d4..5396ad67a7 100644
--- a/src/Avalonia.Controls/RequestBringIntoViewEventArgs.cs
+++ b/src/Avalonia.Controls/RequestBringIntoViewEventArgs.cs
@@ -5,7 +5,7 @@ namespace Avalonia.Controls
 {
     public class RequestBringIntoViewEventArgs : RoutedEventArgs
     {
-        public IVisual TargetObject { get; set; }
+        public IVisual? TargetObject { get; set; }
 
         public Rect TargetRect { get; set; }
     }
diff --git a/src/Avalonia.Controls/Screens.cs b/src/Avalonia.Controls/Screens.cs
index 8a0a0fa728..ef438576a7 100644
--- a/src/Avalonia.Controls/Screens.cs
+++ b/src/Avalonia.Controls/Screens.cs
@@ -9,20 +9,20 @@ namespace Avalonia.Controls
 {
     public class Screens
     {
-        private readonly IScreenImpl _iScreenImpl;
+        private readonly IScreenImpl? _iScreenImpl;
 
-        public int ScreenCount => _iScreenImpl.ScreenCount;
+        public int ScreenCount => _iScreenImpl?.ScreenCount ?? 0;
         public IReadOnlyList<Screen> All => _iScreenImpl?.AllScreens ?? Array.Empty<Screen>();
-        public Screen Primary => All.FirstOrDefault(x => x.Primary);
+        public Screen? Primary => All.FirstOrDefault(x => x.Primary);
 
-        public Screens(IScreenImpl iScreenImpl)
+        public Screens(IScreenImpl? iScreenImpl)
         {
             _iScreenImpl = iScreenImpl;
         }
 
-        public Screen ScreenFromBounds(PixelRect bounds){
+        public Screen? ScreenFromBounds(PixelRect bounds){
         
-            Screen currMaxScreen = null;
+            Screen? currMaxScreen = null;
             double maxAreaSize = 0;
             foreach (Screen screen in All)
             {
@@ -41,12 +41,12 @@ namespace Avalonia.Controls
             return currMaxScreen;
         }
         
-        public Screen ScreenFromPoint(PixelPoint point)
+        public Screen? ScreenFromPoint(PixelPoint point)
         {
             return All.FirstOrDefault(x => x.Bounds.Contains(point));        
         }
 
-        public Screen ScreenFromVisual(IVisual visual)
+        public Screen? ScreenFromVisual(IVisual visual)
         {
             var tl = visual.PointToScreen(visual.Bounds.TopLeft);
             var br = visual.PointToScreen(visual.Bounds.BottomRight);
diff --git a/src/Avalonia.Controls/ScrollViewer.cs b/src/Avalonia.Controls/ScrollViewer.cs
index 10a014e81d..7c4b87e66a 100644
--- a/src/Avalonia.Controls/ScrollViewer.cs
+++ b/src/Avalonia.Controls/ScrollViewer.cs
@@ -199,8 +199,8 @@ namespace Avalonia.Controls
 
         internal const double DefaultSmallChange = 16;
 
-        private IDisposable _childSubscription;
-        private ILogicalScrollable _logicalScrollable;
+        private IDisposable? _childSubscription;
+        private ILogicalScrollable? _logicalScrollable;
         private Size _extent;
         private Vector _offset;
         private Size _viewport;
@@ -210,7 +210,7 @@ namespace Avalonia.Controls
         private Size _largeChange;
         private Size _smallChange = new Size(DefaultSmallChange, DefaultSmallChange);
         private bool _isExpanded;
-        private IDisposable _scrollBarExpandSubscription;
+        private IDisposable? _scrollBarExpandSubscription;
 
         /// <summary>
         /// Initializes static members of the <see cref="ScrollViewer"/> class.
@@ -232,7 +232,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when changes are detected to the scroll position, extent, or viewport size.
         /// </summary>
-        public event EventHandler<ScrollChangedEventArgs> ScrollChanged
+        public event EventHandler<ScrollChangedEventArgs>? ScrollChanged
         {
             add => AddHandler(ScrollChangedEvent, value);
             remove => RemoveHandler(ScrollChangedEvent, value);
@@ -340,7 +340,7 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        public IControl CurrentAnchor => (Presenter as IScrollAnchorProvider)?.CurrentAnchor;
+        public IControl? CurrentAnchor => (Presenter as IScrollAnchorProvider)?.CurrentAnchor;
 
         /// <summary>
         /// Gets the maximum horizontal scrollbar value.
@@ -656,7 +656,7 @@ namespace Avalonia.Controls
             return double.IsNaN(result) ? 0 : result;
         }
 
-        private void ChildChanged(IControl child)
+        private void ChildChanged(IControl? child)
         {
             if (_logicalScrollable is object)
             {
@@ -673,7 +673,7 @@ namespace Avalonia.Controls
             CalculatedPropertiesChanged();
         }
 
-        private void LogicalScrollInvalidated(object sender, EventArgs e)
+        private void LogicalScrollInvalidated(object? sender, EventArgs e)
         {
             CalculatedPropertiesChanged();
         }
@@ -762,9 +762,9 @@ namespace Avalonia.Controls
             _scrollBarExpandSubscription = SubscribeToScrollBars(e);
         }
 
-        private IDisposable SubscribeToScrollBars(TemplateAppliedEventArgs e)
+        private IDisposable? SubscribeToScrollBars(TemplateAppliedEventArgs e)
         {
-            static IObservable<bool> GetExpandedObservable(ScrollBar scrollBar)
+            static IObservable<bool>? GetExpandedObservable(ScrollBar? scrollBar)
             {
                 return scrollBar?.GetObservable(ScrollBar.IsExpandedProperty);
             }
@@ -775,7 +775,7 @@ namespace Avalonia.Controls
             var horizontalExpanded = GetExpandedObservable(horizontalScrollBar);
             var verticalExpanded = GetExpandedObservable(verticalScrollBar);
 
-            IObservable<bool> actualExpanded = null;
+            IObservable<bool>? actualExpanded = null;
 
             if (horizontalExpanded != null && verticalExpanded != null)
             {
@@ -801,7 +801,7 @@ namespace Avalonia.Controls
             IsExpanded = isExpanded;
         }
 
-        private void OnLayoutUpdated(object sender, EventArgs e) => RaiseScrollChanged();
+        private void OnLayoutUpdated(object? sender, EventArgs e) => RaiseScrollChanged();
 
         private void RaiseScrollChanged()
         {
diff --git a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
index 73c0184ebb..edef0a9ff5 100644
--- a/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
+++ b/src/Avalonia.Controls/Selection/InternalSelectionModel.cs
@@ -174,7 +174,7 @@ namespace Avalonia.Controls.Selection
             }
         }
 
-        private void OnSelectionChanged(object sender, SelectionModelSelectionChangedEventArgs e)
+        private void OnSelectionChanged(object? sender, SelectionModelSelectionChangedEventArgs e)
         {
             if (_ignoreModelChanges > 0)
             {
@@ -227,9 +227,9 @@ namespace Avalonia.Controls.Selection
             }
         }
 
-        private void OnSourceReset(object sender, EventArgs e) => SyncFromSelectedItems();
+        private void OnSourceReset(object? sender, EventArgs e) => SyncFromSelectedItems();
 
-        private void OnSelectedItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void OnSelectedItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             if (_ignoreSelectedItemsChanges)
             {
@@ -243,7 +243,7 @@ namespace Avalonia.Controls.Selection
 
             void Remove()
             {
-                foreach (var i in e.OldItems)
+                foreach (var i in e.OldItems!)
                 {
                     var index = IndexOf(Source, i);
 
@@ -263,14 +263,14 @@ namespace Avalonia.Controls.Selection
                 switch (e.Action)
                 {
                     case NotifyCollectionChangedAction.Add:
-                        Add(e.NewItems);
+                        Add(e.NewItems!);
                         break;
                     case NotifyCollectionChangedAction.Remove:
                         Remove();
                         break;
                     case NotifyCollectionChangedAction.Replace:
                         Remove();
-                        Add(e.NewItems);
+                        Add(e.NewItems!);
                         break;
                     case NotifyCollectionChangedAction.Reset:
                         Clear();
diff --git a/src/Avalonia.Controls/Selection/SelectionModel.cs b/src/Avalonia.Controls/Selection/SelectionModel.cs
index 138a765b43..20d5293f8b 100644
--- a/src/Avalonia.Controls/Selection/SelectionModel.cs
+++ b/src/Avalonia.Controls/Selection/SelectionModel.cs
@@ -78,8 +78,7 @@ namespace Avalonia.Controls.Selection
 
         public IReadOnlyList<int> SelectedIndexes => _selectedIndexes ??= new SelectedIndexes<T>(this);
 
-        [MaybeNull, AllowNull]
-        public T SelectedItem
+        public T? SelectedItem
         {
             get
             {
@@ -89,7 +88,7 @@ namespace Avalonia.Controls.Selection
                 }
                 else if (_initSelectedItems is object && _initSelectedItems.Count > 0)
                 {
-                    return (T)_initSelectedItems[0];
+                    return (T?)_initSelectedItems[0];
                 }
 
                 return default;
@@ -394,9 +393,7 @@ namespace Avalonia.Controls.Selection
             {
                 if (SingleSelect)
                 {
-#pragma warning disable CS8604
-                    removed = new List<T> { (T)items[SelectedIndex - index] };
-#pragma warning restore CS8604
+                    removed = new List<T> { (T)items[SelectedIndex - index]! };
                 }
 
                 _selectedIndex = GetFirstSelectedIndexFromRanges();
@@ -466,12 +463,12 @@ namespace Avalonia.Controls.Selection
             {
                 if (e.NewStartingIndex <= _selectedIndex)
                 {
-                    return _selectedIndex + e.NewItems.Count < ItemsView.Count;
+                    return _selectedIndex + e.NewItems!.Count < ItemsView.Count;
                 }
 
                 if (e.NewStartingIndex <= _anchorIndex)
                 {
-                    return _anchorIndex + e.NewItems.Count < ItemsView.Count;
+                    return _anchorIndex + e.NewItems!.Count < ItemsView.Count;
                 }
             }
 
diff --git a/src/Avalonia.Controls/Selection/SelectionNodeBase.cs b/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
index 230575074a..6840821589 100644
--- a/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
+++ b/src/Avalonia.Controls/Selection/SelectionNodeBase.cs
@@ -202,9 +202,7 @@ namespace Avalonia.Controls.Selection
                     {
                         for (var i = range.Begin; i <= range.End; ++i)
                         {
-#pragma warning disable CS8604
-                            removed.Add((T)items[i - index]);
-#pragma warning restore CS8604
+                            removed.Add((T)items[i - index]!);
                         }
                     }
                 }
@@ -244,14 +242,14 @@ namespace Avalonia.Controls.Selection
             {
                 case NotifyCollectionChangedAction.Add:
                     {
-                        var change = OnItemsAdded(e.NewStartingIndex, e.NewItems);
+                        var change = OnItemsAdded(e.NewStartingIndex, e.NewItems!);
                         shiftIndex = change.ShiftIndex;
                         shiftDelta = change.ShiftDelta;
                         break;
                     }
                 case NotifyCollectionChangedAction.Remove:
                     {
-                        var change = OnItemsRemoved(e.OldStartingIndex, e.OldItems);
+                        var change = OnItemsRemoved(e.OldStartingIndex, e.OldItems!);
                         shiftIndex = change.ShiftIndex;
                         shiftDelta = change.ShiftDelta;
                         removed = change.RemovedItems;
@@ -259,8 +257,8 @@ namespace Avalonia.Controls.Selection
                     }
                 case NotifyCollectionChangedAction.Replace:
                     {
-                        var removeChange = OnItemsRemoved(e.OldStartingIndex, e.OldItems);
-                        var addChange = OnItemsAdded(e.NewStartingIndex, e.NewItems);
+                        var removeChange = OnItemsRemoved(e.OldStartingIndex, e.OldItems!);
+                        var addChange = OnItemsAdded(e.NewStartingIndex, e.NewItems!);
                         shiftIndex = removeChange.ShiftIndex;
                         shiftDelta = removeChange.ShiftDelta + addChange.ShiftDelta;
                         removed = removeChange.RemovedItems;
@@ -303,7 +301,7 @@ namespace Avalonia.Controls.Selection
 
                 if (e.NewStartingIndex <= lastIndex)
                 {
-                    return lastIndex + e.NewItems.Count < ItemsView.Count;
+                    return lastIndex + e.NewItems!.Count < ItemsView.Count;
                 }
             }
 
diff --git a/src/Avalonia.Controls/Shapes/Path.cs b/src/Avalonia.Controls/Shapes/Path.cs
index d0ffc27d20..3502b4581f 100644
--- a/src/Avalonia.Controls/Shapes/Path.cs
+++ b/src/Avalonia.Controls/Shapes/Path.cs
@@ -8,7 +8,7 @@ namespace Avalonia.Controls.Shapes
         public static readonly StyledProperty<Geometry> DataProperty =
             AvaloniaProperty.Register<Path, Geometry>(nameof(Data));
 
-        private EventHandler _geometryChangedHandler;
+        private EventHandler? _geometryChangedHandler;
 
         static Path()
         {
@@ -22,7 +22,7 @@ namespace Avalonia.Controls.Shapes
             set { SetValue(DataProperty, value); }
         }
 
-        private EventHandler GeometryChangedHandler => _geometryChangedHandler ??= GeometryChanged;
+        private EventHandler? GeometryChangedHandler => _geometryChangedHandler ??= GeometryChanged;
 
         protected override Geometry CreateDefiningGeometry() => Data;
 
@@ -53,8 +53,8 @@ namespace Avalonia.Controls.Shapes
                 return;
             }
 
-            var oldGeometry = (Geometry)e.OldValue;
-            var newGeometry = (Geometry)e.NewValue;
+            var oldGeometry = (Geometry?)e.OldValue;
+            var newGeometry = (Geometry?)e.NewValue;
 
             if (oldGeometry is object)
             {
@@ -67,7 +67,7 @@ namespace Avalonia.Controls.Shapes
             }
         }
 
-        private void GeometryChanged(object sender, EventArgs e)
+        private void GeometryChanged(object? sender, EventArgs e)
         {
             InvalidateGeometry();
         }
diff --git a/src/Avalonia.Controls/Slider.cs b/src/Avalonia.Controls/Slider.cs
index 6af40b0281..87de04f92f 100644
--- a/src/Avalonia.Controls/Slider.cs
+++ b/src/Avalonia.Controls/Slider.cs
@@ -81,14 +81,14 @@ namespace Avalonia.Controls
 
         // Slider required parts
         private bool _isDragging = false;
-        private Track _track;
-        private Button _decreaseButton;
-        private Button _increaseButton;
-        private IDisposable _decreaseButtonPressDispose;
-        private IDisposable _decreaseButtonReleaseDispose;
-        private IDisposable _increaseButtonSubscription;
-        private IDisposable _increaseButtonReleaseDispose;
-        private IDisposable _pointerMovedDispose;
+        private Track? _track;
+        private Button? _decreaseButton;
+        private Button? _increaseButton;
+        private IDisposable? _decreaseButtonPressDispose;
+        private IDisposable? _decreaseButtonReleaseDispose;
+        private IDisposable? _increaseButtonSubscription;
+        private IDisposable? _increaseButtonReleaseDispose;
+        private IDisposable? _pointerMovedDispose;
 
         private const double Tolerance = 0.0001;
 
@@ -309,7 +309,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void TrackMoved(object sender, PointerEventArgs e)
+        private void TrackMoved(object? sender, PointerEventArgs e)
         {
             if (_isDragging)
             {
@@ -317,12 +317,12 @@ namespace Avalonia.Controls
             }
         }
 
-        private void TrackReleased(object sender, PointerReleasedEventArgs e)
+        private void TrackReleased(object? sender, PointerReleasedEventArgs e)
         {
             _isDragging = false;
         }
 
-        private void TrackPressed(object sender, PointerPressedEventArgs e)
+        private void TrackPressed(object? sender, PointerPressedEventArgs e)
         {
             if (e.GetCurrentPoint(this).Properties.IsLeftButtonPressed)
             {
@@ -333,6 +333,9 @@ namespace Avalonia.Controls
 
         private void MoveToPoint(PointerPoint posOnTrack)
         {
+            if (_track is null)
+                return;
+
             var orient = Orientation == Orientation.Horizontal;
             var thumbLength = (orient 
                 ? _track.Thumb.Bounds.Width 
diff --git a/src/Avalonia.Controls/Spinner.cs b/src/Avalonia.Controls/Spinner.cs
index 5ee13f45c4..cfcf0ee376 100644
--- a/src/Avalonia.Controls/Spinner.cs
+++ b/src/Avalonia.Controls/Spinner.cs
@@ -114,7 +114,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when spinning is initiated by the end-user.
         /// </summary>
-        public event EventHandler<SpinEventArgs> Spin
+        public event EventHandler<SpinEventArgs>? Spin
         {
             add { AddHandler(SpinEvent, value); }
             remove { RemoveHandler(SpinEvent, value); }
@@ -163,8 +163,8 @@ namespace Avalonia.Controls
         {
             if (e.Sender is Spinner spinner)
             {
-                var oldValue = (ValidSpinDirections)e.OldValue;
-                var newValue = (ValidSpinDirections)e.NewValue;
+                var oldValue = (ValidSpinDirections)e.OldValue!;
+                var newValue = (ValidSpinDirections)e.NewValue!;
                 spinner.OnValidSpinDirectionChanged(oldValue, newValue);
             }
         }
diff --git a/src/Avalonia.Controls/SplitView.cs b/src/Avalonia.Controls/SplitView.cs
index 2ea58e6b0d..d2161deb6e 100644
--- a/src/Avalonia.Controls/SplitView.cs
+++ b/src/Avalonia.Controls/SplitView.cs
@@ -118,8 +118,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="PaneBackground"/> property
         /// </summary>
-        public static readonly StyledProperty<IBrush> PaneBackgroundProperty =
-            AvaloniaProperty.Register<SplitView, IBrush>(nameof(PaneBackground));
+        public static readonly StyledProperty<IBrush?> PaneBackgroundProperty =
+            AvaloniaProperty.Register<SplitView, IBrush?>(nameof(PaneBackground));
 
