Merge pull request #7477 from AvaloniaUI/fixes/avalonia-controls-nullability

Added nullable annotations to Avalonia.Controls.

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");

src/Avalonia.Controls/Application.cs

@@ -13,7 +13,6 @@ using Avalonia.Platform;
 using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.Threading;
-#nullable enable
 
 namespace Avalonia
 {
@@ -177,13 +176,13 @@ 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;

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

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;
     }
 }

src/Avalonia.Controls/ApplicationLifetimes/ISingleViewApplicationLifetime.cs

@@ -2,6 +2,6 @@ namespace Avalonia.Controls.ApplicationLifetimes
 {
     public interface ISingleViewApplicationLifetime : IApplicationLifetime
     {
-        Control MainView { get; set; }
+        Control? MainView { get; set; }
     }
 }

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;

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>

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); }

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)
             {

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)
             {

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)
                 {

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

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;
                     }

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);
 

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

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)
             {
@@ -917,15 +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;
-                        Contract.Requires<ArgumentNullException>(selectedDate != null);
+                        DateTime selectedDate = (DateTime)b.DataContext!;
                         _isMouseLeftButtonDown = true;
                         // null check is added for unit tests
                         if (e != null)
@@ -1027,7 +1022,7 @@ 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 +1036,11 @@ 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 +1089,13 @@ 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 +1112,9 @@ 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 +1126,13 @@ 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 +1148,11 @@ 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);
             }
         }

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)

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;

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;

src/Avalonia.Controls/Chrome/CaptionButtons.cs

@@ -3,8 +3,6 @@ using System.Reactive.Disposables;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 
-#nullable enable
-
 namespace Avalonia.Controls.Chrome
 {
     /// <summary>

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -3,8 +3,6 @@ using System.Reactive.Disposables;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 
-#nullable enable
-
 namespace Avalonia.Controls.Chrome
 {
     /// <summary>
@@ -36,7 +34,7 @@ namespace Avalonia.Controls.Chrome
                     }
                 }
 
-                IsVisible = window.PlatformImpl.NeedsManagedDecorations;
+                IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
             }
         }
 

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)
                 {

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;

src/Avalonia.Controls/ContextMenu.cs

@@ -14,8 +14,6 @@ using Avalonia.Interactivity;
 using Avalonia.Layout;
 using Avalonia.Styling;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -368,7 +366,7 @@ namespace Avalonia.Controls
             });
         }
 
-        private void PopupOpened(object sender, EventArgs e)
+        private void PopupOpened(object? sender, EventArgs e)
         {
             _previousFocus = FocusManager.Instance?.Current;
             Focus();
@@ -376,12 +374,12 @@ 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 +409,7 @@ namespace Avalonia.Controls
             _popupHostChangedHandler?.Invoke(null);
         }
 
-        private void PopupKeyUp(object sender, KeyEventArgs e)
+        private void PopupKeyUp(object? sender, KeyEventArgs e)
         {
             if (IsOpen)
             {
@@ -426,7 +424,7 @@ 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 +437,7 @@ 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)

src/Avalonia.Controls/ContextRequestedEventArgs.cs

@@ -1,8 +1,6 @@
 using Avalonia.Input;
 using Avalonia.Interactivity;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>

src/Avalonia.Controls/Control.cs

@@ -11,8 +11,6 @@ using Avalonia.Rendering;
 using Avalonia.Styling;
 using Avalonia.VisualTree;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -129,7 +127,7 @@ 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);

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));
         }

src/Avalonia.Controls/Converters/CornerRadiusFilterConverter.cs

@@ -1,5 +1,4 @@
-#nullable enable
-using System;
+using System;
 using System.Globalization;
 
 using Avalonia.Data.Converters;

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();
         }

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 ||

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();
         }

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;
         }

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);
         }

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);
         }
     }
 }

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)
         {

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;
     }
 }

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;
         }
     }
 }

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);
         }
     }
 }

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)
             {

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);

src/Avalonia.Controls/DefinitionList.cs

@@ -2,8 +2,6 @@ using System.Collections;
 using System.Collections.Specialized;
 using Avalonia.Collections;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     public abstract class DefinitionList<T> : AvaloniaList<T> where T : DefinitionBase
@@ -36,7 +34,7 @@ namespace Avalonia.Controls
             }
         }
 
-        internal void OnCollectionChanged(object sender, NotifyCollectionChangedEventArgs e)
+        internal void OnCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             var idx = 0;
 
