Nullability fixes for Avalonia.Controls

src/Avalonia.Base/Input/Platform/IClipboard.cs

@@ -6,9 +6,9 @@ namespace Avalonia.Input.Platform
     [NotClientImplementable]
     public interface IClipboard
     {
-        Task<string> GetTextAsync();
+        Task<string?> GetTextAsync();
 
-        Task SetTextAsync(string text);
+        Task SetTextAsync(string? text);
 
         Task ClearAsync();
 
@@ -16,6 +16,6 @@ namespace Avalonia.Input.Platform
         
         Task<string[]> GetFormatsAsync();
         
-        Task<object> GetDataAsync(string format);
+        Task<object?> GetDataAsync(string format);
     }
 }

src/Avalonia.Base/Media/Color.cs

@@ -331,7 +331,7 @@ namespace Avalonia.Media
         /// <summary>
         /// Parses the given string representing a CSS color value into a new <see cref="Color"/>.
         /// </summary>
-        private static bool TryParseCssFormat(string s, out Color color)
+        private static bool TryParseCssFormat(string? s, out Color color)
         {
             bool prefixMatched = false;
 

src/Avalonia.Base/Platform/Internal/AssemblyDescriptor.cs

@@ -1,4 +1,5 @@
-using System.Collections.Generic;
+using System;
+using System.Collections.Generic;
 using System.IO;
 using System.Linq;
 using System.Reflection;
@@ -18,7 +19,7 @@ internal class AssemblyDescriptor : IAssemblyDescriptor
 {
     public AssemblyDescriptor(Assembly assembly)
     {
-        Assembly = assembly;
+        Assembly = assembly ?? throw new ArgumentNullException(nameof(assembly));
         Resources = assembly.GetManifestResourceNames()
             .ToDictionary(n => n, n => (IAssetDescriptor)new AssemblyResourceDescriptor(assembly, n));
         Name = assembly.GetName().Name;

src/Avalonia.Controls/ApplicationLifetimes/ClassicDesktopStyleApplicationLifetime.cs

@@ -1,6 +1,5 @@
 using System;
 using System.Collections.Generic;
-using System.ComponentModel;
 using System.Linq;
 using System.Runtime.CompilerServices;
 using System.Threading;
@@ -17,32 +16,34 @@ namespace Avalonia.Controls.ApplicationLifetimes
         private int _exitCode;
         private CancellationTokenSource? _cts;
         private bool _isShuttingDown;
-        private HashSet<Window> _windows = new HashSet<Window>();
+        private readonly HashSet<Window> _windows = new();
+
+        private static ClassicDesktopStyleApplicationLifetime? s_activeLifetime;
 
-        private static ClassicDesktopStyleApplicationLifetime? _activeLifetime;
         static ClassicDesktopStyleApplicationLifetime()
         {
             Window.WindowOpenedEvent.AddClassHandler(typeof(Window), OnWindowOpened);
-            Window.WindowClosedEvent.AddClassHandler(typeof(Window), WindowClosedEvent);
+            Window.WindowClosedEvent.AddClassHandler(typeof(Window), OnWindowClosed);
         }
 
-        private static void WindowClosedEvent(object? sender, RoutedEventArgs e)
+        private static void OnWindowClosed(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Remove((Window)sender!);
-            _activeLifetime?.HandleWindowClosed((Window)sender!);
+            var window = (Window)sender!;
+            s_activeLifetime?._windows.Remove(window);
+            s_activeLifetime?.HandleWindowClosed(window);
         }
 
         private static void OnWindowOpened(object? sender, RoutedEventArgs e)
         {
-            _activeLifetime?._windows.Add((Window)sender!);
+            s_activeLifetime?._windows.Add((Window)sender!);
         }
 
         public ClassicDesktopStyleApplicationLifetime()
         {
-            if (_activeLifetime != null)
+            if (s_activeLifetime != null)
                 throw new InvalidOperationException(
                     "Can not have multiple active ClassicDesktopStyleApplicationLifetime instances and the previously created one was not disposed");
-            _activeLifetime = this;
+            s_activeLifetime = this;
         }
 