         /// <summary>
         /// Defines the <see cref="PanePlacement"/> property
@@ -130,8 +130,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Pane"/> property
         /// </summary>
-        public static readonly StyledProperty<object> PaneProperty =
-            AvaloniaProperty.Register<SplitView, object>(nameof(Pane));
+        public static readonly StyledProperty<object?> PaneProperty =
+            AvaloniaProperty.Register<SplitView, object?>(nameof(Pane));
 
         /// <summary>
         /// Defines the <see cref="PaneTemplate"/> property.
@@ -152,8 +152,8 @@ namespace Avalonia.Controls
             AvaloniaProperty.Register<SplitView, SplitViewTemplateSettings>(nameof(TemplateSettings));
 
         private bool _isPaneOpen;
-        private Panel _pane;
-        private IDisposable _pointerDisposable;
+        private Panel? _pane;
+        private IDisposable? _pointerDisposable;
 
         public SplitView()
         {
@@ -207,12 +207,12 @@ namespace Avalonia.Controls
 
                 if (value)
                 {
-                    OnPaneOpening(this, null);
+                    OnPaneOpening(this, EventArgs.Empty);
                     SetAndRaise(IsPaneOpenProperty, ref _isPaneOpen, value);
 
                     PseudoClasses.Add(":open");
                     PseudoClasses.Remove(":closed");
-                    OnPaneOpened(this, null);
+                    OnPaneOpened(this, EventArgs.Empty);
                 }
                 else
                 {
@@ -224,7 +224,7 @@ namespace Avalonia.Controls
 
                         PseudoClasses.Add(":closed");
                         PseudoClasses.Remove(":open");
-                        OnPaneClosed(this, null);
+                        OnPaneClosed(this, EventArgs.Empty);
                     }
                 }
             }
@@ -242,7 +242,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the background of the pane
         /// </summary>
-        public IBrush PaneBackground
+        public IBrush? PaneBackground
         {
             get => GetValue(PaneBackgroundProperty);
             set => SetValue(PaneBackgroundProperty, value);
@@ -261,7 +261,7 @@ namespace Avalonia.Controls
         /// Gets or sets the Pane for the SplitView
         /// </summary>
         [DependsOn(nameof(PaneTemplate))]
-        public object Pane
+        public object? Pane
         {
             get => GetValue(PaneProperty);
             set => SetValue(PaneProperty, value);
@@ -300,22 +300,22 @@ namespace Avalonia.Controls
         /// <summary>
         /// Fired when the pane is closed
         /// </summary>
-        public event EventHandler<EventArgs> PaneClosed;
+        public event EventHandler<EventArgs>? PaneClosed;
 
         /// <summary>
         /// Fired when the pane is closing
         /// </summary>
-        public event EventHandler<SplitViewPaneClosingEventArgs> PaneClosing;
+        public event EventHandler<SplitViewPaneClosingEventArgs>? PaneClosing;
 
         /// <summary>
         /// Fired when the pane is opened
         /// </summary>
-        public event EventHandler<EventArgs> PaneOpened;
+        public event EventHandler<EventArgs>? PaneOpened;
 
         /// <summary>
         /// Fired when the pane is opening
         /// </summary>
-        public event EventHandler<EventArgs> PaneOpening;
+        public event EventHandler<EventArgs>? PaneOpening;
 
         protected override bool RegisterContentPresenter(IContentPresenter presenter)
         {
@@ -352,7 +352,7 @@ namespace Avalonia.Controls
             _pointerDisposable?.Dispose();
         }
 
-        private void PointerPressedOutside(object sender, PointerPressedEventArgs e)
+        private void PointerPressedOutside(object? sender, PointerPressedEventArgs e)
         {
             if (!IsPaneOpen)
             {
@@ -417,7 +417,7 @@ namespace Avalonia.Controls
 
         private void OnCompactPaneLengthChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var newLen = (double)e.NewValue;
+            var newLen = (double)e.NewValue!;
             var displayMode = DisplayMode;
             if (displayMode == SplitViewDisplayMode.CompactInline)
             {
@@ -432,16 +432,16 @@ namespace Avalonia.Controls
 
         private void OnPanePlacementChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldState = e.OldValue.ToString().ToLower();
-            var newState = e.NewValue.ToString().ToLower();
+            var oldState = e.OldValue!.ToString()!.ToLower();
+            var newState = e.NewValue!.ToString()!.ToLower();
             PseudoClasses.Remove($":{oldState}");
             PseudoClasses.Add($":{newState}");
         }
 
         private void OnDisplayModeChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var oldState = e.OldValue.ToString().ToLower();
-            var newState = e.NewValue.ToString().ToLower();
+            var oldState = e.OldValue!.ToString()!.ToLower();
+            var newState = e.NewValue!.ToString()!.ToLower();
 
             PseudoClasses.Remove($":{oldState}");
             PseudoClasses.Add($":{newState}");
@@ -460,7 +460,7 @@ namespace Avalonia.Controls
 
         private void OnUseLightDismissChanged(AvaloniaPropertyChangedEventArgs e)
         {
-            var mode = (bool)e.NewValue;
+            var mode = (bool)e.NewValue!;
             PseudoClasses.Set(":lightdismiss", mode);
         }
 
diff --git a/src/Avalonia.Controls/StackPanel.cs b/src/Avalonia.Controls/StackPanel.cs
index 9e087b7fd3..feb425a9c3 100644
--- a/src/Avalonia.Controls/StackPanel.cs
+++ b/src/Avalonia.Controls/StackPanel.cs
@@ -60,7 +60,7 @@ namespace Avalonia.Controls
         /// <param name="from">The control from which movement begins.</param>
         /// <param name="wrap">Whether to wrap around when the first or last item is reached.</param>
         /// <returns>The control.</returns>
-        IInputElement INavigableContainer.GetControl(NavigationDirection direction, IInputElement from, bool wrap)
+        IInputElement? INavigableContainer.GetControl(NavigationDirection direction, IInputElement? from, bool wrap)
         {
             var result = GetControlInDirection(direction, from as IControl);
 
@@ -109,7 +109,7 @@ namespace Avalonia.Controls
         /// <param name="direction">The movement direction.</param>
         /// <param name="from">The control from which movement begins.</param>
         /// <returns>The control.</returns>
-        protected virtual IInputElement GetControlInDirection(NavigationDirection direction, IControl from)
+        protected virtual IInputElement? GetControlInDirection(NavigationDirection direction, IControl? from)
         {
             var horiz = Orientation == Orientation.Horizontal;
             int index = from != null ? Children.IndexOf(from) : -1;
diff --git a/src/Avalonia.Controls/TabControl.cs b/src/Avalonia.Controls/TabControl.cs
index 306a9d3e6a..019bdfb98a 100644
--- a/src/Avalonia.Controls/TabControl.cs
+++ b/src/Avalonia.Controls/TabControl.cs
@@ -38,20 +38,20 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="ContentTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> ContentTemplateProperty =
+        public static readonly StyledProperty<IDataTemplate?> ContentTemplateProperty =
             ContentControl.ContentTemplateProperty.AddOwner<TabControl>();
 
         /// <summary>
         /// The selected content property
         /// </summary>
-        public static readonly StyledProperty<object> SelectedContentProperty =
-            AvaloniaProperty.Register<TabControl, object>(nameof(SelectedContent));
+        public static readonly StyledProperty<object?> SelectedContentProperty =
+            AvaloniaProperty.Register<TabControl, object?>(nameof(SelectedContent));
 
         /// <summary>
         /// The selected content template property
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> SelectedContentTemplateProperty =
-            AvaloniaProperty.Register<TabControl, IDataTemplate>(nameof(SelectedContentTemplate));
+        public static readonly StyledProperty<IDataTemplate?> SelectedContentTemplateProperty =
+            AvaloniaProperty.Register<TabControl, IDataTemplate?>(nameof(SelectedContentTemplate));
 
         /// <summary>
         /// The default value for the <see cref="ItemsControl.ItemsPanel"/> property.
@@ -100,7 +100,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the default data template used to display the content of the selected tab.
         /// </summary>
-        public IDataTemplate ContentTemplate
+        public IDataTemplate? ContentTemplate
         {
             get { return GetValue(ContentTemplateProperty); }
             set { SetValue(ContentTemplateProperty, value); }
@@ -112,7 +112,7 @@ namespace Avalonia.Controls
         /// <value>
         /// The content of the selected tab.
         /// </value>
-        public object SelectedContent
+        public object? SelectedContent
         {
             get { return GetValue(SelectedContentProperty); }
             internal set { SetValue(SelectedContentProperty, value); }
@@ -124,15 +124,15 @@ namespace Avalonia.Controls
         /// <value>
         /// The content template of the selected tab.
         /// </value>
-        public IDataTemplate SelectedContentTemplate
+        public IDataTemplate? SelectedContentTemplate
         {
             get { return GetValue(SelectedContentTemplateProperty); }
             internal set { SetValue(SelectedContentTemplateProperty, value); }
         }
 
-        internal ItemsPresenter ItemsPresenterPart { get; private set; }
+        internal ItemsPresenter? ItemsPresenterPart { get; private set; }
 
-        internal IContentPresenter ContentPart { get; private set; }
+        internal IContentPresenter? ContentPart { get; private set; }
 
         /// <inheritdoc/>
         IAvaloniaList<ILogical> IContentPresenterHost.LogicalChildren => LogicalChildren;
@@ -164,7 +164,7 @@ namespace Avalonia.Controls
             else
             {
                 var container = SelectedItem as IContentControl ??
-                    ItemContainerGenerator.ContainerFromIndex(SelectedIndex) as IContentControl;
+                    ItemContainerGenerator!.ContainerFromIndex(SelectedIndex) as IContentControl;
                 SelectedContentTemplate = container?.ContentTemplate;
                 SelectedContent = container?.Content;
             }
diff --git a/src/Avalonia.Controls/Templates/DataTemplateExtensions.cs b/src/Avalonia.Controls/Templates/DataTemplateExtensions.cs
index 82a1758c26..49184bf7df 100644
--- a/src/Avalonia.Controls/Templates/DataTemplateExtensions.cs
+++ b/src/Avalonia.Controls/Templates/DataTemplateExtensions.cs
@@ -21,7 +21,7 @@ namespace Avalonia.Controls.Templates
         /// <returns>The data template or null if no matching data template was found.</returns>
         public static IDataTemplate? FindDataTemplate(
             this IControl control,
-            object data,
+            object? data,
             IDataTemplate? primary = null)
         {
             if (primary?.Match(data) == true)
diff --git a/src/Avalonia.Controls/Templates/FuncDataTemplate.cs b/src/Avalonia.Controls/Templates/FuncDataTemplate.cs
index 5dff13997e..10da2cfea3 100644
--- a/src/Avalonia.Controls/Templates/FuncDataTemplate.cs
+++ b/src/Avalonia.Controls/Templates/FuncDataTemplate.cs
@@ -7,13 +7,13 @@ namespace Avalonia.Controls.Templates
     /// <summary>
     /// Builds a control for a piece of data.
     /// </summary>
-    public class FuncDataTemplate : FuncTemplate<object, IControl>, IRecyclingDataTemplate
+    public class FuncDataTemplate : FuncTemplate<object?, IControl?>, IRecyclingDataTemplate
     {
         /// <summary>
         /// The default data template used in the case where no matching data template is found.
         /// </summary>
         public static readonly FuncDataTemplate Default =
-            new FuncDataTemplate<object>(
+            new FuncDataTemplate<object?>(
                 (data, s) =>
                 {
                     if (data != null)
@@ -56,7 +56,7 @@ namespace Avalonia.Controls.Templates
         /// <summary>
         /// The implementation of the <see cref="Match"/> method.
         /// </summary>
-        private readonly Func<object, bool> _match;
+        private readonly Func<object?, bool> _match;
         private readonly bool _supportsRecycling;
 
         /// <summary>
@@ -69,7 +69,7 @@ namespace Avalonia.Controls.Templates
         /// <param name="supportsRecycling">Whether the control can be recycled.</param>
         public FuncDataTemplate(
             Type type,
-            Func<object, INameScope, IControl> build,
+            Func<object?, INameScope, IControl?> build,
             bool supportsRecycling = false)
             : this(o => IsInstance(o, type), build, supportsRecycling)
         {
@@ -86,14 +86,12 @@ namespace Avalonia.Controls.Templates
         /// </param>
         /// <param name="supportsRecycling">Whether the control can be recycled.</param>
         public FuncDataTemplate(
-            Func<object, bool> match,
-            Func<object, INameScope, IControl> build,
+            Func<object?, bool> match,
+            Func<object?, INameScope, IControl?> build,
             bool supportsRecycling = false)
             : base(build)
         {
-            Contract.Requires<ArgumentNullException>(match != null);
-
-            _match = match;
+            _match = match ?? throw new ArgumentNullException(nameof(match));
             _supportsRecycling = supportsRecycling;
         }
 
@@ -104,7 +102,7 @@ namespace Avalonia.Controls.Templates
         /// <returns>
         /// True if the data template can build a control for the data, otherwise false.
         /// </returns>
-        public bool Match(object data)
+        public bool Match(object? data)
         {
             return _match(data);
         }
@@ -116,13 +114,13 @@ namespace Avalonia.Controls.Templates
         /// <param name="existing">An optional control to recycle.</param>
         /// <returns>
         /// The <paramref name="existing"/> control if supplied and applicable to
-        /// <paramref name="data"/>, otherwise a new control.
+        /// <paramref name="data"/>, otherwise a new control or null.
         /// </returns>
         /// <remarks>
         /// The caller should ensure that any control passed to <paramref name="existing"/>
         /// originated from the same data template.
         /// </remarks>
-        public IControl Build(object data, IControl existing)
+        public IControl? Build(object? data, IControl? existing)
         {
             return _supportsRecycling && existing is object ? existing : Build(data);
         }
@@ -135,7 +133,7 @@ namespace Avalonia.Controls.Templates
         /// <returns>
         /// True if <paramref name="o"/> is of type <paramref name="t"/>, otherwise false.
         /// </returns>
-        private static bool IsInstance(object o, Type t)
+        private static bool IsInstance(object? o, Type t)
         {
             return t.IsInstanceOfType(o);
         }
diff --git a/src/Avalonia.Controls/Templates/FuncDataTemplate`1.cs b/src/Avalonia.Controls/Templates/FuncDataTemplate`1.cs
index 8e7b290247..6076e774a7 100644
--- a/src/Avalonia.Controls/Templates/FuncDataTemplate`1.cs
+++ b/src/Avalonia.Controls/Templates/FuncDataTemplate`1.cs
@@ -17,7 +17,7 @@ namespace Avalonia.Controls.Templates
         /// A function which when passed an object of <typeparamref name="T"/> returns a control.
         /// </param>
         /// <param name="supportsRecycling">Whether the control can be recycled.</param>
-        public FuncDataTemplate(Func<T, INameScope, IControl> build, bool supportsRecycling = false)
+        public FuncDataTemplate(Func<T, INameScope, IControl?> build, bool supportsRecycling = false)
             : base(o => TypeUtilities.CanCast<T>(o), CastBuild(build), supportsRecycling)
         {
         }
@@ -63,9 +63,9 @@ namespace Avalonia.Controls.Templates
         /// </summary>
         /// <param name="f">The strongly typed function.</param>
         /// <returns>The weakly typed function.</returns>
-        private static Func<object, bool> CastMatch(Func<T, bool> f)
+        private static Func<object?, bool> CastMatch(Func<T, bool> f)
         {
-            return o => TypeUtilities.CanCast<T>(o) && f((T)o);
+            return o => TypeUtilities.CanCast<T>(o) && f((T)o!);
         }
 