@@ -62,7 +60,7 @@ namespace Avalonia.Controls
 
             for (var i = 0; i < count; i++)
             {
-                var definition = (DefinitionBase) items[i];
+                var definition = (DefinitionBase) items[i]!;
 
                 if (wasRemoved)
                 {

src/Avalonia.Controls/Diagnostics/IPopupHostProvider.cs

@@ -1,8 +1,6 @@
 using System;
 using Avalonia.Controls.Primitives;
 
-#nullable enable
-
 namespace Avalonia.Controls.Diagnostics
 {
     /// <summary>

src/Avalonia.Controls/Diagnostics/ToolTipDiagnostics.cs

@@ -1,6 +1,4 @@
-#nullable enable
-
-namespace Avalonia.Controls.Diagnostics
+namespace Avalonia.Controls.Diagnostics
 {
     /// <summary>
     /// Helper class to provide diagnostics information for <see cref="ToolTip"/>.

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;
     }
 }

src/Avalonia.Controls/Expander.cs

@@ -1,11 +1,8 @@
 using System.Threading;
-
 using Avalonia.Animation;
 using Avalonia.Controls.Metadata;
 using Avalonia.Controls.Primitives;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>

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:

src/Avalonia.Controls/Flyouts/Flyout.cs

@@ -1,8 +1,6 @@
 using Avalonia.Controls.Primitives;
 using Avalonia.Metadata;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     public class Flyout : FlyoutBase

src/Avalonia.Controls/Flyouts/FlyoutBase.cs

@@ -8,8 +8,6 @@ using Avalonia.Input.Raw;
 using Avalonia.Layout;
 using Avalonia.Logging;
 
-#nullable enable
-
 namespace Avalonia.Controls.Primitives
 {
     public abstract class FlyoutBase : AvaloniaObject, IPopupHostProvider
@@ -267,7 +265,7 @@ 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 +333,7 @@ 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 +343,7 @@ namespace Avalonia.Controls.Primitives
 
         protected virtual void OnClosed()
         {
-            Closed?.Invoke(this, null);
+            Closed?.Invoke(this, EventArgs.Empty);
         }
 
         /// <summary>
@@ -368,14 +366,14 @@ 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 +381,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        private void OnPopupClosed(object sender, EventArgs e)
+        private void OnPopupClosed(object? sender, EventArgs e)
         {
             HideCore(false);
 
@@ -391,7 +389,7 @@ 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 +528,10 @@ 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)

src/Avalonia.Controls/Flyouts/MenuFlyout.cs

@@ -4,8 +4,6 @@ using Avalonia.Controls.Primitives;
 using Avalonia.Controls.Templates;
 using Avalonia.Metadata;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     public class MenuFlyout : FlyoutBase
@@ -18,7 +16,7 @@ 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 +33,7 @@ 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 +49,7 @@ namespace Avalonia.Controls
         }
 
         private Classes? _classes;
-        private IEnumerable _items;
+        private IEnumerable? _items;
         private IDataTemplate? _itemTemplate;
 
         protected override Control CreatePresenter()

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.

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"/>.

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;
 

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;
 

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);

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;

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)
             {

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);

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);
@@ -598,7 +599,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Contract.Requires<NullReferenceException>(_data != null);
+            Debug.Assert(_data != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!ColumnDefinitionsDirty)
@@ -620,7 +621,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Contract.Requires<NullReferenceException>(_data != null);
+            Debug.Assert(_data != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!RowDefinitionsDirty)
@@ -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)
                 {

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)
             {

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;

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);
     }
 }

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; }
     }
-}
+}

src/Avalonia.Controls/IControl.cs

@@ -3,8 +3,6 @@ using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.VisualTree;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>

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; }
     }
 }

src/Avalonia.Controls/IMenuElement.cs

@@ -1,8 +1,6 @@
 using System.Collections.Generic;
 using Avalonia.Input;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>

src/Avalonia.Controls/IMenuItem.cs

@@ -1,6 +1,4 @@
-#nullable enable
-
-namespace Avalonia.Controls
+namespace Avalonia.Controls
 {
     /// <summary>
     /// Represents a <see cref="MenuItem"/>.

src/Avalonia.Controls/IScrollAnchorProvider.cs

@@ -1,6 +1,4 @@
-#nullable enable
-
-namespace Avalonia.Controls
+namespace Avalonia.Controls
 {
     /// <summary>
     /// Specifies a contract for a scrolling control that supports scroll anchoring.

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.

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);
         }

src/Avalonia.Controls/ItemsSourceView.cs

@@ -7,12 +7,9 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using System.Linq;
 using Avalonia.Controls.Utils;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -196,7 +193,7 @@ 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 +236,7 @@ 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();

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);

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.