         /// <inheritdoc/>
@@ -65,9 +66,10 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// <inheritdoc/>
         public Window? MainWindow { get; set; }
 
+        /// <inheritdoc />
         public IReadOnlyList<Window> Windows => _windows.ToArray();
 
-        private void HandleWindowClosed(Window window)
+        private void HandleWindowClosed(Window? window)
         {
             if (window == null)
                 return;
@@ -130,8 +132,8 @@ namespace Avalonia.Controls.ApplicationLifetimes
 
         public void Dispose()
         {
-            if (_activeLifetime == this)
-                _activeLifetime = null;
+            if (s_activeLifetime == this)
+                s_activeLifetime = null;
         }
 
         private bool DoShutdown(

src/Avalonia.Controls/ApplicationLifetimes/IClassicDesktopStyleApplicationLifetime.cs

@@ -40,7 +40,10 @@ namespace Avalonia.Controls.ApplicationLifetimes
         /// The main window.
         /// </value>
         Window? MainWindow { get; set; }
-        
+
+        /// <summary>
+        /// Gets the list of all open windows in the application.
+        /// </summary>
         IReadOnlyList<Window> Windows { get; }
 
         /// <summary>

src/Avalonia.Controls/AutoCompleteBox/AutoCompleteBox.cs

@@ -1711,7 +1711,7 @@ 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;

src/Avalonia.Controls/Automation/AutomationProperties.cs

@@ -38,8 +38,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.AcceleratorKey attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> AcceleratorKeyProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AcceleratorKeyProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AcceleratorKey",
                 typeof(AutomationProperties));
 
@@ -54,16 +54,16 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.AccessKey attached property
         /// </summary>
-        public static readonly AttachedProperty<string> AccessKeyProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AccessKeyProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AccessKey",
                 typeof(AutomationProperties));
 
         /// <summary>
         /// Defines the AutomationProperties.AutomationId attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> AutomationIdProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> AutomationIdProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "AutomationId",
                 typeof(AutomationProperties));
 
@@ -78,8 +78,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.HelpText attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> HelpTextProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> HelpTextProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "HelpText",
                 typeof(AutomationProperties));
 
@@ -122,16 +122,16 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.ItemStatus attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> ItemStatusProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> ItemStatusProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "ItemStatus",
                 typeof(AutomationProperties));
 
         /// <summary>
         /// Defines the AutomationProperties.ItemType attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> ItemTypeProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> ItemTypeProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "ItemType",
                 typeof(AutomationProperties));
 
@@ -155,8 +155,8 @@ namespace Avalonia.Automation
         /// <summary>
         /// Defines the AutomationProperties.Name attached attached property.
         /// </summary>
-        public static readonly AttachedProperty<string> NameProperty =
-            AvaloniaProperty.RegisterAttached<StyledElement, string>(
+        public static readonly AttachedProperty<string?> NameProperty =
+            AvaloniaProperty.RegisterAttached<StyledElement, string?>(
                 "Name",
                 typeof(AutomationProperties));
 
@@ -193,25 +193,17 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAcceleratorKey(StyledElement element, string value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AcceleratorKeyProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AcceleratorKey property from a StyledElement.
         /// </summary>
-        public static string GetAcceleratorKey(StyledElement element)
+        public static string? GetAcceleratorKey(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(AcceleratorKeyProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(AcceleratorKeyProperty);
         }
 
         /// <summary>
@@ -219,11 +211,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAccessibilityView(StyledElement element, AccessibilityView value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AccessibilityViewProperty, value);
         }
 
@@ -232,11 +220,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static AccessibilityView GetAccessibilityView(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(AccessibilityViewProperty);
         }
 
@@ -245,50 +229,34 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetAccessKey(StyledElement element, string value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AccessKeyProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AccessKey property from a StyledElement.
         /// </summary>
-        public static string GetAccessKey(StyledElement element)
+        public static string? GetAccessKey(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(AccessKeyProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(AccessKeyProperty);
         }
 
         /// <summary>
         /// Helper for setting AutomationId property on a StyledElement. 
         /// </summary>
-        public static void SetAutomationId(StyledElement element, string value)
+        public static void SetAutomationId(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(AutomationIdProperty, value);
         }
 