         /// <summary>
@@ -74,9 +74,9 @@ namespace Avalonia.Controls.Templates
         /// <typeparam name="TResult">The strong data type.</typeparam>
         /// <param name="f">The strongly typed function.</param>
         /// <returns>The weakly typed function.</returns>
-        private static Func<object, INameScope, TResult> CastBuild<TResult>(Func<T, INameScope, TResult> f)
+        private static Func<object?, INameScope, TResult> CastBuild<TResult>(Func<T, INameScope, TResult> f)
         {
-            return (o, s) => f((T)o, s);
+            return (o, s) => f((T)o!, s);
         }
     }
 }
diff --git a/src/Avalonia.Controls/Templates/FuncTemplateNameScopeExtensions.cs b/src/Avalonia.Controls/Templates/FuncTemplateNameScopeExtensions.cs
index 76bc298e30..64e6b6c714 100644
--- a/src/Avalonia.Controls/Templates/FuncTemplateNameScopeExtensions.cs
+++ b/src/Avalonia.Controls/Templates/FuncTemplateNameScopeExtensions.cs
@@ -7,6 +7,9 @@ namespace Avalonia.Controls.Templates
         public static T RegisterInNameScope<T>(this T control, INameScope scope)
         where T : StyledElement
         {
+            if (control.Name is null)
+                throw new ArgumentException("RegisterInNameScope must be called on a control with non-null name.");
+
             scope.Register(control.Name, control);
             return control;
         }
diff --git a/src/Avalonia.Controls/Templates/FuncTemplate`1.cs b/src/Avalonia.Controls/Templates/FuncTemplate`1.cs
index e1818b07d5..ec4e077660 100644
--- a/src/Avalonia.Controls/Templates/FuncTemplate`1.cs
+++ b/src/Avalonia.Controls/Templates/FuncTemplate`1.cs
@@ -17,9 +17,7 @@ namespace Avalonia.Controls.Templates
         /// <param name="func">The function used to create the control.</param>
         public FuncTemplate(Func<TControl> func)
         {
-            Contract.Requires<ArgumentNullException>(func != null);
-
-            _func = func;
+            _func = func ?? throw new ArgumentNullException(nameof(func));
         }
 
         /// <summary>
@@ -35,4 +33,4 @@ namespace Avalonia.Controls.Templates
 
         object ITemplate.Build() => Build();
     }
-}
\ No newline at end of file
+}
diff --git a/src/Avalonia.Controls/Templates/FuncTemplate`2.cs b/src/Avalonia.Controls/Templates/FuncTemplate`2.cs
index cd0e3ad603..c310386c77 100644
--- a/src/Avalonia.Controls/Templates/FuncTemplate`2.cs
+++ b/src/Avalonia.Controls/Templates/FuncTemplate`2.cs
@@ -10,7 +10,7 @@ namespace Avalonia.Controls.Templates
     /// <typeparam name="TParam">The type of the parameter.</typeparam>
     /// <typeparam name="TControl">The type of control.</typeparam>
     public class FuncTemplate<TParam, TControl> : ITemplate<TParam, TControl>
-        where TControl : IControl
+        where TControl : IControl?
     {
         private readonly Func<TParam, INameScope, TControl> _func;
 
diff --git a/src/Avalonia.Controls/Templates/FuncTreeDataTemplate.cs b/src/Avalonia.Controls/Templates/FuncTreeDataTemplate.cs
index 4e40d87d47..50fed100d8 100644
--- a/src/Avalonia.Controls/Templates/FuncTreeDataTemplate.cs
+++ b/src/Avalonia.Controls/Templates/FuncTreeDataTemplate.cs
@@ -9,7 +9,7 @@ namespace Avalonia.Controls.Templates
     /// </summary>
     public class FuncTreeDataTemplate : FuncDataTemplate, ITreeDataTemplate
     {
-        private readonly Func<object, IEnumerable> _itemsSelector;
+        private readonly Func<object?, IEnumerable> _itemsSelector;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="FuncTreeDataTemplate"/> class.
@@ -24,8 +24,8 @@ namespace Avalonia.Controls.Templates
         /// </param>
         public FuncTreeDataTemplate(
             Type type,
-            Func<object, INameScope, IControl> build,
-            Func<object, IEnumerable> itemsSelector)
+            Func<object?, INameScope, IControl> build,
+            Func<object?, IEnumerable> itemsSelector)
             : this(o => IsInstance(o, type), build, itemsSelector)
         {
         }
@@ -43,9 +43,9 @@ namespace Avalonia.Controls.Templates
         /// A function which when passed a matching object returns the child items.
         /// </param>
         public FuncTreeDataTemplate(
-            Func<object, bool> match,
-            Func<object, INameScope, IControl> build,
-            Func<object, IEnumerable> itemsSelector)
+            Func<object?, bool> match,
+            Func<object?, INameScope, IControl?> build,
+            Func<object?, IEnumerable> itemsSelector)
             : base(match, build)
         {
             _itemsSelector = itemsSelector;
@@ -58,7 +58,7 @@ namespace Avalonia.Controls.Templates
         /// <returns>The child items, or null if no child items.</returns>
         public InstancedBinding ItemsSelector(object item)
         {
-            return InstancedBinding.OneTime(this?._itemsSelector(item));
+            return InstancedBinding.OneTime(_itemsSelector(item));
         }
 
         /// <summary>
@@ -69,7 +69,7 @@ namespace Avalonia.Controls.Templates
         /// <returns>
         /// True if <paramref name="o"/> is of type <paramref name="t"/>, otherwise false.
         /// </returns>
-        private static bool IsInstance(object o, Type t)
+        private static bool IsInstance(object? o, Type t)
         {
             return t.IsInstanceOfType(o);
         }
diff --git a/src/Avalonia.Controls/Templates/FuncTreeDataTemplate`1.cs b/src/Avalonia.Controls/Templates/FuncTreeDataTemplate`1.cs
index c366107be4..d4ecdd6cf0 100644
--- a/src/Avalonia.Controls/Templates/FuncTreeDataTemplate`1.cs
+++ b/src/Avalonia.Controls/Templates/FuncTreeDataTemplate`1.cs
@@ -57,7 +57,7 @@ namespace Avalonia.Controls.Templates
         /// </summary>
         /// <param name="f">The typed function.</param>
         /// <returns>The untyped function.</returns>
-        private static Func<object, bool> CastMatch(Func<T, bool> f)
+        private static Func<object?, bool> CastMatch(Func<T, bool> f)
         {
             return o => (o is T) && f((T)o);
         }
@@ -68,9 +68,9 @@ namespace Avalonia.Controls.Templates
         /// <typeparam name="TResult">The result.</typeparam>
         /// <param name="f">The typed function.</param>
         /// <returns>The untyped function.</returns>
-        private static Func<object, INameScope, TResult> Cast<TResult>(Func<T, INameScope, TResult> f)
+        private static Func<object?, INameScope, TResult> Cast<TResult>(Func<T, INameScope, TResult> f)
         {
-            return (o, s) => f((T)o, s);
+            return (o, s) => f((T)o!, s);
         }
         
         /// <summary>
@@ -79,9 +79,9 @@ namespace Avalonia.Controls.Templates
         /// <typeparam name="TResult">The result.</typeparam>
         /// <param name="f">The typed function.</param>
         /// <returns>The untyped function.</returns>
-        private static Func<object, TResult> Cast<TResult>(Func<T, TResult> f)
+        private static Func<object?, TResult> Cast<TResult>(Func<T, TResult> f)
         {
-            return o => f((T)o);
+            return o => f((T)o!);
         }
     }
 }
diff --git a/src/Avalonia.Controls/Templates/IDataTemplate.cs b/src/Avalonia.Controls/Templates/IDataTemplate.cs
index 0368748a0b..3d54eeb613 100644
--- a/src/Avalonia.Controls/Templates/IDataTemplate.cs
+++ b/src/Avalonia.Controls/Templates/IDataTemplate.cs
@@ -7,7 +7,7 @@ namespace Avalonia.Controls.Templates
     /// <summary>
     /// Interface representing a template used to build a control for a piece of data.
     /// </summary>
-    public interface IDataTemplate : ITemplate<object, IControl>
+    public interface IDataTemplate : ITemplate<object?, IControl?>
     {
         /// <summary>
         /// Checks to see if this data template matches the specified data.
@@ -16,6 +16,6 @@ namespace Avalonia.Controls.Templates
         /// <returns>
         /// True if the data template can build a control for the data, otherwise false.
         /// </returns>
-        bool Match(object data);
+        bool Match(object? data);
     }
 }
diff --git a/src/Avalonia.Controls/Templates/IRecyclingDataTemplate.cs b/src/Avalonia.Controls/Templates/IRecyclingDataTemplate.cs
index 25956a9c9a..aafee33199 100644
--- a/src/Avalonia.Controls/Templates/IRecyclingDataTemplate.cs
+++ b/src/Avalonia.Controls/Templates/IRecyclingDataTemplate.cs
@@ -14,12 +14,12 @@ namespace Avalonia.Controls.Templates
         /// <param name="existing">An optional control to recycle.</param>
         /// <returns>
         /// The <paramref name="existing"/> control if supplied and applicable to
-        /// <paramref name="data"/>, otherwise a new control.
+        /// <paramref name="data"/>, otherwise a new control or null.
         /// </returns>
         /// <remarks>
         /// The caller should ensure that any control passed to <paramref name="existing"/>
         /// originated from the same data template.
         /// </remarks>
-        IControl Build(object data, IControl? existing);
+        IControl? Build(object? data, IControl? existing);
     }
 }
diff --git a/src/Avalonia.Controls/Templates/ITreeDataTemplate.cs b/src/Avalonia.Controls/Templates/ITreeDataTemplate.cs
index 60da65304f..22d332b6d8 100644
--- a/src/Avalonia.Controls/Templates/ITreeDataTemplate.cs
+++ b/src/Avalonia.Controls/Templates/ITreeDataTemplate.cs
@@ -15,6 +15,6 @@ namespace Avalonia.Controls.Templates
         /// An <see cref="InstancedBinding"/> holding the items, or an observable that tracks the
         /// items. May return null if no child items.
         /// </returns>
-        InstancedBinding ItemsSelector(object item);
+        InstancedBinding? ItemsSelector(object item);
     }
 }
diff --git a/src/Avalonia.Controls/TextBlock.cs b/src/Avalonia.Controls/TextBlock.cs
index 14cde774f4..73d635d114 100644
--- a/src/Avalonia.Controls/TextBlock.cs
+++ b/src/Avalonia.Controls/TextBlock.cs
@@ -15,7 +15,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Background"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> BackgroundProperty =
+        public static readonly StyledProperty<IBrush?> BackgroundProperty =
             Border.BackgroundProperty.AddOwner<TextBlock>();
 
         /// <summary>
@@ -65,8 +65,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Foreground"/> property.
         /// </summary>
-        public static readonly AttachedProperty<IBrush> ForegroundProperty =
-            AvaloniaProperty.RegisterAttached<TextBlock, Control, IBrush>(
+        public static readonly AttachedProperty<IBrush?> ForegroundProperty =
+            AvaloniaProperty.RegisterAttached<TextBlock, Control, IBrush?>(
                 nameof(Foreground),
                 Brushes.Black,
                 inherits: true);
@@ -91,8 +91,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Text"/> property.
         /// </summary>
-        public static readonly DirectProperty<TextBlock, string> TextProperty =
-            AvaloniaProperty.RegisterDirect<TextBlock, string>(
+        public static readonly DirectProperty<TextBlock, string?> TextProperty =
+            AvaloniaProperty.RegisterDirect<TextBlock, string?>(
                 nameof(Text),
                 o => o.Text,
                 (o, v) => o.Text = v);
@@ -118,11 +118,11 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="TextDecorations"/> property.
         /// </summary>
-        public static readonly StyledProperty<TextDecorationCollection> TextDecorationsProperty =
-            AvaloniaProperty.Register<TextBlock, TextDecorationCollection>(nameof(TextDecorations));
+        public static readonly StyledProperty<TextDecorationCollection?> TextDecorationsProperty =
+            AvaloniaProperty.Register<TextBlock, TextDecorationCollection?>(nameof(TextDecorations));
 
-        private string _text;
-        private TextLayout _textLayout;
+        private string? _text;
+        private TextLayout? _textLayout;
         private Size _constraint;
 
         /// <summary>
@@ -159,7 +159,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the <see cref="TextLayout"/> used to render the text.
         /// </summary>
-        public TextLayout TextLayout
+        public TextLayout? TextLayout
         {
             get
             {
@@ -179,7 +179,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a brush used to paint the control's background.
         /// </summary>
-        public IBrush Background
+        public IBrush? Background
         {
             get { return GetValue(BackgroundProperty); }
             set { SetValue(BackgroundProperty, value); }
@@ -189,7 +189,7 @@ namespace Avalonia.Controls
         /// Gets or sets the text.
         /// </summary>
         [Content]
-        public string Text
+        public string? Text
         {
             get { return _text; }
             set { SetAndRaise(TextProperty, ref _text, value); }
@@ -234,7 +234,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets a brush used to paint the text.
         /// </summary>
-        public IBrush Foreground
+        public IBrush? Foreground
         {
             get { return GetValue(ForegroundProperty); }
             set { SetValue(ForegroundProperty, value); }
@@ -288,7 +288,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the text decorations.
         /// </summary>
-        public TextDecorationCollection TextDecorations
+        public TextDecorationCollection? TextDecorations
         {
             get => GetValue(TextDecorationsProperty);
             set => SetValue(TextDecorationsProperty, value);
@@ -339,7 +339,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="control">The control.</param>
         /// <returns>The foreground.</returns>
-        public static IBrush GetForeground(Control control)
+        public static IBrush? GetForeground(Control control)
         {
             return control.GetValue(ForegroundProperty);
         }
@@ -394,7 +394,7 @@ namespace Avalonia.Controls
         /// <param name="control">The control.</param>
         /// <param name="value">The property value to set.</param>
         /// <returns>The font family.</returns>
-        public static void SetForeground(Control control, IBrush value)
+        public static void SetForeground(Control control, IBrush? value)
         {
             control.SetValue(ForegroundProperty, value);
         }
@@ -444,7 +444,7 @@ namespace Avalonia.Controls
         /// <param name="constraint">The constraint of the text.</param>
         /// <param name="text">The text to format.</param>
         /// <returns>A <see cref="TextLayout"/> object.</returns>
-        protected virtual TextLayout CreateTextLayout(Size constraint, string text)
+        protected virtual TextLayout? CreateTextLayout(Size constraint, string? text)
         {
             if (constraint == Size.Empty)
             {
@@ -455,7 +455,7 @@ namespace Avalonia.Controls
                 text ?? string.Empty,
                 new Typeface(FontFamily, FontStyle, FontWeight),
                 FontSize,
-                Foreground,
+                Foreground ?? Brushes.Transparent,
                 TextAlignment,
                 TextWrapping,
                 TextTrimming,
diff --git a/src/Avalonia.Controls/TextBox.cs b/src/Avalonia.Controls/TextBox.cs
index 20d8a94c1a..a4c0dd08ba 100644
--- a/src/Avalonia.Controls/TextBox.cs
+++ b/src/Avalonia.Controls/TextBox.cs
@@ -23,13 +23,13 @@ namespace Avalonia.Controls
     [PseudoClasses(":empty")]
     public class TextBox : TemplatedControl, UndoRedoHelper<TextBox.UndoRedoState>.IUndoRedoHost
     {
-        public static KeyGesture CutGesture { get; } = AvaloniaLocator.Current
+        public static KeyGesture? CutGesture { get; } = AvaloniaLocator.Current
             .GetService<PlatformHotkeyConfiguration>()?.Cut.FirstOrDefault();
 
-        public static KeyGesture CopyGesture { get; } = AvaloniaLocator.Current
+        public static KeyGesture? CopyGesture { get; } = AvaloniaLocator.Current
             .GetService<PlatformHotkeyConfiguration>()?.Copy.FirstOrDefault();
 
-        public static KeyGesture PasteGesture { get; } = AvaloniaLocator.Current
+        public static KeyGesture? PasteGesture { get; } = AvaloniaLocator.Current
             .GetService<PlatformHotkeyConfiguration>()?.Paste.FirstOrDefault();
 
         public static readonly StyledProperty<bool> AcceptsReturnProperty =
@@ -50,14 +50,14 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<char> PasswordCharProperty =
             AvaloniaProperty.Register<TextBox, char>(nameof(PasswordChar));
 
-        public static readonly StyledProperty<IBrush> SelectionBrushProperty =
-            AvaloniaProperty.Register<TextBox, IBrush>(nameof(SelectionBrushProperty));
+        public static readonly StyledProperty<IBrush?> SelectionBrushProperty =
+            AvaloniaProperty.Register<TextBox, IBrush?>(nameof(SelectionBrushProperty));
 
-        public static readonly StyledProperty<IBrush> SelectionForegroundBrushProperty =
-            AvaloniaProperty.Register<TextBox, IBrush>(nameof(SelectionForegroundBrushProperty));
+        public static readonly StyledProperty<IBrush?> SelectionForegroundBrushProperty =
+            AvaloniaProperty.Register<TextBox, IBrush?>(nameof(SelectionForegroundBrushProperty));
 
-        public static readonly StyledProperty<IBrush> CaretBrushProperty =
-            AvaloniaProperty.Register<TextBox, IBrush>(nameof(CaretBrushProperty));
+        public static readonly StyledProperty<IBrush?> CaretBrushProperty =
+            AvaloniaProperty.Register<TextBox, IBrush?>(nameof(CaretBrushProperty));
 
         public static readonly DirectProperty<TextBox, int> SelectionStartProperty =
             AvaloniaProperty.RegisterDirect<TextBox, int>(
@@ -74,7 +74,7 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<int> MaxLengthProperty =
             AvaloniaProperty.Register<TextBox, int>(nameof(MaxLength), defaultValue: 0);
 
-        public static readonly DirectProperty<TextBox, string> TextProperty =
+        public static readonly DirectProperty<TextBox, string?> TextProperty =
             TextBlock.TextProperty.AddOwnerWithDataValidation<TextBox>(
                 o => o.Text,
                 (o, v) => o.Text = v,
@@ -99,8 +99,8 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<TextWrapping> TextWrappingProperty =
             TextBlock.TextWrappingProperty.AddOwner<TextBox>();
 