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;

src/Avalonia.Controls/MaskedTextBox.cs

@@ -8,8 +8,6 @@ using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Styling;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     public class MaskedTextBox : TextBox, IStyleable
@@ -241,7 +239,7 @@ namespace Avalonia.Controls
             switch (e.Key)
             {
                 case Key.Delete:
-                    if (CaretIndex < Text.Length)
+                    if (CaretIndex < Text?.Length)
                     {
                         if (MaskProvider.RemoveAt(CaretIndex))
                         {
@@ -286,7 +284,7 @@ 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 +381,11 @@ 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++;
                     }

src/Avalonia.Controls/Menu.cs

@@ -5,8 +5,6 @@ using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.Layout;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>

src/Avalonia.Controls/MenuBase.cs

@@ -8,8 +8,6 @@ using Avalonia.Input;
 using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -83,12 +81,13 @@ 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 +96,7 @@ namespace Avalonia.Controls
         {
             get
             {
-                return ItemContainerGenerator.Containers
+                return ItemContainerGenerator!.Containers
                     .Select(x => x.ContainerControl)
                     .OfType<IMenuItem>();
             }
@@ -111,7 +110,7 @@ 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 +119,7 @@ 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); }

src/Avalonia.Controls/MenuItem.cs

@@ -14,8 +14,6 @@ using Avalonia.Interactivity;
 using Avalonia.LogicalTree;
 using Avalonia.VisualTree;
 
-#nullable enable
-
 namespace Avalonia.Controls
 {
     /// <summary>
@@ -42,20 +40,20 @@ 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 +162,7 @@ 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 +174,7 @@ 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 +186,7 @@ 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 +195,7 @@ 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 +223,7 @@ 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 +232,7 @@ 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 +245,7 @@ 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 +308,12 @@ 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 +322,7 @@ namespace Avalonia.Controls
         {
             get
             {
-                return ItemContainerGenerator.Containers
+                return ItemContainerGenerator!.Containers
                     .Select(x => x.ContainerControl)
                     .OfType<IMenuItem>();
             }
@@ -390,7 +388,7 @@ 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 +559,7 @@ 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 +653,7 @@ namespace Avalonia.Controls
 
             if (value)
             {
-                foreach (var item in Items.OfType<MenuItem>())
+                foreach (var item in Items!.OfType<MenuItem>())
                 {
                     item.TryUpdateCanExecute();
                 }
@@ -677,13 +675,13 @@ 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 +691,7 @@ 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;
         }

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))
             {

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 =>
             {

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;

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)
             {

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);

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();
         }
     }
 }

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

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);

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; }
     }
 }

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; }
     }
 }

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();
         }
 

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();
             }

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);

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;
                     }

src/Avalonia.Controls/Platform/DefaultMenuInteractionHandler.cs

@@ -4,12 +4,11 @@ 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;
 
-#nullable enable
-
 namespace Avalonia.Controls.Platform
 {
     /// <summary>
@@ -68,8 +67,8 @@ 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 +117,7 @@ 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 +127,7 @@ 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 +137,7 @@ 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 +266,7 @@ 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 +287,7 @@ 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 +332,7 @@ 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 +367,12 @@ 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 +398,7 @@ 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 +409,7 @@ 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 +427,18 @@ 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();
         }

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; }

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; }
     }
 }

src/Avalonia.Controls/Platform/ISystemDialogImpl.cs

@@ -1,7 +1,5 @@
 using System.Threading.Tasks;
 
-#nullable enable
-
 namespace Avalonia.Controls.Platform
 {
     /// <summary>

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.

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; }
     }
 }

src/Avalonia.Controls/Platform/ITopLevelNativeMenuExporter.cs

@@ -1,8 +1,6 @@
 using System;
 using Avalonia.Platform;
 
-#nullable enable
-
 namespace Avalonia.Controls.Platform
 {
     public interface INativeMenuExporter
@@ -24,6 +22,6 @@ namespace Avalonia.Controls.Platform
     
     public interface ITopLevelImplWithNativeMenuExporter : ITopLevelImpl
     {
-        ITopLevelNativeMenuExporter NativeMenuExporter { get; }
+        ITopLevelNativeMenuExporter? NativeMenuExporter { get; }
     }
 }

src/Avalonia.Controls/Platform/ITrayIconImpl.cs

@@ -1,8 +1,6 @@
 using System;
 using Avalonia.Controls.Platform;
 
-#nullable enable
-
 namespace Avalonia.Platform
 {
     public interface ITrayIconImpl : IDisposable

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.