         /// <summary>
         /// Helper for reading AutomationId property from a StyledElement.
         /// </summary>
-        public static string GetAutomationId(StyledElement element)
+        public static string? GetAutomationId(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(AutomationIdProperty);
         }
 
@@ -297,11 +265,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetControlTypeOverride(StyledElement element, AutomationControlType? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ControlTypeOverrideProperty, value);
         }
 
@@ -310,38 +274,26 @@ namespace Avalonia.Automation
         /// </summary>
         public static AutomationControlType? GetControlTypeOverride(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(ControlTypeOverrideProperty);
         }
 
         /// <summary>
         /// Helper for setting HelpText property on a StyledElement. 
         /// </summary>
-        public static void SetHelpText(StyledElement element, string value)
+        public static void SetHelpText(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(HelpTextProperty, value);
         }
 
         /// <summary>
         /// Helper for reading HelpText property from a StyledElement.
         /// </summary>
-        public static string GetHelpText(StyledElement element)
+        public static string? GetHelpText(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(HelpTextProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(HelpTextProperty);
         }
 
         /// <summary>
@@ -349,11 +301,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsColumnHeader(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsColumnHeaderProperty, value);
         }
 
@@ -362,12 +310,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsColumnHeader(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsColumnHeaderProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsColumnHeaderProperty);
         }
 
         /// <summary>
@@ -375,11 +319,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsRequiredForForm(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsRequiredForFormProperty, value);
         }
 
@@ -388,12 +328,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsRequiredForForm(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsRequiredForFormProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsRequiredForFormProperty);
         }
 
         /// <summary>
@@ -401,12 +337,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static bool GetIsRowHeader(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((bool)element.GetValue(IsRowHeaderProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsRowHeaderProperty);
         }
 
         /// <summary>
@@ -414,11 +346,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsRowHeader(StyledElement element, bool value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsRowHeaderProperty, value);
         }
 
@@ -427,11 +355,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetIsOffscreenBehavior(StyledElement element, IsOffscreenBehavior value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(IsOffscreenBehaviorProperty, value);
         }
 
@@ -440,64 +364,44 @@ namespace Avalonia.Automation
         /// </summary>
         public static IsOffscreenBehavior GetIsOffscreenBehavior(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((IsOffscreenBehavior)element.GetValue(IsOffscreenBehaviorProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(IsOffscreenBehaviorProperty);
         }
 
         /// <summary>
         /// Helper for setting ItemStatus property on a StyledElement. 
         /// </summary>
-        public static void SetItemStatus(StyledElement element, string value)
+        public static void SetItemStatus(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ItemStatusProperty, value);
         }
 
         /// <summary>
         /// Helper for reading ItemStatus property from a StyledElement.
         /// </summary>
-        public static string GetItemStatus(StyledElement element)
+        public static string? GetItemStatus(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(ItemStatusProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(ItemStatusProperty);
         }
 
         /// <summary>
         /// Helper for setting ItemType property on a StyledElement. 
         /// </summary>
-        public static void SetItemType(StyledElement element, string value)
+        public static void SetItemType(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(ItemTypeProperty, value);
         }
 
         /// <summary>
         /// Helper for reading ItemType property from a StyledElement.
         /// </summary>
-        public static string GetItemType(StyledElement element)
+        public static string? GetItemType(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(ItemTypeProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(ItemTypeProperty);
         }
 
         /// <summary>
@@ -505,11 +409,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetLabeledBy(StyledElement element, Control value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(LabeledByProperty, value);
         }
 
@@ -518,11 +418,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static Control GetLabeledBy(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             return element.GetValue(LabeledByProperty);
         }
 
@@ -531,11 +427,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetLiveSetting(StyledElement element, AutomationLiveSetting value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(LiveSettingProperty, value);
         }
 
@@ -544,38 +436,26 @@ namespace Avalonia.Automation
         /// </summary>
         public static AutomationLiveSetting GetLiveSetting(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((AutomationLiveSetting)element.GetValue(LiveSettingProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(LiveSettingProperty);
         }
 