-        public static readonly StyledProperty<string> WatermarkProperty =
-            AvaloniaProperty.Register<TextBox, string>(nameof(Watermark));
+        public static readonly StyledProperty<string?> WatermarkProperty =
+            AvaloniaProperty.Register<TextBox, string?>(nameof(Watermark));
 
         public static readonly StyledProperty<bool> UseFloatingWatermarkProperty =
             AvaloniaProperty.Register<TextBox, bool>(nameof(UseFloatingWatermark));
@@ -159,10 +159,10 @@ namespace Avalonia.Controls
 
         readonly struct UndoRedoState : IEquatable<UndoRedoState>
         {
-            public string Text { get; }
+            public string? Text { get; }
             public int CaretPosition { get; }
 
-            public UndoRedoState(string text, int caretPosition)
+            public UndoRedoState(string? text, int caretPosition)
             {
                 Text = text;
                 CaretPosition = caretPosition;
@@ -170,16 +170,16 @@ namespace Avalonia.Controls
 
             public bool Equals(UndoRedoState other) => ReferenceEquals(Text, other.Text) || Equals(Text, other.Text);
 
-            public override bool Equals(object obj) => obj is UndoRedoState other && Equals(other);
+            public override bool Equals(object? obj) => obj is UndoRedoState other && Equals(other);
 
-            public override int GetHashCode() => Text.GetHashCode();
+            public override int GetHashCode() => Text?.GetHashCode() ?? 0;
         }
 
-        private string _text;
+        private string? _text;
         private int _caretIndex;
         private int _selectionStart;
         private int _selectionEnd;
-        private TextPresenter _presenter;
+        private TextPresenter? _presenter;
         private TextBoxTextInputMethodClient _imClient = new TextBoxTextInputMethodClient();
         private UndoRedoHelper<UndoRedoState> _undoRedoHelper;
         private bool _isUndoingRedoing;
@@ -268,19 +268,19 @@ namespace Avalonia.Controls
             set => SetValue(PasswordCharProperty, value);
         }
 
-        public IBrush SelectionBrush
+        public IBrush? SelectionBrush
         {
             get => GetValue(SelectionBrushProperty);
             set => SetValue(SelectionBrushProperty, value);
         }
 
-        public IBrush SelectionForegroundBrush
+        public IBrush? SelectionForegroundBrush
         {
             get => GetValue(SelectionForegroundBrushProperty);
             set => SetValue(SelectionForegroundBrushProperty, value);
         }
 
-        public IBrush CaretBrush
+        public IBrush? CaretBrush
         {
             get => GetValue(CaretBrushProperty);
             set => SetValue(CaretBrushProperty, value);
@@ -337,7 +337,7 @@ namespace Avalonia.Controls
         }
 
         [Content]
-        public string Text
+        public string? Text
         {
             get { return _text; }
             set
@@ -400,7 +400,7 @@ namespace Avalonia.Controls
             set { SetValue(TextAlignmentProperty, value); }
         }
 
-        public string Watermark
+        public string? Watermark
         {
             get { return GetValue(WatermarkProperty); }
             set { SetValue(WatermarkProperty, value); }
@@ -512,19 +512,19 @@ namespace Avalonia.Controls
             }
         }
 
-        public event EventHandler<RoutedEventArgs> CopyingToClipboard
+        public event EventHandler<RoutedEventArgs>? CopyingToClipboard
         {
             add => AddHandler(CopyingToClipboardEvent, value);
             remove => RemoveHandler(CopyingToClipboardEvent, value);
         }
 
-        public event EventHandler<RoutedEventArgs> CuttingToClipboard
+        public event EventHandler<RoutedEventArgs>? CuttingToClipboard
         {
             add => AddHandler(CuttingToClipboardEvent, value);
             remove => RemoveHandler(CuttingToClipboardEvent, value);
         }
 
-        public event EventHandler<RoutedEventArgs> PastingFromClipboard
+        public event EventHandler<RoutedEventArgs>? PastingFromClipboard
         {
             add => AddHandler(PastingFromClipboardEvent, value);
             remove => RemoveHandler(PastingFromClipboardEvent, value);
@@ -615,7 +615,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void HandleTextInput(string input)
+        private void HandleTextInput(string? input)
         {
             if (IsReadOnly)
             {
@@ -671,8 +671,11 @@ namespace Avalonia.Controls
             }
         }
 
-        public string RemoveInvalidCharacters(string text)
+        public string? RemoveInvalidCharacters(string? text)
         {
+            if (text is null)
+                return null;
+
             for (var i = 0; i < invalidCharacters.Length; i++)
             {
                 text = text.Replace(invalidCharacters[i], string.Empty);
@@ -694,7 +697,7 @@ namespace Avalonia.Controls
             if (!eventArgs.Handled)
             {
                 SnapshotUndoRedo();
-                await ((IClipboard)AvaloniaLocator.Current.GetService(typeof(IClipboard)))
+                await ((IClipboard)AvaloniaLocator.Current.GetRequiredService(typeof(IClipboard)))
                     .SetTextAsync(text);
                 DeleteSelection();
             }
@@ -712,7 +715,7 @@ namespace Avalonia.Controls
             RaiseEvent(eventArgs);
             if (!eventArgs.Handled)
             {
-                await ((IClipboard)AvaloniaLocator.Current.GetService(typeof(IClipboard)))
+                await ((IClipboard)AvaloniaLocator.Current.GetRequiredService(typeof(IClipboard)))
                     .SetTextAsync(text);
             }
         }
@@ -726,7 +729,7 @@ namespace Avalonia.Controls
                 return;
             }
 
-            var text = await ((IClipboard)AvaloniaLocator.Current.GetService(typeof(IClipboard))).GetTextAsync();
+            var text = await ((IClipboard)AvaloniaLocator.Current.GetRequiredService(typeof(IClipboard))).GetTextAsync();
 
             if (string.IsNullOrEmpty(text))
             {
@@ -746,7 +749,7 @@ namespace Avalonia.Controls
             bool handled = false;
             var modifiers = e.KeyModifiers;
 
-            var keymap = AvaloniaLocator.Current.GetService<PlatformHotkeyConfiguration>();
+            var keymap = AvaloniaLocator.Current.GetRequiredService<PlatformHotkeyConfiguration>();
 
             bool Match(List<KeyGesture> gestures) => gestures.Any(g => g.Matches(e));
             bool DetectSelection() => e.KeyModifiers.HasAllFlags(keymap.SelectionModifiers);
@@ -1006,7 +1009,7 @@ namespace Avalonia.Controls
             if (text != null && clickInfo.Properties.IsLeftButtonPressed && !(clickInfo.Pointer?.Captured is Border))
             {
                 var point = e.GetPosition(_presenter);
-                var index = _presenter.GetCaretIndex(point);
+                var index = _presenter!.GetCaretIndex(point);
                 var clickToSelect = index != CaretIndex && e.KeyModifiers.HasFlag(KeyModifiers.Shift);
 
                 if (!clickToSelect)
@@ -1096,7 +1099,7 @@ namespace Avalonia.Controls
 
         private int CoerceCaretIndex(int value) => CoerceCaretIndex(value, Text);
 
-        private int CoerceCaretIndex(int value, string text)
+        private int CoerceCaretIndex(int value, string? text)
         {
             if (text == null)
             {
@@ -1127,29 +1130,6 @@ namespace Avalonia.Controls
             Text = string.Empty;
         }
 
-        private int DeleteCharacter(int index)
-        {
-            var start = index + 1;
-            var text = Text;
-            var c = text[index];
-            var result = 1;
-
-            if (c == '\n' && index > 0 && text[index - 1] == '\r')
-            {
-                --index;
-                ++result;
-            }
-            else if (c == '\r' && index < text.Length - 1 && text[index + 1] == '\n')
-            {
-                ++start;
-                ++result;
-            }
-
-            Text = text.Substring(0, index) + text.Substring(start);
-
-            return result;
-        }
-
         private void MoveHorizontal(int direction, bool wholeWord, bool isSelecting)
         {
             var text = Text ?? string.Empty;
@@ -1327,7 +1307,7 @@ namespace Avalonia.Controls
                 {
                     var start = Math.Min(selectionStart, selectionEnd);
                     var end = Math.Max(selectionStart, selectionEnd);
-                    var text = Text;
+                    var text = Text!;
                     SetTextInternal(text.Substring(0, start) + text.Substring(end), raiseTextChanged);
                     CaretIndex = start;
                     ClearSelection();
diff --git a/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs b/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
index 334db2cafd..ef583e5d81 100644
--- a/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
+++ b/src/Avalonia.Controls/TextBoxTextInputMethodClient.cs
@@ -8,9 +8,9 @@ namespace Avalonia.Controls
 {
     internal class TextBoxTextInputMethodClient : ITextInputMethodClient
     {
-        private InputElement _parent;
-        private TextPresenter _presenter;
-        private IDisposable _subscription;
+        private InputElement? _parent;
+        private TextPresenter? _presenter;
+        private IDisposable? _subscription;
         public Rect CursorRectangle
         {
             get
@@ -28,17 +28,17 @@ namespace Avalonia.Controls
             }
         }
 
-        public event EventHandler CursorRectangleChanged;
-        public IVisual TextViewVisual => _presenter;
-        public event EventHandler TextViewVisualChanged;
+        public event EventHandler? CursorRectangleChanged;
+        public IVisual TextViewVisual => _presenter!;
+        public event EventHandler? TextViewVisualChanged;
         public bool SupportsPreedit => false;
         public void SetPreeditText(string text) => throw new NotSupportedException();
 
         public bool SupportsSurroundingText => false;
         public TextInputMethodSurroundingText SurroundingText => throw new NotSupportedException();
-        public event EventHandler SurroundingTextChanged { add { } remove { } }
-        public string TextBeforeCursor => null;
-        public string TextAfterCursor => null;
+        public event EventHandler? SurroundingTextChanged { add { } remove { } }
+        public string? TextBeforeCursor => null;
+        public string? TextAfterCursor => null;
 
         private void OnCaretIndexChanged(int index) => CursorRectangleChanged?.Invoke(this, EventArgs.Empty);
 
diff --git a/src/Avalonia.Controls/ToggleSwitch.cs b/src/Avalonia.Controls/ToggleSwitch.cs
index 662a355dac..f33f2b9df3 100644
--- a/src/Avalonia.Controls/ToggleSwitch.cs
+++ b/src/Avalonia.Controls/ToggleSwitch.cs
@@ -12,8 +12,8 @@ namespace Avalonia.Controls
     [PseudoClasses(":dragging")]
     public class ToggleSwitch : ToggleButton
     {
-        private Panel _knobsPanel;
-        private Panel _switchKnob;
+        private Panel? _knobsPanel;
+        private Panel? _switchKnob;
         private bool _knobsPanelPressed = false;
         private Point _switchStartPoint = new Point();
         private double _initLeft = -1;
@@ -43,31 +43,31 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="OffContent"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> OffContentProperty =
-         AvaloniaProperty.Register<ToggleSwitch, object>(nameof(OffContent), defaultValue: "Off");
+        public static readonly StyledProperty<object?> OffContentProperty =
+         AvaloniaProperty.Register<ToggleSwitch, object?>(nameof(OffContent), defaultValue: "Off");
 
         /// <summary>
         /// Defines the <see cref="OffContentTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> OffContentTemplateProperty =
-            AvaloniaProperty.Register<ToggleSwitch, IDataTemplate>(nameof(OffContentTemplate));
+        public static readonly StyledProperty<IDataTemplate?> OffContentTemplateProperty =
+            AvaloniaProperty.Register<ToggleSwitch, IDataTemplate?>(nameof(OffContentTemplate));
 
         /// <summary>
         /// Defines the <see cref="OnContent"/> property.
         /// </summary>
-        public static readonly StyledProperty<object> OnContentProperty =
-            AvaloniaProperty.Register<ToggleSwitch, object>(nameof(OnContent), defaultValue: "On");
+        public static readonly StyledProperty<object?> OnContentProperty =
+            AvaloniaProperty.Register<ToggleSwitch, object?>(nameof(OnContent), defaultValue: "On");
 
         /// <summary>
         /// Defines the <see cref="OnContentTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate> OnContentTemplateProperty =
-            AvaloniaProperty.Register<ToggleSwitch, IDataTemplate>(nameof(OnContentTemplate));
+        public static readonly StyledProperty<IDataTemplate?> OnContentTemplateProperty =
+            AvaloniaProperty.Register<ToggleSwitch, IDataTemplate?>(nameof(OnContentTemplate));
 
         /// <summary>
         /// Gets or Sets the Content that is displayed when in the On State.
         /// </summary>
-        public object OnContent
+        public object? OnContent
         {
             get { return GetValue(OnContentProperty); }
             set { SetValue(OnContentProperty, value); }
@@ -76,19 +76,19 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or Sets the Content that is displayed when in the Off State.
         /// </summary>
-        public object OffContent
+        public object? OffContent
         {
             get { return GetValue(OffContentProperty); }
             set { SetValue(OffContentProperty, value); }
         }
 
-        public IContentPresenter OffContentPresenter
+        public IContentPresenter? OffContentPresenter
         {
             get;
             private set;
         }
 
-        public IContentPresenter OnContentPresenter
+        public IContentPresenter? OnContentPresenter
         {
             get;
             private set;
@@ -97,7 +97,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or Sets the <see cref="IDataTemplate"/> used to display the <see cref="OffContent"/>.
         /// </summary>
-        public IDataTemplate OffContentTemplate
+        public IDataTemplate? OffContentTemplate
         {
             get { return GetValue(OffContentTemplateProperty); }
             set { SetValue(OffContentTemplateProperty, value); }
@@ -106,7 +106,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or Sets the <see cref="IDataTemplate"/> used to display the <see cref="OnContent"/>.
         /// </summary>
-        public IDataTemplate OnContentTemplate
+        public IDataTemplate? OnContentTemplate
         {
             get { return GetValue(OnContentTemplateProperty); }
             set { SetValue(OnContentTemplateProperty, value); }
@@ -162,7 +162,7 @@ namespace Avalonia.Controls
             base.OnApplyTemplate(e);
 
             _switchKnob = e.NameScope.Find<Panel>("SwitchKnob");
-            _knobsPanel = e.NameScope.Find<Panel>("MovingKnobs");
+            _knobsPanel = e.NameScope.Get<Panel>("MovingKnobs");
             
             _knobsPanel.PointerPressed += KnobsPanel_PointerPressed;
             _knobsPanel.PointerReleased += KnobsPanel_PointerReleased;
@@ -174,20 +174,20 @@ namespace Avalonia.Controls
             }
         }
         
-        private void KnobsPanel_PointerPressed(object sender, Input.PointerPressedEventArgs e)
+        private void KnobsPanel_PointerPressed(object? sender, Input.PointerPressedEventArgs e)
         {
             _switchStartPoint = e.GetPosition(_switchKnob);
-            _initLeft = Canvas.GetLeft(_knobsPanel);
+            _initLeft = Canvas.GetLeft(_knobsPanel!);
             _isDragging = false;
             _knobsPanelPressed = true;
         }
 
-        private void KnobsPanel_PointerReleased(object sender, Input.PointerReleasedEventArgs e)
+        private void KnobsPanel_PointerReleased(object? sender, Input.PointerReleasedEventArgs e)
         {
             if (_isDragging)
             {
-                bool shouldBecomeChecked = Canvas.GetLeft(_knobsPanel) >= (_switchKnob.Bounds.Width / 2);
-                _knobsPanel.ClearValue(Canvas.LeftProperty);
+                bool shouldBecomeChecked = Canvas.GetLeft(_knobsPanel!) >= (_switchKnob!.Bounds.Width / 2);
+                _knobsPanel!.ClearValue(Canvas.LeftProperty);
 
                 PseudoClasses.Set(":dragging", false);
 
@@ -210,7 +210,7 @@ namespace Avalonia.Controls
             _knobsPanelPressed = false;
         }
 
-        private void KnobsPanel_PointerMoved(object sender, Input.PointerEventArgs e)
+        private void KnobsPanel_PointerMoved(object? sender, Input.PointerEventArgs e)
         {
             if (_knobsPanelPressed)
             {
@@ -224,7 +224,7 @@ namespace Avalonia.Controls
 
                 if (_isDragging)
                 {
-                    Canvas.SetLeft(_knobsPanel, System.Math.Min(_switchKnob.Bounds.Width, System.Math.Max(0, (_initLeft + difference.X))));
+                    Canvas.SetLeft(_knobsPanel!, System.Math.Min(_switchKnob!.Bounds.Width, System.Math.Max(0, (_initLeft + difference.X))));
                 }
             }
         }
diff --git a/src/Avalonia.Controls/ToolTip.cs b/src/Avalonia.Controls/ToolTip.cs
index ab310d60ef..95011a4425 100644
--- a/src/Avalonia.Controls/ToolTip.cs
+++ b/src/Avalonia.Controls/ToolTip.cs
@@ -297,7 +297,7 @@ namespace Avalonia.Controls
         {
             if (host is PopupRoot pr)
             {
-                pr.PlatformImpl.SetWindowManagerAddShadowHint(hint);
+                pr.PlatformImpl?.SetWindowManagerAddShadowHint(hint);
             }
         }
 
diff --git a/src/Avalonia.Controls/ToolTipService.cs b/src/Avalonia.Controls/ToolTipService.cs
index f6458c2112..a3100ff296 100644
--- a/src/Avalonia.Controls/ToolTipService.cs
+++ b/src/Avalonia.Controls/ToolTipService.cs
@@ -12,7 +12,7 @@ namespace Avalonia.Controls
     {
         public static ToolTipService Instance { get; } = new ToolTipService();
 
-        private DispatcherTimer _timer;
+        private DispatcherTimer? _timer;
 
         private ToolTipService() { }
 
@@ -46,7 +46,7 @@ namespace Avalonia.Controls
                 {
                     var tip = control.GetValue(ToolTip.ToolTipProperty);
 
-                    tip.Content = e.NewValue;
+                    tip!.Content = e.NewValue;
                 }
             }
         }
@@ -67,9 +67,9 @@ namespace Avalonia.Controls
             }
         }
         
-        private void ControlDetaching(object sender, VisualTreeAttachmentEventArgs e)
+        private void ControlDetaching(object? sender, VisualTreeAttachmentEventArgs e)
         {
-            var control = (Control)sender;
+            var control = (Control)sender!;
             control.DetachedFromVisualTree -= ControlDetaching;
             control.EffectiveViewportChanged -= ControlEffectiveViewportChanged;
             Close(control);
@@ -80,11 +80,11 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void ControlPointerEnter(object sender, PointerEventArgs e)
+        private void ControlPointerEnter(object? sender, PointerEventArgs e)
         {
             StopTimer();
 
-            var control = (Control)sender;
+            var control = (Control)sender!;
             var showDelay = ToolTip.GetShowDelay(control);
             if (showDelay == 0)
             {
@@ -101,15 +101,15 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void ControlPointerLeave(object sender, PointerEventArgs e)
+        private void ControlPointerLeave(object? sender, PointerEventArgs e)
         {
-            var control = (Control)sender;
+            var control = (Control)sender!;
             Close(control);
         }
 
-        private void ControlEffectiveViewportChanged(object sender, Layout.EffectiveViewportChangedEventArgs e)
+        private void ControlEffectiveViewportChanged(object? sender, Layout.EffectiveViewportChangedEventArgs e)
         {
-            var control = (Control)sender;
+            var control = (Control)sender!;
             var toolTip = control.GetValue(ToolTip.ToolTipProperty);
             toolTip?.RecalculatePosition(control);
         }
diff --git a/src/Avalonia.Controls/TopLevel.cs b/src/Avalonia.Controls/TopLevel.cs
index 9eb35e0548..6bba889748 100644
--- a/src/Avalonia.Controls/TopLevel.cs
+++ b/src/Avalonia.Controls/TopLevel.cs
@@ -51,8 +51,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="IInputRoot.PointerOverElement"/> property.
         /// </summary>
-        public static readonly StyledProperty<IInputElement> PointerOverElementProperty =
-            AvaloniaProperty.Register<TopLevel, IInputElement>(nameof(IInputRoot.PointerOverElement));
+        public static readonly StyledProperty<IInputElement?> PointerOverElementProperty =
+            AvaloniaProperty.Register<TopLevel, IInputElement?>(nameof(IInputRoot.PointerOverElement));
 
         /// <summary>
         /// Defines the <see cref="TransparencyLevelHint"/> property.
@@ -80,16 +80,16 @@ namespace Avalonia.Controls
                 (s, h) => s.ResourcesChanged -= h
             );
 
-        private readonly IInputManager _inputManager;
-        private readonly IAccessKeyHandler _accessKeyHandler;
-        private readonly IKeyboardNavigationHandler _keyboardNavigationHandler;
-        private readonly IPlatformRenderInterface _renderInterface;
-        private readonly IGlobalStyles _globalStyles;
+        private readonly IInputManager? _inputManager;
+        private readonly IAccessKeyHandler? _accessKeyHandler;
+        private readonly IKeyboardNavigationHandler? _keyboardNavigationHandler;
+        private readonly IPlatformRenderInterface? _renderInterface;
+        private readonly IGlobalStyles? _globalStyles;
         private Size _clientSize;
         private Size? _frameSize;
         private WindowTransparencyLevel _actualTransparencyLevel;
-        private ILayoutManager _layoutManager;
-        private Border _transparencyFallbackBorder;
+        private ILayoutManager? _layoutManager;
+        private Border? _transparencyFallbackBorder;
 
         /// <summary>
         /// Initializes static members of the <see cref="TopLevel"/> class.
@@ -104,7 +104,7 @@ namespace Avalonia.Controls
                 {
                     if (tl.PlatformImpl != null)
                     {
-                        tl.PlatformImpl.SetTransparencyLevelHint((WindowTransparencyLevel)e.NewValue);
+                        tl.PlatformImpl.SetTransparencyLevelHint((WindowTransparencyLevel)e.NewValue!);
                         tl.HandleTransparencyLevelChanged(tl.PlatformImpl.TransparencyLevel);
                     }
                 });
@@ -126,7 +126,7 @@ namespace Avalonia.Controls
         /// <param name="dependencyResolver">
         /// The dependency resolver to use. If null the default dependency resolver will be used.
         /// </param>
-        public TopLevel(ITopLevelImpl impl, IAvaloniaDependencyResolver dependencyResolver)
+        public TopLevel(ITopLevelImpl impl, IAvaloniaDependencyResolver? dependencyResolver)
         {
             if (impl == null)
             {
@@ -155,6 +155,11 @@ namespace Avalonia.Controls
             {
                 Renderer.SceneInvalidated += SceneInvalidated;
             }
+            else
+            {
+                // Prevent nullable error.
+                Renderer = null!;
+            }
 
             impl.SetInputRoot(this);
 
@@ -195,12 +200,12 @@ namespace Avalonia.Controls
         /// <summary>
         /// Fired when the window is opened.
         /// </summary>
-        public event EventHandler Opened;
+        public event EventHandler? Opened;
 
         /// <summary>
         /// Fired when the window is closed.
         /// </summary>
-        public event EventHandler Closed;
+        public event EventHandler? Closed;
 
         /// <summary>
         /// Gets or sets the client size of the window.
@@ -261,8 +266,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
-        [CanBeNull]
-        public ITopLevelImpl PlatformImpl { get; private set; }
+        public ITopLevelImpl? PlatformImpl { get; private set; }
         
         /// <summary>
         /// Gets the renderer for the window.
@@ -272,26 +276,26 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the access key handler for the window.
         /// </summary>
-        IAccessKeyHandler IInputRoot.AccessKeyHandler => _accessKeyHandler;
+        IAccessKeyHandler IInputRoot.AccessKeyHandler => _accessKeyHandler!;
 
         /// <summary>
         /// Gets or sets the keyboard navigation handler for the window.
         /// </summary>
-        IKeyboardNavigationHandler IInputRoot.KeyboardNavigationHandler => _keyboardNavigationHandler;
+        IKeyboardNavigationHandler IInputRoot.KeyboardNavigationHandler => _keyboardNavigationHandler!;
 
         /// <summary>
         /// Gets or sets the input element that the pointer is currently over.
         /// </summary>
-        IInputElement IInputRoot.PointerOverElement
+        IInputElement? IInputRoot.PointerOverElement
         {
             get { return GetValue(PointerOverElementProperty); }
             set { SetValue(PointerOverElementProperty, value); }
         }
 
         /// <inheritdoc/>
-        IMouseDevice IInputRoot.MouseDevice => PlatformImpl?.MouseDevice;
+        IMouseDevice? IInputRoot.MouseDevice => PlatformImpl?.MouseDevice;
 
-        void IWeakEventSubscriber<ResourcesChangedEventArgs>.OnEvent(object sender, WeakEvent ev, ResourcesChangedEventArgs e)
+        void IWeakEventSubscriber<ResourcesChangedEventArgs>.OnEvent(object? sender, WeakEvent ev, ResourcesChangedEventArgs e)
         {
             ((ILogical)this).NotifyResourcesChanged(e);
         }
@@ -311,7 +315,7 @@ namespace Avalonia.Controls
         /// <inheritdoc/>
         double IRenderRoot.RenderScaling => PlatformImpl?.RenderScaling ?? 1;
 
-        IStyleHost IStyleHost.StylingParent => _globalStyles;
+        IStyleHost IStyleHost.StylingParent => _globalStyles!;
 
         IRenderTarget IRenderRoot.CreateRenderTarget() => CreateRenderTarget();
 
@@ -320,7 +324,7 @@ namespace Avalonia.Controls
         {
             if(PlatformImpl == null)
                 throw new InvalidOperationException("Can't create render target, PlatformImpl is null (might be already disposed)");
-            return _renderInterface.CreateRenderTarget(PlatformImpl.Surfaces);
+            return _renderInterface!.CreateRenderTarget(PlatformImpl.Surfaces);
         }
 
         /// <inheritdoc/>
@@ -367,7 +371,7 @@ namespace Avalonia.Controls
             }
 
             Renderer?.Dispose();
-            Renderer = null;
+            Renderer = null!;
             
             (this as IInputRoot).MouseDevice?.TopLevelClosed(this);
             PlatformImpl = null;
@@ -394,7 +398,7 @@ namespace Avalonia.Controls
         protected virtual void HandleResized(Size clientSize, PlatformResizeReason reason)
         {
             ClientSize = clientSize;
-            FrameSize = PlatformImpl.FrameSize;
+            FrameSize = PlatformImpl!.FrameSize;
             Width = clientSize.Width;
             Height = clientSize.Height;
             LayoutManager.ExecuteLayoutPass();
@@ -457,6 +461,9 @@ namespace Avalonia.Controls
         {
             base.OnApplyTemplate(e);
 
+            if (PlatformImpl is null)
+                return;
+
             _transparencyFallbackBorder = e.NameScope.Find<Border>("PART_TransparencyFallback");
 
             HandleTransparencyLevelChanged(PlatformImpl.TransparencyLevel);
@@ -481,7 +488,7 @@ namespace Avalonia.Controls
         /// <typeparam name="T">The service type.</typeparam>
         /// <param name="resolver">The resolver.</param>
         /// <returns>The service.</returns>
-        private T TryGetService<T>(IAvaloniaDependencyResolver resolver) where T : class
+        private T? TryGetService<T>(IAvaloniaDependencyResolver resolver) where T : class
         {
             var result = resolver.GetService<T>();
 
@@ -502,27 +509,27 @@ namespace Avalonia.Controls
         /// <param name="e">The event args.</param>
         private void HandleInput(RawInputEventArgs e)
         {
-            _inputManager.ProcessInput(e);
+            _inputManager?.ProcessInput(e);
         }
 
-        private void SceneInvalidated(object sender, SceneInvalidatedEventArgs e)
+        private void SceneInvalidated(object? sender, SceneInvalidatedEventArgs e)
         {
-            (this as IInputRoot).MouseDevice.SceneInvalidated(this, e.DirtyRect);
+            (this as IInputRoot).MouseDevice?.SceneInvalidated(this, e.DirtyRect);
         }
 
         void PlatformImpl_LostFocus()
         {
-            var focused = (IVisual)FocusManager.Instance.Current;
+            var focused = (IVisual?)FocusManager.Instance?.Current;
             if (focused == null)
                 return;
             while (focused.VisualParent != null)
                 focused = focused.VisualParent;
 
             if (focused == this)
-                KeyboardDevice.Instance.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None);
+                KeyboardDevice.Instance?.SetFocusedElement(null, NavigationMethod.Unspecified, KeyModifiers.None);
         }
 
-        ITextInputMethodImpl ITextInputMethodRoot.InputMethod =>
+        ITextInputMethodImpl? ITextInputMethodRoot.InputMethod =>
             (PlatformImpl as ITopLevelImplWithTextInputMethod)?.TextInputMethod;
     }
 }
diff --git a/src/Avalonia.Controls/TrayIcon.cs b/src/Avalonia.Controls/TrayIcon.cs
index 03ffe830ed..02ab6228fa 100644
--- a/src/Avalonia.Controls/TrayIcon.cs
+++ b/src/Avalonia.Controls/TrayIcon.cs
@@ -110,7 +110,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Icon"/> property.
         /// </summary>
-        public static readonly StyledProperty<WindowIcon> IconProperty =
+        public static readonly StyledProperty<WindowIcon?> IconProperty =
             Window.IconProperty.AddOwner<TrayIcon>();
 
         /// <summary>
@@ -160,7 +160,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the icon of the TrayIcon.
         /// </summary>
-        public WindowIcon Icon
+        public WindowIcon? Icon
         {
             get => GetValue(IconProperty);
             set => SetValue(IconProperty, value);
@@ -186,7 +186,7 @@ namespace Avalonia.Controls
 
         public INativeMenuExporter? NativeMenuExporter => _impl?.MenuExporter;
 
-        private static void Lifetime_Exit(object sender, ControlledApplicationLifetimeExitEventArgs e)
+        private static void Lifetime_Exit(object? sender, ControlledApplicationLifetimeExitEventArgs e)
         {
             var app = Application.Current ?? throw new InvalidOperationException("Application not yet initialized.");
             var trayIcons = GetIcons(app);
@@ -194,9 +194,10 @@ namespace Avalonia.Controls
             RemoveIcons(trayIcons);
         }
 
-        private static void Icons_CollectionChanged(object sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
+        private static void Icons_CollectionChanged(object? sender, System.Collections.Specialized.NotifyCollectionChangedEventArgs e)
         {
-            RemoveIcons(e.OldItems.Cast<TrayIcon>());
+            if (e.OldItems is not null)
+                RemoveIcons(e.OldItems.Cast<TrayIcon>());
         }
 
         private static void RemoveIcons(IEnumerable<TrayIcon> icons)
@@ -213,7 +214,7 @@ namespace Avalonia.Controls
 
             if (change.Property == IconProperty)
             {
-                _impl?.SetIcon(Icon.PlatformImpl);
+                _impl?.SetIcon(Icon?.PlatformImpl);
             }
             else if (change.Property == IsVisibleProperty)
             {
diff --git a/src/Avalonia.Controls/TreeView.cs b/src/Avalonia.Controls/TreeView.cs
index 03f6fd3aaa..9e0a31ad64 100644
--- a/src/Avalonia.Controls/TreeView.cs
+++ b/src/Avalonia.Controls/TreeView.cs
@@ -33,7 +33,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="SelectedItem"/> property.
         /// </summary>
-        public static readonly DirectProperty<TreeView, object> SelectedItemProperty =
+        public static readonly DirectProperty<TreeView, object?> SelectedItemProperty =
             SelectingItemsControl.SelectedItemProperty.AddOwner<TreeView>(
                 o => o.SelectedItem,
                 (o, v) => o.SelectedItem = v);
@@ -42,7 +42,8 @@ namespace Avalonia.Controls
         /// Defines the <see cref="SelectedItems"/> property.
         /// </summary>
         public static readonly DirectProperty<TreeView, IList> SelectedItemsProperty =
-            ListBox.SelectedItemsProperty.AddOwner<TreeView>(
+            AvaloniaProperty.RegisterDirect<TreeView, IList>(
+                nameof(SelectedItems),
                 o => o.SelectedItems,
                 (o, v) => o.SelectedItems = v);
 
@@ -53,8 +54,8 @@ namespace Avalonia.Controls
             ListBox.SelectionModeProperty.AddOwner<TreeView>();
 
         private static readonly IList Empty = Array.Empty<object>();
-        private object _selectedItem;
-        private IList _selectedItems;
+        private object? _selectedItem;
+        private IList? _selectedItems;
         private bool _syncingSelectedItems;
 
         /// <summary>
@@ -68,7 +69,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Occurs when the control's selection changes.
         /// </summary>
-        public event EventHandler<SelectionChangedEventArgs> SelectionChanged
+        public event EventHandler<SelectionChangedEventArgs>? SelectionChanged
         {
             add => AddHandler(SelectingItemsControl.SelectionChangedEvent, value);
             remove => RemoveHandler(SelectingItemsControl.SelectionChangedEvent, value);
@@ -78,7 +79,7 @@ namespace Avalonia.Controls
         /// Gets the <see cref="ITreeItemContainerGenerator"/> for the tree view.
         /// </summary>
         public new ITreeItemContainerGenerator ItemContainerGenerator =>
-            (ITreeItemContainerGenerator)base.ItemContainerGenerator;
+            (ITreeItemContainerGenerator)base.ItemContainerGenerator!;
 
         /// <summary>
         /// Gets or sets a value indicating whether to automatically scroll to newly selected items.
@@ -105,7 +106,7 @@ namespace Avalonia.Controls
         /// Note that setting this property only currently works if the item is expanded to be visible.
         /// To select non-expanded nodes use `Selection.SelectedIndex`.
         /// </remarks>
-        public object SelectedItem
+        public object? SelectedItem
         {
             get => _selectedItem;
             set
@@ -187,7 +188,7 @@ namespace Avalonia.Controls
         /// </remarks>
         public void SelectAll()
         {
-            SynchronizeItems(SelectedItems, ItemContainerGenerator.Index.Items);
+            SynchronizeItems(SelectedItems, ItemContainerGenerator.Index!.Items);
         }
 
         /// <summary>
@@ -228,20 +229,20 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void SelectedItemsCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        private void SelectedItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
-            IList added = null;
-            IList removed = null;
+            IList? added = null;
+            IList? removed = null;
 