         /// <summary>
         /// Helper for setting Name property on a StyledElement. 
         /// </summary>
-        public static void SetName(StyledElement element, string value)
+        public static void SetName(StyledElement element, string? value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(NameProperty, value);
         }
 
         /// <summary>
         /// Helper for reading Name property from a StyledElement.
         /// </summary>
-        public static string GetName(StyledElement element)
+        public static string? GetName(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((string)element.GetValue(NameProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(NameProperty);
         }
 
         /// <summary>
@@ -583,11 +463,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetPositionInSet(StyledElement element, int value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(PositionInSetProperty, value);
         }
 
@@ -596,12 +472,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static int GetPositionInSet(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((int)element.GetValue(PositionInSetProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(PositionInSetProperty);
         }
 
         /// <summary>
@@ -609,11 +481,7 @@ namespace Avalonia.Automation
         /// </summary>
         public static void SetSizeOfSet(StyledElement element, int value)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
+            _ = element ?? throw new ArgumentNullException(nameof(element));
             element.SetValue(SizeOfSetProperty, value);
         }
 
@@ -622,12 +490,8 @@ namespace Avalonia.Automation
         /// </summary>
         public static int GetSizeOfSet(StyledElement element)
         {
-            if (element == null)
-            {
-                throw new ArgumentNullException(nameof(element));
-            }
-
-            return ((int)element.GetValue(SizeOfSetProperty));
+            _ = element ?? throw new ArgumentNullException(nameof(element));
+            return element.GetValue(SizeOfSetProperty);
         }
     }
 }

src/Avalonia.Controls/Calendar/Calendar.cs

@@ -237,11 +237,11 @@ namespace Avalonia.Controls
         private DateTime _selectedYear;
 
         private DateTime _displayDate = DateTime.Today;
-        private DateTime? _displayDateStart = null;
-        private DateTime? _displayDateEnd = null;
+        private DateTime? _displayDateStart;
+        private DateTime? _displayDateEnd;
 
         private bool _isShiftPressed;
-        private bool _displayDateIsChanging = false;
+        private bool _displayDateIsChanging;
 
         internal CalendarDayButton? FocusButton { get; set; }
         internal CalendarButton? FocusCalendarButton { get; set; }
@@ -291,7 +291,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                throw new ArgumentOutOfRangeException("d", "Invalid DayOfWeek");
+                throw new ArgumentOutOfRangeException(nameof(e), "Invalid DayOfWeek");
             }
         }
 
@@ -346,10 +346,10 @@ namespace Avalonia.Controls
             }
         }
 
-        public static readonly StyledProperty<IBrush> HeaderBackgroundProperty =
-            AvaloniaProperty.Register<Calendar, IBrush>(nameof(HeaderBackground));
+        public static readonly StyledProperty<IBrush?> HeaderBackgroundProperty =
+            AvaloniaProperty.Register<Calendar, IBrush?>(nameof(HeaderBackground));
 
-        public IBrush HeaderBackground
+        public IBrush? HeaderBackground
         {
             get { return GetValue(HeaderBackgroundProperty); }
             set { SetValue(HeaderBackgroundProperty, value); }
@@ -478,7 +478,7 @@ namespace Avalonia.Controls
             }
             else
             {
-                throw new ArgumentOutOfRangeException("d", "Invalid SelectionMode");
+                throw new ArgumentOutOfRangeException(nameof(e), "Invalid SelectionMode");
             }
         }
 
@@ -574,7 +574,7 @@ namespace Avalonia.Controls
                     }
                     else
                     {
-                        throw new ArgumentOutOfRangeException("d", "SelectedDate value is not valid.");
+                        throw new ArgumentOutOfRangeException(nameof(e), "SelectedDate value is not valid.");
                     }
                 }
                 else

src/Avalonia.Controls/Calendar/CalendarBlackoutDatesCollection.cs

@@ -15,7 +15,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// The Calendar whose dates this object represents.
         /// </summary>
-        private Calendar _owner;
+        private readonly Calendar _owner;
         