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
 
-                    SelectedItemsAdded(e.NewItems.Cast<object>().ToArray());
+                    SelectedItemsAdded(e.NewItems!.Cast<object>().ToArray());
 
                     if (AutoScrollToSelectedItem)
                     {
-                        var container = (TreeViewItem)ItemContainerGenerator.Index.ContainerFromItem(e.NewItems[0]);
+                        var container = ItemContainerGenerator.Index!.ContainerFromItem(e.NewItems![0]!);
 
                         container?.BringIntoView();
                     }
@@ -271,7 +272,7 @@ namespace Avalonia.Controls
                         }
                     }
 
-                    foreach (var item in e.OldItems)
+                    foreach (var item in e.OldItems!)
                     {
                         MarkItemSelected(item, false);
                     }
@@ -281,7 +282,7 @@ namespace Avalonia.Controls
                     break;
                 case NotifyCollectionChangedAction.Reset:
 
-                    foreach (IControl container in ItemContainerGenerator.Index.Containers)
+                    foreach (IControl container in ItemContainerGenerator.Index!.Containers)
                     {
                         MarkContainerSelected(container, false);
                     }
@@ -300,12 +301,12 @@ namespace Avalonia.Controls
                     break;
                 case NotifyCollectionChangedAction.Replace:
 
-                    foreach (var item in e.OldItems)
+                    foreach (var item in e.OldItems!)
                     {
                         MarkItemSelected(item, false);
                     }
 
-                    foreach (var item in e.NewItems)
+                    foreach (var item in e.NewItems!)
                     {
                         MarkItemSelected(item, true);
                     }
@@ -336,7 +337,7 @@ namespace Avalonia.Controls
 
         private void MarkItemSelected(object item, bool selected)
         {
-            var container = ItemContainerGenerator.Index.ContainerFromItem(item);
+            var container = ItemContainerGenerator.Index!.ContainerFromItem(item)!;
 
             MarkContainerSelected(container, selected);
         }
@@ -369,7 +370,7 @@ namespace Avalonia.Controls
                 incc.CollectionChanged -= SelectedItemsCollectionChanged;
             }
         }
-        (bool handled, IInputElement next) ICustomKeyboardNavigation.GetNext(IInputElement element,
+        (bool handled, IInputElement? next) ICustomKeyboardNavigation.GetNext(IInputElement element,
             NavigationDirection direction)
         {
             if (direction == NavigationDirection.Next || direction == NavigationDirection.Previous)
@@ -377,7 +378,7 @@ namespace Avalonia.Controls
                 if (!this.IsVisualAncestorOf(element))
                 {
                     var result = _selectedItem != null ?
-                        ItemContainerGenerator.Index.ContainerFromItem(_selectedItem) :
+                        ItemContainerGenerator.Index!.ContainerFromItem(_selectedItem) :
                         ItemContainerGenerator.ContainerFromIndex(0);
                     
                     return (result != null, result); // SelectedItem may not be in the treeview.
@@ -393,7 +394,7 @@ namespace Avalonia.Controls
         protected override IItemContainerGenerator CreateItemContainerGenerator()
         {
             var result = CreateTreeItemContainerGenerator();
-            result.Index.Materialized += ContainerMaterialized;
+            result.Index!.Materialized += ContainerMaterialized;
             return result;
         }
 
@@ -416,7 +417,7 @@ namespace Avalonia.Controls
             if (e.NavigationMethod == NavigationMethod.Directional)
             {
                 e.Handled = UpdateSelectionFromEventSource(
-                    e.Source,
+                    e.Source!,
                     true,
                     e.KeyModifiers.HasAllFlags(KeyModifiers.Shift));
             }
@@ -431,13 +432,13 @@ namespace Avalonia.Controls
                 if (SelectedItem != null)
                 {
                     var next = GetContainerInDirection(
-                        GetContainerFromEventSource(e.Source),
+                        GetContainerFromEventSource(e.Source!),
                         direction.Value,
                         true);
 
                     if (next != null)
                     {
-                        FocusManager.Instance.Focus(next, NavigationMethod.Directional);
+                        FocusManager.Instance?.Focus(next, NavigationMethod.Directional);
                         e.Handled = true;
                     }
                 }
@@ -450,9 +451,9 @@ namespace Avalonia.Controls
             if (!e.Handled)
             {
                 var keymap = AvaloniaLocator.Current.GetService<PlatformHotkeyConfiguration>();
-                bool Match(List<KeyGesture> gestures) => gestures.Any(g => g.Matches(e));
+                bool Match(List<KeyGesture>? gestures) => gestures?.Any(g => g.Matches(e)) ?? false;
 
-                if (this.SelectionMode == SelectionMode.Multiple && Match(keymap.SelectAll))
+                if (this.SelectionMode == SelectionMode.Multiple && Match(keymap?.SelectAll))
                 {
                     SelectAll();
                     e.Handled = true;
@@ -460,47 +461,47 @@ namespace Avalonia.Controls
             }
         }
 
-        private TreeViewItem GetContainerInDirection(
-            TreeViewItem from,
+        private TreeViewItem? GetContainerInDirection(
+            TreeViewItem? from,
             NavigationDirection direction,
             bool intoChildren)
         {
-            IItemContainerGenerator parentGenerator = GetParentContainerGenerator(from);
+            IItemContainerGenerator? parentGenerator = GetParentContainerGenerator(from);
 
             if (parentGenerator == null)
             {
                 return null;
             }
 
-            var index = parentGenerator.IndexFromContainer(from);
-            var parent = from.Parent as ItemsControl;
-            TreeViewItem result = null;
+            var index = from is not null ? parentGenerator.IndexFromContainer(from) : -1;
+            var parent = from?.Parent as ItemsControl;
+            TreeViewItem? result = null;
 
             switch (direction)
             {
                 case NavigationDirection.Up:
                     if (index > 0)
                     {
-                        var previous = (TreeViewItem)parentGenerator.ContainerFromIndex(index - 1);
+                        var previous = (TreeViewItem)parentGenerator.ContainerFromIndex(index - 1)!;
                         result = previous.IsExpanded && previous.ItemCount > 0 ?
-                            (TreeViewItem)previous.ItemContainerGenerator.ContainerFromIndex(previous.ItemCount - 1) :
+                            (TreeViewItem)previous.ItemContainerGenerator.ContainerFromIndex(previous.ItemCount - 1)! :
                             previous;
                     }
                     else
                     {
-                        result = from.Parent as TreeViewItem;
+                        result = from?.Parent as TreeViewItem;
                     }
 
                     break;
 
                 case NavigationDirection.Down:
-                    if (from.IsExpanded && intoChildren && from.ItemCount > 0)
+                    if (from?.IsExpanded == true && intoChildren && from.ItemCount > 0)
                     {
-                        result = (TreeViewItem)from.ItemContainerGenerator.ContainerFromIndex(0);
+                        result = (TreeViewItem)from.ItemContainerGenerator.ContainerFromIndex(0)!;
                     }
                     else if (index < parent?.ItemCount - 1)
                     {
-                        result = (TreeViewItem)parentGenerator.ContainerFromIndex(index + 1);
+                        result = (TreeViewItem)parentGenerator.ContainerFromIndex(index + 1)!;
                     }
                     else if (parent is TreeViewItem parentItem)
                     {
@@ -549,18 +550,18 @@ namespace Avalonia.Controls
             bool toggleModifier = false,
             bool rightButton = false)
         {
-            var item = ItemContainerGenerator.Index.ItemFromContainer(container);
+            var item = ItemContainerGenerator.Index!.ItemFromContainer(container);
 
             if (item == null)
             {
                 return;
             }
 
-            IControl selectedContainer = null;
+            IControl? selectedContainer = null;
 
             if (SelectedItem != null)
             {
-                selectedContainer = ItemContainerGenerator.Index.ContainerFromItem(SelectedItem);
+                selectedContainer = ItemContainerGenerator.Index!.ContainerFromItem(SelectedItem);
             }
 
             var mode = SelectionMode;
@@ -607,7 +608,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private static IItemContainerGenerator GetParentContainerGenerator(TreeViewItem item)
+        private static IItemContainerGenerator? GetParentContainerGenerator(TreeViewItem? item)
         {
             if (item == null)
             {
@@ -632,12 +633,12 @@ namespace Avalonia.Controls
         /// <param name="nodeA">Nodes to find.</param>
         /// <param name="nodeB">Node to find.</param>
         /// <returns>Found first node.</returns>
-        private static TreeViewItem FindFirstNode(TreeView treeView, TreeViewItem nodeA, TreeViewItem nodeB)
+        private static TreeViewItem? FindFirstNode(TreeView treeView, TreeViewItem nodeA, TreeViewItem nodeB)
         {
             return FindInContainers(treeView.ItemContainerGenerator, nodeA, nodeB);
         }
 
-        private static TreeViewItem FindInContainers(ITreeItemContainerGenerator containerGenerator,
+        private static TreeViewItem? FindInContainers(ITreeItemContainerGenerator containerGenerator,
             TreeViewItem nodeA,
             TreeViewItem nodeB)
         {
@@ -645,7 +646,7 @@ namespace Avalonia.Controls
 
             foreach (ItemContainerInfo container in containers)
             {
-                TreeViewItem node = FindFirstNode(container.ContainerControl as TreeViewItem, nodeA, nodeB);
+                TreeViewItem? node = FindFirstNode(container.ContainerControl as TreeViewItem, nodeA, nodeB);
 
                 if (node != null)
                 {
@@ -656,14 +657,14 @@ namespace Avalonia.Controls
             return null;
         }
 
-        private static TreeViewItem FindFirstNode(TreeViewItem node, TreeViewItem nodeA, TreeViewItem nodeB)
+        private static TreeViewItem? FindFirstNode(TreeViewItem? node, TreeViewItem nodeA, TreeViewItem nodeB)
         {
             if (node == null)
             {
                 return null;
             }
 
-            TreeViewItem match = node == nodeA ? nodeA : node == nodeB ? nodeB : null;
+            TreeViewItem? match = node == nodeA ? nodeA : node == nodeB ? nodeB : null;
 
             if (match != null)
             {
@@ -679,7 +680,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="from">From container.</param>
         /// <param name="to">To container.</param>
-        private List<object> GetItemsInRange(TreeViewItem from, TreeViewItem to)
+        private List<object> GetItemsInRange(TreeViewItem? from, TreeViewItem? to)
         {
             var items = new List<object>();
 
@@ -688,7 +689,7 @@ namespace Avalonia.Controls
                 return items;
             }
 
-            TreeViewItem firstItem = FindFirstNode(this, from, to);
+            TreeViewItem? firstItem = FindFirstNode(this, from, to);
 
             if (firstItem == null)
             {
@@ -707,11 +708,11 @@ namespace Avalonia.Controls
                 wasReversed = true;
             }
 
-            TreeViewItem node = from;
+            TreeViewItem? node = from;
 
             while (node != to)
             {
-                var item = ItemContainerGenerator.Index.ItemFromContainer(node);
+                var item = ItemContainerGenerator.Index!.ItemFromContainer(node);
 
                 if (item != null)
                 {
@@ -721,7 +722,7 @@ namespace Avalonia.Controls
                 node = GetContainerInDirection(node, NavigationDirection.Down, true);
             }
 
-            var toItem = ItemContainerGenerator.Index.ItemFromContainer(to);
+            var toItem = ItemContainerGenerator.Index!.ItemFromContainer(to);
 
             if (toItem != null)
             {
@@ -772,7 +773,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="eventSource">The control that raised the event.</param>
         /// <returns>The container or null if the event did not originate in a container.</returns>
-        protected TreeViewItem GetContainerFromEventSource(IInteractive eventSource)
+        protected TreeViewItem? GetContainerFromEventSource(IInteractive eventSource)
         {
             var item = ((IVisual)eventSource).GetSelfAndVisualAncestors()
                 .OfType<TreeViewItem>()
@@ -794,7 +795,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="sender">The event sender.</param>
         /// <param name="e">The event args.</param>
-        private void ContainerMaterialized(object sender, ItemContainerEventArgs e)
+        private void ContainerMaterialized(object? sender, ItemContainerEventArgs e)
         {
             var selectedItem = SelectedItem;
 
diff --git a/src/Avalonia.Controls/TreeViewItem.cs b/src/Avalonia.Controls/TreeViewItem.cs
index 88fb3cd9f1..79e8a199f9 100644
--- a/src/Avalonia.Controls/TreeViewItem.cs
+++ b/src/Avalonia.Controls/TreeViewItem.cs
@@ -40,8 +40,8 @@ namespace Avalonia.Controls
         private static readonly ITemplate<IPanel> DefaultPanel =
             new FuncTemplate<IPanel>(() => new StackPanel());
 
-        private TreeView _treeView;
-        private IControl _header;
+        private TreeView? _treeView;
+        private IControl? _header;
         private bool _isExpanded;
         private int _level;
 
@@ -89,7 +89,7 @@ namespace Avalonia.Controls
         /// Gets the <see cref="ITreeItemContainerGenerator"/> for the tree view.
         /// </summary>
         public new ITreeItemContainerGenerator ItemContainerGenerator =>
-            (ITreeItemContainerGenerator)base.ItemContainerGenerator;
+            (ITreeItemContainerGenerator)base.ItemContainerGenerator!;
 
         /// <inheritdoc/>
         protected override IItemContainerGenerator CreateItemContainerGenerator() => CreateTreeItemContainerGenerator<TreeViewItem>();
@@ -174,7 +174,7 @@ namespace Avalonia.Controls
             _header = e.NameScope.Find<IControl>("PART_Header");
         }
 
-        private static int CalculateDistanceFromLogicalParent<T>(ILogical logical, int @default = -1) where T : class
+        private static int CalculateDistanceFromLogicalParent<T>(ILogical? logical, int @default = -1) where T : class
         {
             var result = 0;
 
diff --git a/src/Avalonia.Controls/Utils/AncestorFinder.cs b/src/Avalonia.Controls/Utils/AncestorFinder.cs
index 42e0d85dcd..b15e072052 100644
--- a/src/Avalonia.Controls/Utils/AncestorFinder.cs
+++ b/src/Avalonia.Controls/Utils/AncestorFinder.cs
@@ -12,11 +12,11 @@ namespace Avalonia.Controls.Utils
         {
             private readonly IStyledElement _control;
             private readonly Type _ancestorType;
-            public IObservable<IStyledElement> Observable => _subject;
-            private readonly Subject<IStyledElement> _subject = new Subject<IStyledElement>();
+            public IObservable<IStyledElement?> Observable => _subject;
+            private readonly Subject<IStyledElement?> _subject = new Subject<IStyledElement?>();
 
-            private FinderNode _child;
-            private IDisposable _disposable;
+            private FinderNode? _child;
+            private IDisposable? _disposable;
 
             public FinderNode(IStyledElement control, Type ancestorType)
             {
@@ -29,7 +29,7 @@ namespace Avalonia.Controls.Utils
                 _disposable = _control.GetObservable(Control.ParentProperty).Subscribe(OnValueChanged);
             }
 
-            private void OnValueChanged(IStyledElement next)
+            private void OnValueChanged(IStyledElement? next)
             {
                 if (next == null || _ancestorType.IsAssignableFrom(next.GetType()))
                     _subject.OnNext(next);
@@ -42,26 +42,26 @@ namespace Avalonia.Controls.Utils
                 }
             }
 
-            private void OnChildValueChanged(IStyledElement control) => _subject.OnNext(control);
+            private void OnChildValueChanged(IStyledElement? control) => _subject.OnNext(control);
 
 
             public void Dispose()
             {
                 _child?.Dispose();
                 _subject.Dispose();
-                _disposable.Dispose();
+                _disposable?.Dispose();
             }
         }
 
-        public static IObservable<T> Create<T>(IStyledElement control)
+        public static IObservable<T?> Create<T>(IStyledElement control)
             where T : IStyledElement
         {
-            return Create(control, typeof(T)).Cast<T>();
+            return Create(control, typeof(T)).Select(x => (T?)x);
         }
 
-        public static IObservable<IStyledElement> Create(IStyledElement control, Type ancestorType)
+        public static IObservable<IStyledElement?> Create(IStyledElement control, Type ancestorType)
         {
-            return new AnonymousObservable<IStyledElement>(observer =>
+            return new AnonymousObservable<IStyledElement?>(observer =>
             {
                 var finder = new FinderNode(control, ancestorType);
                 var subscription = finder.Observable.Subscribe(observer);
diff --git a/src/Avalonia.Controls/Utils/BorderRenderHelper.cs b/src/Avalonia.Controls/Utils/BorderRenderHelper.cs
index 201dc8eaf0..ecc7cd98bb 100644
--- a/src/Avalonia.Controls/Utils/BorderRenderHelper.cs
+++ b/src/Avalonia.Controls/Utils/BorderRenderHelper.cs
@@ -12,8 +12,8 @@ namespace Avalonia.Controls.Utils
     {
         private bool _useComplexRendering;
         private bool? _backendSupportsIndividualCorners;
-        private StreamGeometry _backgroundGeometryCache;
-        private StreamGeometry _borderGeometryCache;
+        private StreamGeometry? _backgroundGeometryCache;
+        private StreamGeometry? _borderGeometryCache;
         private Size _size;
         private Thickness _borderThickness;
         private CornerRadius _cornerRadius;
@@ -22,7 +22,7 @@ namespace Avalonia.Controls.Utils
 
         void Update(Size finalSize, Thickness borderThickness, CornerRadius cornerRadius)
         {
-            _backendSupportsIndividualCorners ??= AvaloniaLocator.Current.GetService<IPlatformRenderInterface>()
+            _backendSupportsIndividualCorners ??= AvaloniaLocator.Current.GetRequiredService<IPlatformRenderInterface>()
                 .SupportsIndividualRoundRects;
             _size = finalSize;
             _borderThickness = borderThickness;
@@ -41,8 +41,8 @@ namespace Avalonia.Controls.Utils
 
                 var boundRect = new Rect(finalSize);
                 var innerRect = boundRect.Deflate(borderThickness);
-                BorderGeometryKeypoints backgroundKeypoints = null;
-                StreamGeometry backgroundGeometry = null;
+                BorderGeometryKeypoints? backgroundKeypoints = null;
+                StreamGeometry? backgroundGeometry = null;
 
                 if (innerRect.Width != 0 && innerRect.Height != 0)
                 {
@@ -73,7 +73,7 @@ namespace Avalonia.Controls.Utils
 
                         if (backgroundGeometry != null)
                         {
-                            CreateGeometry(ctx, innerRect, backgroundKeypoints);
+                            CreateGeometry(ctx, innerRect, backgroundKeypoints!);
                         }
                     }
 
@@ -88,9 +88,9 @@ namespace Avalonia.Controls.Utils
 
         public void Render(DrawingContext context,
             Size finalSize, Thickness borderThickness, CornerRadius cornerRadius,
-            IBrush background, IBrush borderBrush, BoxShadows boxShadows, double borderDashOffset = 0,
+            IBrush? background, IBrush? borderBrush, BoxShadows boxShadows, double borderDashOffset = 0,
             PenLineCap borderLineCap = PenLineCap.Flat, PenLineJoin borderLineJoin = PenLineJoin.Miter,
-            AvaloniaList<double> borderDashArray = null)
+            AvaloniaList<double>? borderDashArray = null)
         {
             if (_size != finalSize
                 || _borderThickness != borderThickness
@@ -101,9 +101,9 @@ namespace Avalonia.Controls.Utils
                 borderDashArray);
         }
 
-        void RenderCore(DrawingContext context, IBrush background, IBrush borderBrush, BoxShadows boxShadows,
+        void RenderCore(DrawingContext context, IBrush? background, IBrush? borderBrush, BoxShadows boxShadows,
             double borderDashOffset, PenLineCap borderLineCap, PenLineJoin borderLineJoin,
-            AvaloniaList<double> borderDashArray)
+            AvaloniaList<double>? borderDashArray)
         {
             if (_useComplexRendering)
             {
@@ -122,7 +122,7 @@ namespace Avalonia.Controls.Utils
             else
             {
                 var borderThickness = _borderThickness.Top;
-                IPen pen = null;
+                IPen? pen = null;
 
 
                 ImmutableDashStyle? dashStyle = null;
diff --git a/src/Avalonia.Controls/Utils/IEnumerableUtils.cs b/src/Avalonia.Controls/Utils/IEnumerableUtils.cs
index fa5a09e245..5f2605936c 100644
--- a/src/Avalonia.Controls/Utils/IEnumerableUtils.cs
+++ b/src/Avalonia.Controls/Utils/IEnumerableUtils.cs
@@ -12,7 +12,7 @@ namespace Avalonia.Controls.Utils
             return items.IndexOf(item) != -1;
         }
 
-        public static bool TryGetCountFast(this IEnumerable items, out int count)
+        public static bool TryGetCountFast(this IEnumerable? items, out int count)
         {
             if (items != null)
             {
@@ -32,7 +32,7 @@ namespace Avalonia.Controls.Utils
             return false;
         }
 
-        public static int Count(this IEnumerable items)
+        public static int Count(this IEnumerable? items)
         {
             if (TryGetCountFast(items, out var count))
             {
@@ -50,7 +50,7 @@ namespace Avalonia.Controls.Utils
 
         public static int IndexOf(this IEnumerable items, object item)
         {
-            Contract.Requires<ArgumentNullException>(items != null);
+            _ = items ?? throw new ArgumentNullException(nameof(items));
 
             var list = items as IList;
 
@@ -76,9 +76,9 @@ namespace Avalonia.Controls.Utils
             }
         }
 
-        public static object ElementAt(this IEnumerable items, int index)
+        public static object? ElementAt(this IEnumerable items, int index)
         {
-            Contract.Requires<ArgumentNullException>(items != null);
+            _ = items ?? throw new ArgumentNullException(nameof(items));
 
             var list = items as IList;
 
diff --git a/src/Avalonia.Controls/Utils/ISelectionAdapter.cs b/src/Avalonia.Controls/Utils/ISelectionAdapter.cs
index 3c1006a12e..c5fb12197f 100644
--- a/src/Avalonia.Controls/Utils/ISelectionAdapter.cs
+++ b/src/Avalonia.Controls/Utils/ISelectionAdapter.cs
@@ -21,14 +21,14 @@ namespace Avalonia.Controls.Utils
         /// Gets or sets the selected item.
         /// </summary>
         /// <value>The currently selected item.</value>
-        object SelectedItem { get; set; }
+        object? SelectedItem { get; set; }
 
         /// <summary>
         /// Occurs when the
         /// <see cref="P:Avalonia.Controls.Utils.ISelectionAdapter.SelectedItem" />
         /// property value changes.
         /// </summary>
-        event EventHandler<SelectionChangedEventArgs> SelectionChanged;
+        event EventHandler<SelectionChangedEventArgs>? SelectionChanged;
         
         /// <summary>
         /// Gets or sets a collection that is used to generate content for the
@@ -36,18 +36,18 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <value>The collection that is used to generate content for the
         /// selection adapter.</value>
-        IEnumerable Items { get; set; }
+        IEnumerable? Items { get; set; }
 
         /// <summary>
         /// Occurs when a selected item is not cancelled and is committed as the
         /// selected item.
         /// </summary>
-        event EventHandler<RoutedEventArgs> Commit;
+        event EventHandler<RoutedEventArgs>? Commit;
 
         /// <summary>
         /// Occurs when a selection has been canceled.
         /// </summary>
-        event EventHandler<RoutedEventArgs> Cancel;
+        event EventHandler<RoutedEventArgs>? Cancel;
 
         /// <summary>
         /// Provides handling for the
diff --git a/src/Avalonia.Controls/Utils/SelectingItemsControlSelectionAdapter.cs b/src/Avalonia.Controls/Utils/SelectingItemsControlSelectionAdapter.cs
index 7f2008c739..0288f99dce 100644
--- a/src/Avalonia.Controls/Utils/SelectingItemsControlSelectionAdapter.cs
+++ b/src/Avalonia.Controls/Utils/SelectingItemsControlSelectionAdapter.cs
@@ -22,7 +22,7 @@ namespace Avalonia.Controls.Utils
         /// <summary>
         /// The SelectingItemsControl instance.
         /// </summary>
-        private SelectingItemsControl _selector;
+        private SelectingItemsControl? _selector;
 
         /// <summary>
         /// Gets or sets a value indicating whether the selection change event 
@@ -38,7 +38,7 @@ namespace Avalonia.Controls.Utils
         /// <value>The underlying
         /// <see cref="T:Avalonia.Controls.Primitives.SelectingItemsControl" />
         /// control.</value>
-        public SelectingItemsControl SelectorControl
+        public SelectingItemsControl? SelectorControl
         {
             get { return _selector; }
 
@@ -65,19 +65,19 @@ namespace Avalonia.Controls.Utils
         /// <see cref="P:Avalonia.Controls.Utils.SelectingItemsControlSelectionAdapter.SelectedItem" />
         /// property value changes.
         /// </summary>
-        public event EventHandler<SelectionChangedEventArgs> SelectionChanged;
+        public event EventHandler<SelectionChangedEventArgs>? SelectionChanged;
 
         /// <summary>
         /// Occurs when an item is selected and is committed to the underlying
         /// <see cref="T:Avalonia.Controls.Primitives.SelectingItemsControl" />
         /// control.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Commit;
+        public event EventHandler<RoutedEventArgs>? Commit;
 
         /// <summary>
         /// Occurs when a selection is canceled before it is committed.
         /// </summary>
-        public event EventHandler<RoutedEventArgs> Cancel;
+        public event EventHandler<RoutedEventArgs>? Cancel;
 
         /// <summary>
         /// Initializes a new instance of the
@@ -109,7 +109,7 @@ namespace Avalonia.Controls.Utils
         /// Gets or sets the selected item of the selection adapter.
         /// </summary>
         /// <value>The selected item of the underlying selection adapter.</value>
-        public object SelectedItem
+        public object? SelectedItem
         {
             get
             {
@@ -140,7 +140,7 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <value>The collection used to generate content for the selection
         /// adapter.</value>
-        public IEnumerable Items
+        public IEnumerable? Items
         {
             get
             {
@@ -163,7 +163,7 @@ namespace Avalonia.Controls.Utils
         {
             if (SelectorControl != null)
             {
-                ScrollViewer sv = SelectorControl.GetLogicalDescendants().OfType<ScrollViewer>().FirstOrDefault();
+                var sv = SelectorControl.GetLogicalDescendants().OfType<ScrollViewer>().FirstOrDefault();
                 if (sv != null)
                 {
                     sv.Offset = new Vector(0, 0);
@@ -176,7 +176,7 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void OnSelectorPointerReleased(object sender, PointerReleasedEventArgs e)
+        private void OnSelectorPointerReleased(object? sender, PointerReleasedEventArgs e)
         {
             if (e.InitialPressMouseButton == MouseButton.Left)
             {
@@ -189,7 +189,7 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The selection changed event data.</param>
-        private void OnSelectionChanged(object sender, SelectionChangedEventArgs e)
+        private void OnSelectionChanged(object? sender, SelectionChangedEventArgs e)
         {
             if (IgnoringSelectionChanged)
             {
@@ -293,7 +293,7 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void OnCommit(object sender, RoutedEventArgs e)
+        private void OnCommit(object? sender, RoutedEventArgs e)
         {
             Commit?.Invoke(sender, e);
 
@@ -315,7 +315,7 @@ namespace Avalonia.Controls.Utils
         /// </summary>
         /// <param name="sender">The source object.</param>
         /// <param name="e">The event data.</param>
-        private void OnCancel(object sender, RoutedEventArgs e)
+        private void OnCancel(object? sender, RoutedEventArgs e)
         {
             Cancel?.Invoke(sender, e);
 
diff --git a/src/Avalonia.Controls/Utils/UndoRedoHelper.cs b/src/Avalonia.Controls/Utils/UndoRedoHelper.cs
index fd1ca54b57..0d5048c080 100644
--- a/src/Avalonia.Controls/Utils/UndoRedoHelper.cs
+++ b/src/Avalonia.Controls/Utils/UndoRedoHelper.cs
@@ -20,7 +20,7 @@ namespace Avalonia.Controls.Utils
 
         private readonly LinkedList<TState> _states = new LinkedList<TState>();
 
-        private LinkedListNode<TState> _currentNode;
+        private LinkedListNode<TState>? _currentNode;
 
         /// <summary>
         /// Maximum number of states this helper can store for undo/redo.
@@ -44,13 +44,13 @@ namespace Avalonia.Controls.Utils
 
         public bool IsLastState => _currentNode != null && _currentNode.Next == null;
 
-        public bool TryGetLastState(out TState _state)
+        public bool TryGetLastState(out TState? _state)
         {
             _state = default(TState);
             if (!IsLastState)
                 return false;
 
-            _state = _currentNode.Value;
+            _state = _currentNode!.Value;
             return true;
         }
 
@@ -86,7 +86,7 @@ namespace Avalonia.Controls.Utils
         public void Snapshot()
         {
             var current = _host.UndoRedoState;
-            if (_currentNode == null || !_currentNode.Value.Equals(current))
+            if (_currentNode == null || !_currentNode.Value!.Equals(current))
             {
                 if (_currentNode?.Next != null)
                     DiscardRedo();
diff --git a/src/Avalonia.Controls/ValidatingToplevel.cs b/src/Avalonia.Controls/ValidatingToplevel.cs
index d1edd28ebc..a68920059f 100644
--- a/src/Avalonia.Controls/ValidatingToplevel.cs
+++ b/src/Avalonia.Controls/ValidatingToplevel.cs
@@ -59,31 +59,31 @@ internal class ValidatingToplevelImpl : ITopLevelImpl, ITopLevelImplWithNativeCo
     public double RenderScaling => Inner.RenderScaling;
     public IEnumerable<object> Surfaces => Inner.Surfaces;
 
-    public Action<RawInputEventArgs> Input
+    public Action<RawInputEventArgs>? Input
     {
         get => Inner.Input;
         set => Inner.Input = value;
     }
 
-    public Action<Rect> Paint
+    public Action<Rect>? Paint
     {
         get => Inner.Paint;
         set => Inner.Paint = value;
     }
 
-    public Action<Size, PlatformResizeReason> Resized
+    public Action<Size, PlatformResizeReason>? Resized
     {
         get => Inner.Resized;
         set => Inner.Resized = value;
     }
 
-    public Action<double> ScalingChanged
+    public Action<double>? ScalingChanged
     {
         get => Inner.ScalingChanged;
         set => Inner.ScalingChanged = value;
     }
 
-    public Action<WindowTransparencyLevel> TransparencyLevelChanged
+    public Action<WindowTransparencyLevel>? TransparencyLevelChanged
     {
         get => Inner.TransparencyLevelChanged;
         set => Inner.TransparencyLevelChanged = value;
@@ -99,15 +99,15 @@ internal class ValidatingToplevelImpl : ITopLevelImpl, ITopLevelImplWithNativeCo
 
     public PixelPoint PointToScreen(Point point) => Inner.PointToScreen(point);
 
-    public void SetCursor(ICursorImpl cursor) => Inner.SetCursor(cursor);
+    public void SetCursor(ICursorImpl? cursor) => Inner.SetCursor(cursor);
 
-    public Action Closed
+    public Action? Closed
     {
         get => Inner.Closed;
         set => Inner.Closed = value;
     }
 
-    public Action LostFocus
+    public Action? LostFocus
     {
         get => Inner.LostFocus;
         set => Inner.LostFocus = value;
@@ -115,7 +115,7 @@ internal class ValidatingToplevelImpl : ITopLevelImpl, ITopLevelImplWithNativeCo
 
     // Exception: for some reason we are notifying platform mouse device from TopLevel.cs
     public IMouseDevice MouseDevice => _impl.MouseDevice;
-    public IPopupImpl CreatePopup() => Inner.CreatePopup();
+    public IPopupImpl? CreatePopup() => Inner.CreatePopup();
 
     public void SetTransparencyLevelHint(WindowTransparencyLevel transparencyLevel) =>
         Inner.SetTransparencyLevelHint(transparencyLevel);
@@ -123,12 +123,12 @@ internal class ValidatingToplevelImpl : ITopLevelImpl, ITopLevelImplWithNativeCo
 
     public WindowTransparencyLevel TransparencyLevel => Inner.TransparencyLevel;
     public AcrylicPlatformCompensationLevels AcrylicCompensationLevels => Inner.AcrylicCompensationLevels;
-    public INativeControlHostImpl NativeControlHost => (Inner as ITopLevelImplWithNativeControlHost)?.NativeControlHost;
+    public INativeControlHostImpl? NativeControlHost => (Inner as ITopLevelImplWithNativeControlHost)?.NativeControlHost;
 
-    public ITopLevelNativeMenuExporter NativeMenuExporter =>
+    public ITopLevelNativeMenuExporter? NativeMenuExporter =>
         (Inner as ITopLevelImplWithNativeMenuExporter)?.NativeMenuExporter;
 
-    public ITextInputMethodImpl TextInputMethod => (Inner as ITopLevelImplWithTextInputMethod)?.TextInputMethod;
+    public ITextInputMethodImpl? TextInputMethod => (Inner as ITopLevelImplWithTextInputMethod)?.TextInputMethod;
 }
 
 internal class ValidatingWindowBaseImpl : ValidatingToplevelImpl, IWindowBaseImpl
@@ -167,7 +167,7 @@ internal class ValidatingWindowBaseImpl : ValidatingToplevelImpl, IWindowBaseImp
     public double DesktopScaling => Inner.DesktopScaling;
     public PixelPoint Position => Inner.Position;
 
-    public Action<PixelPoint> PositionChanged
+    public Action<PixelPoint>? PositionChanged
     {
         get => Inner.PositionChanged;
         set => Inner.PositionChanged = value;
@@ -175,13 +175,13 @@ internal class ValidatingWindowBaseImpl : ValidatingToplevelImpl, IWindowBaseImp
 
     public void Activate() => Inner.Activate();
 
-    public Action Deactivated
+    public Action? Deactivated
     {
         get => Inner.Deactivated;
         set => Inner.Deactivated = value;
     }
 
-    public Action Activated
+    public Action? Activated
     {
         get => Inner.Activated;
         set => Inner.Deactivated = value;
@@ -234,7 +234,7 @@ internal class ValidatingWindowImpl : ValidatingWindowBaseImpl, IWindowImpl
         set => Inner.WindowStateChanged = value;
     }
 
-    public void SetTitle(string title) => Inner.SetTitle(title);
+    public void SetTitle(string? title) => Inner.SetTitle(title);
 
     public void SetParent(IWindowImpl parent)
     {
@@ -259,7 +259,7 @@ internal class ValidatingWindowImpl : ValidatingWindowBaseImpl, IWindowImpl
 
     public void SetSystemDecorations(SystemDecorations enabled) => Inner.SetSystemDecorations(enabled);
 
-    public void SetIcon(IWindowIconImpl icon) => Inner.SetIcon(icon);
+    public void SetIcon(IWindowIconImpl? icon) => Inner.SetIcon(icon);
 
     public void ShowTaskbarIcon(bool value) => Inner.ShowTaskbarIcon(value);
 
diff --git a/src/Avalonia.Controls/VirtualizingStackPanel.cs b/src/Avalonia.Controls/VirtualizingStackPanel.cs
index da8bbe4fcf..34b774e23f 100644
--- a/src/Avalonia.Controls/VirtualizingStackPanel.cs
+++ b/src/Avalonia.Controls/VirtualizingStackPanel.cs
@@ -27,7 +27,7 @@ namespace Avalonia.Controls
             }
         }
 
-        IVirtualizingController IVirtualizingPanel.Controller { get; set; }
+        IVirtualizingController? IVirtualizingPanel.Controller { get; set; }
         int IVirtualizingPanel.OverflowCount => _canBeRemoved;
         Orientation IVirtualizingPanel.ScrollDirection => Orientation;
         double IVirtualizingPanel.AverageItemSize => _averageItemSize;
@@ -70,7 +70,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private IVirtualizingController Controller => ((IVirtualizingPanel)this).Controller;
+        private IVirtualizingController? Controller => ((IVirtualizingPanel)this).Controller;
 
         void IVirtualizingPanel.ForceInvalidateMeasure()
         {
@@ -103,14 +103,14 @@ namespace Avalonia.Controls
             return result;
         }
 
-        protected override void ChildrenChanged(object sender, NotifyCollectionChangedEventArgs e)
+        protected override void ChildrenChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             base.ChildrenChanged(sender, e);
 
             switch (e.Action)
             {
                 case NotifyCollectionChangedAction.Add:
-                    foreach (IControl control in e.NewItems)
+                    foreach (IControl control in e.NewItems!)
                     {
                         UpdateAdd(control);
                     }
@@ -118,7 +118,7 @@ namespace Avalonia.Controls
                     break;
 
                 case NotifyCollectionChangedAction.Remove:
-                    foreach (IControl control in e.OldItems)
+                    foreach (IControl control in e.OldItems!)
                     {
                         UpdateRemove(control);
                     }
@@ -127,7 +127,7 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override IInputElement GetControlInDirection(NavigationDirection direction, IControl from)
+        protected override IInputElement? GetControlInDirection(NavigationDirection direction, IControl? from)
         {
             if (from == null)
                 return null;
diff --git a/src/Avalonia.Controls/Window.cs b/src/Avalonia.Controls/Window.cs
index b9ca594efa..a5f48bd4a5 100644
--- a/src/Avalonia.Controls/Window.cs
+++ b/src/Avalonia.Controls/Window.cs
@@ -147,14 +147,14 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Title"/> property.
         /// </summary>
-        public static readonly StyledProperty<string> TitleProperty =
-            AvaloniaProperty.Register<Window, string>(nameof(Title), "Window");
+        public static readonly StyledProperty<string?> TitleProperty =
+            AvaloniaProperty.Register<Window, string?>(nameof(Title), "Window");
 
         /// <summary>
         /// Defines the <see cref="Icon"/> property.
         /// </summary>
-        public static readonly StyledProperty<WindowIcon> IconProperty =
-            AvaloniaProperty.Register<Window, WindowIcon>(nameof(Icon));
+        public static readonly StyledProperty<WindowIcon?> IconProperty =
+            AvaloniaProperty.Register<Window, WindowIcon?>(nameof(Icon));
 
         /// <summary>
         /// Defines the <see cref="WindowStartupLocation"/> property.
@@ -183,7 +183,7 @@ namespace Avalonia.Controls
 
 
         private readonly NameScope _nameScope = new NameScope();
-        private object _dialogResult;
+        private object? _dialogResult;
         private readonly Size _maxPlatformClientSize;
         private WindowStartupLocation _windowStartupLocation;
 
@@ -193,35 +193,35 @@ namespace Avalonia.Controls
         static Window()
         {
             BackgroundProperty.OverrideDefaultValue(typeof(Window), Brushes.White);
-            TitleProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetTitle((string)e.NewValue));
-            ShowInTaskbarProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.ShowTaskbarIcon((bool)e.NewValue));
+            TitleProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetTitle((string?)e.NewValue));
+            ShowInTaskbarProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.ShowTaskbarIcon((bool)e.NewValue!));
 
-            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetIcon(((WindowIcon)e.NewValue)?.PlatformImpl));
+            IconProperty.Changed.AddClassHandler<Window>((s, e) => s.PlatformImpl?.SetIcon(((WindowIcon?)e.NewValue)?.PlatformImpl));
 
-            CanResizeProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.CanResize((bool)e.NewValue));
+            CanResizeProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.CanResize((bool)e.NewValue!));
 
             WindowStateProperty.Changed.AddClassHandler<Window>(
-                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.WindowState = (WindowState)e.NewValue; });
+                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.WindowState = (WindowState)e.NewValue!; });
 
             ExtendClientAreaToDecorationsHintProperty.Changed.AddClassHandler<Window>(
-                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.SetExtendClientAreaToDecorationsHint((bool)e.NewValue); });
+                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.SetExtendClientAreaToDecorationsHint((bool)e.NewValue!); });
 
             ExtendClientAreaChromeHintsProperty.Changed.AddClassHandler<Window>(
                 (w, e) =>
                 {
                     if (w.PlatformImpl != null)
                     {
-                        w.PlatformImpl.SetExtendClientAreaChromeHints((ExtendClientAreaChromeHints)e.NewValue);
+                        w.PlatformImpl.SetExtendClientAreaChromeHints((ExtendClientAreaChromeHints)e.NewValue!);
                     }
                 });
 
             ExtendClientAreaTitleBarHeightHintProperty.Changed.AddClassHandler<Window>(
-                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.SetExtendClientAreaTitleBarHeightHint((double)e.NewValue); });
+                (w, e) => { if (w.PlatformImpl != null) w.PlatformImpl.SetExtendClientAreaTitleBarHeightHint((double)e.NewValue!); });
 
-            MinWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size((double)e.NewValue, w.MinHeight), new Size(w.MaxWidth, w.MaxHeight)));
-            MinHeightProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, (double)e.NewValue), new Size(w.MaxWidth, w.MaxHeight)));
-            MaxWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size((double)e.NewValue, w.MaxHeight)));
-            MaxHeightProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size(w.MaxWidth, (double)e.NewValue)));
+            MinWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size((double)e.NewValue!, w.MinHeight), new Size(w.MaxWidth, w.MaxHeight)));
+            MinHeightProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, (double)e.NewValue!), new Size(w.MaxWidth, w.MaxHeight)));
+            MaxWidthProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size((double)e.NewValue!, w.MaxHeight)));
+            MaxHeightProperty.Changed.AddClassHandler<Window>((w, e) => w.PlatformImpl?.SetMinMaxSize(new Size(w.MinWidth, w.MinHeight), new Size(w.MaxWidth, (double)e.NewValue!)));
         }
 
         /// <summary>
@@ -253,8 +253,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets the platform-specific window implementation.
         /// </summary>
-        [CanBeNull]
-        public new IWindowImpl PlatformImpl => (IWindowImpl)base.PlatformImpl;
+        public new IWindowImpl? PlatformImpl => (IWindowImpl?)base.PlatformImpl;
 
         /// <summary>
         /// Gets or sets a value indicating how the window will size itself to fit its content.
@@ -280,7 +279,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the title of the window.
         /// </summary>
-        public string Title
+        public string? Title
         {
             get { return GetValue(TitleProperty); }
             set { SetValue(TitleProperty, value); }
@@ -416,7 +415,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the icon of the window.
         /// </summary>
-        public WindowIcon Icon
+        public WindowIcon? Icon
         {
             get { return GetValue(IconProperty); }
             set { SetValue(IconProperty, value); }
@@ -460,7 +459,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Fired before a window is closed.
         /// </summary>
-        public event EventHandler<CancelEventArgs> Closing;
+        public event EventHandler<CancelEventArgs>? Closing;
 
         /// <summary>
         /// Closes the window.
@@ -538,7 +537,7 @@ namespace Avalonia.Controls
             PlatformImpl?.Dispose();
         }
 
-        private bool ShouldCancelClose(CancelEventArgs args = null)
+        private bool ShouldCancelClose(CancelEventArgs? args = null)
         {
             if (args is null)
             {
@@ -582,8 +581,8 @@ namespace Avalonia.Controls
         protected virtual void ExtendClientAreaToDecorationsChanged(bool isExtended)
         {
             IsExtendedIntoWindowDecorations = isExtended;
-            WindowDecorationMargin = PlatformImpl.ExtendedMargins;
-            OffScreenMargin = PlatformImpl.OffScreenMargin;
+            WindowDecorationMargin = PlatformImpl?.ExtendedMargins ?? default;
+            OffScreenMargin = PlatformImpl?.OffScreenMargin ?? default;
         }
 
         /// <summary>
@@ -644,7 +643,7 @@ namespace Avalonia.Controls
             ShowCore(parent);
         }
 
-        private void ShowCore(Window parent)
+        private void ShowCore(Window? parent)
         {
             if (PlatformImpl == null)
             {
@@ -688,9 +687,9 @@ namespace Avalonia.Controls
 
             LayoutManager.ExecuteInitialLayoutPass();
 
-            if (parent != null)
+            if (PlatformImpl != null && parent?.PlatformImpl is not null)
             {
-                PlatformImpl?.SetParent(parent.PlatformImpl);
+                PlatformImpl.SetParent(parent.PlatformImpl);
             }
 
             Owner = parent;
@@ -786,7 +785,7 @@ namespace Avalonia.Controls
                 .Subscribe(_ =>
                 {
                     owner.Activate();
-                    result.SetResult((TResult)(_dialogResult ?? default(TResult)));
+                    result.SetResult((TResult)(_dialogResult ?? default(TResult)!));
                 });
 
             OnOpened(EventArgs.Empty);
@@ -806,7 +805,7 @@ namespace Avalonia.Controls
                 }
             }
 
-            PlatformImpl.SetEnabled(isEnabled);
+            PlatformImpl?.SetEnabled(isEnabled);
         }
 
         private void AddChild(Window window, bool isDialog)
@@ -832,7 +831,7 @@ namespace Avalonia.Controls
 
         private void OnGotInputWhenDisabled()
         {
-            Window firstDialogChild = null;
+            Window? firstDialogChild = null;
 
             foreach (var (child, isDialog)  in _children)
             {
@@ -853,7 +852,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void SetWindowStartupLocation(IWindowBaseImpl owner = null)
+        private void SetWindowStartupLocation(IWindowBaseImpl? owner = null)
         {
             var scaling = owner?.DesktopScaling ?? PlatformImpl?.DesktopScaling ?? 1;
 
diff --git a/src/Avalonia.Controls/WindowBase.cs b/src/Avalonia.Controls/WindowBase.cs
index 2207d0550a..4464491020 100644
--- a/src/Avalonia.Controls/WindowBase.cs
+++ b/src/Avalonia.Controls/WindowBase.cs
@@ -30,8 +30,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Owner"/> property.
         /// </summary>
-        public static readonly DirectProperty<WindowBase, WindowBase> OwnerProperty =
-            AvaloniaProperty.RegisterDirect<WindowBase, WindowBase>(
+        public static readonly DirectProperty<WindowBase, WindowBase?> OwnerProperty =
+            AvaloniaProperty.RegisterDirect<WindowBase, WindowBase?>(
                 nameof(Owner),
                 o => o.Owner,
                 (o, v) => o.Owner = v);
@@ -42,7 +42,7 @@ namespace Avalonia.Controls
         private bool _hasExecutedInitialLayoutPass;
         private bool _isActive;
         private bool _ignoreVisibilityChange;
-        private WindowBase _owner;
+        private WindowBase? _owner;
 
         static WindowBase()
         {
@@ -50,14 +50,14 @@ namespace Avalonia.Controls
             IsVisibleProperty.Changed.AddClassHandler<WindowBase>((x,e) => x.IsVisibleChanged(e));
 
             
-            TopmostProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetTopmost((bool)e.NewValue));
+            TopmostProperty.Changed.AddClassHandler<WindowBase>((w, e) => w.PlatformImpl?.SetTopmost((bool)e.NewValue!));
         }
 
         public WindowBase(IWindowBaseImpl impl) : this(impl, AvaloniaLocator.Current)
         {
         }
 
-        public WindowBase(IWindowBaseImpl impl, IAvaloniaDependencyResolver dependencyResolver) : base(ValidatingWindowBaseImpl.Wrap(impl), dependencyResolver)
+        public WindowBase(IWindowBaseImpl impl, IAvaloniaDependencyResolver? dependencyResolver) : base(ValidatingWindowBaseImpl.Wrap(impl), dependencyResolver)
         {
             Screens = new Screens(PlatformImpl?.Screen);
             var wrapped = PlatformImpl!;
@@ -69,20 +69,19 @@ namespace Avalonia.Controls
         /// <summary>
         /// Fired when the window is activated.
         /// </summary>
-        public event EventHandler Activated;
+        public event EventHandler? Activated;
 
         /// <summary>
         /// Fired when the window is deactivated.
         /// </summary>
-        public event EventHandler Deactivated;
+        public event EventHandler? Deactivated;
 
         /// <summary>
         /// Fired when the window position is changed.
         /// </summary>
-        public event EventHandler<PixelPointEventArgs> PositionChanged;
+        public event EventHandler<PixelPointEventArgs>? PositionChanged;
 
-        [CanBeNull]
-        public new IWindowBaseImpl PlatformImpl => (IWindowBaseImpl) base.PlatformImpl;
+        public new IWindowBaseImpl? PlatformImpl => (IWindowBaseImpl?) base.PlatformImpl;
 
         /// <summary>
         /// Gets a value that indicates whether the window is active.
@@ -101,7 +100,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the owner of the window.
         /// </summary>
-        public WindowBase Owner
+        public WindowBase? Owner
         {
             get { return _owner; }
             protected set { SetAndRaise(OwnerProperty, ref _owner, value); }
@@ -219,7 +218,7 @@ namespace Avalonia.Controls
         protected override void HandleResized(Size clientSize, PlatformResizeReason reason)
         {
             ClientSize = clientSize;
-            FrameSize = PlatformImpl.FrameSize;
+            FrameSize = PlatformImpl?.FrameSize;
             LayoutManager.ExecuteLayoutPass();
             Renderer?.Resized(clientSize);
         }
@@ -308,7 +307,7 @@ namespace Avalonia.Controls
         {
             if (!_ignoreVisibilityChange)
             {
-                if ((bool)e.NewValue)
+                if ((bool)e.NewValue!)
                 {
                     Show();
                 }
diff --git a/src/Avalonia.Controls/WindowIcon.cs b/src/Avalonia.Controls/WindowIcon.cs
index 9e4329372a..20ea872cad 100644
--- a/src/Avalonia.Controls/WindowIcon.cs
+++ b/src/Avalonia.Controls/WindowIcon.cs
@@ -11,17 +11,17 @@ namespace Avalonia.Controls
     {
         public WindowIcon(IBitmap bitmap)
         {
-            PlatformImpl = AvaloniaLocator.Current.GetService<IPlatformIconLoader>().LoadIcon(bitmap.PlatformImpl.Item);
+            PlatformImpl = AvaloniaLocator.Current.GetRequiredService<IPlatformIconLoader>().LoadIcon(bitmap.PlatformImpl.Item);
         }
 
         public WindowIcon(string fileName)
         {
-            PlatformImpl = AvaloniaLocator.Current.GetService<IPlatformIconLoader>().LoadIcon(fileName);
+            PlatformImpl = AvaloniaLocator.Current.GetRequiredService<IPlatformIconLoader>().LoadIcon(fileName);
         }
 
         public WindowIcon(Stream stream)
         {
-            PlatformImpl = AvaloniaLocator.Current.GetService<IPlatformIconLoader>().LoadIcon(stream);
+            PlatformImpl = AvaloniaLocator.Current.GetRequiredService<IPlatformIconLoader>().LoadIcon(stream);
         }
 
         public IWindowIconImpl PlatformImpl { get; }
diff --git a/src/Avalonia.Controls/WrapPanel.cs b/src/Avalonia.Controls/WrapPanel.cs
index d06a71a9f8..b7061824f0 100644
--- a/src/Avalonia.Controls/WrapPanel.cs
+++ b/src/Avalonia.Controls/WrapPanel.cs
@@ -79,12 +79,12 @@ namespace Avalonia.Controls
         /// <param name="from">The control from which movement begins.</param>
         /// <param name="wrap">Whether to wrap around when the first or last item is reached.</param>
         /// <returns>The control.</returns>
-        IInputElement INavigableContainer.GetControl(NavigationDirection direction, IInputElement from, bool wrap)
+        IInputElement? INavigableContainer.GetControl(NavigationDirection direction, IInputElement? from, bool wrap)
         {
             var orientation = Orientation;
             var children = Children;
             bool horiz = orientation == Orientation.Horizontal;
-            int index = Children.IndexOf((IControl)from);
+            int index = from is not null ? Children.IndexOf((IControl)from) : -1;
 
             switch (direction)
             {