         /// <summary>
         /// Initializes a new instance of the
@@ -79,13 +79,13 @@ namespace Avalonia.Controls.Primitives
 
             if (DateTime.Compare(end, start) > -1)
             {
-                rangeStart = DateTimeHelper.DiscardTime(start).Value;
-                rangeEnd = DateTimeHelper.DiscardTime(end).Value;
+                rangeStart = DateTimeHelper.DiscardTime(start);
+                rangeEnd = DateTimeHelper.DiscardTime(end);
             }
             else
             {
-                rangeStart = DateTimeHelper.DiscardTime(end).Value;
-                rangeEnd = DateTimeHelper.DiscardTime(start).Value;
+                rangeStart = DateTimeHelper.DiscardTime(end);
+                rangeEnd = DateTimeHelper.DiscardTime(start);
             }
 
             int count = Count;
@@ -144,7 +144,7 @@ namespace Avalonia.Controls.Primitives
 
             if (!IsValid(item))
             {
-                throw new ArgumentOutOfRangeException("Value is not valid.");
+                throw new ArgumentOutOfRangeException(nameof(item), "Value is not valid.");
             }
 
             base.InsertItem(index, item);
@@ -186,7 +186,7 @@ namespace Avalonia.Controls.Primitives
 
             if (!IsValid(item))
             {
-                throw new ArgumentOutOfRangeException("Value is not valid.");
+                throw new ArgumentOutOfRangeException(nameof(item), "Value is not valid.");
             }
 
             base.SetItem(index, item);

src/Avalonia.Controls/Calendar/CalendarItem.cs

@@ -44,30 +44,30 @@ namespace Avalonia.Controls.Primitives
         private ITemplate<Control>? _dayTitleTemplate;
         
         private DateTime _currentMonth;
-        private bool _isMouseLeftButtonDown = false;
-        private bool _isMouseLeftButtonDownYearView = false;
-        private bool _isControlPressed = false;
+        private bool _isMouseLeftButtonDown;
+        private bool _isMouseLeftButtonDownYearView;
+        private bool _isControlPressed;
 
-        private System.Globalization.Calendar _calendar = new System.Globalization.GregorianCalendar();
-
-        private PointerPressedEventArgs? _downEventArg;
-        private PointerPressedEventArgs? _downEventArgYearView;
+        private readonly System.Globalization.Calendar _calendar = new GregorianCalendar();
 
         internal Calendar? Owner { get; set; }
         internal CalendarDayButton? CurrentButton { get; set; }
 
-        public static readonly StyledProperty<IBrush> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
-        public IBrush HeaderBackground
+        public static readonly StyledProperty<IBrush?> HeaderBackgroundProperty = Calendar.HeaderBackgroundProperty.AddOwner<CalendarItem>();
+
+        public IBrush? HeaderBackground
         {
             get { return GetValue(HeaderBackgroundProperty); }
             set { SetValue(HeaderBackgroundProperty, value); }
         }
+
         public static readonly DirectProperty<CalendarItem, ITemplate<Control>?> DayTitleTemplateProperty =
                 AvaloniaProperty.RegisterDirect<CalendarItem, ITemplate<Control>?>(
                     nameof(DayTitleTemplate),
                     o => o.DayTitleTemplate,
                     (o,v) => o.DayTitleTemplate = v,
                     defaultBindingMode: BindingMode.OneTime);
+
         public ITemplate<Control>? DayTitleTemplate
         {
             get { return _dayTitleTemplate; }
@@ -178,7 +178,7 @@ namespace Avalonia.Controls.Primitives
                 {
                     if (_dayTitleTemplate != null)
                     {
-                        var cell = (Control) _dayTitleTemplate.Build();
+                        var cell = _dayTitleTemplate.Build();
                         cell.DataContext = string.Empty;
                         cell.SetValue(Grid.RowProperty, 0);
                         cell.SetValue(Grid.ColumnProperty, i);
@@ -308,16 +308,13 @@ namespace Avalonia.Controls.Primitives
             for (int childIndex = 0; childIndex < Calendar.ColumnsPerMonth; childIndex++)
             {
                 var daytitle = MonthView!.Children[childIndex];
-                if (daytitle != null)
+                if (Owner != null)
                 {
-                    if (Owner != null)
-                    {
-                        daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)Owner.FirstDayOfWeek) % NumberOfDaysPerWeek];
-                    }
-                    else
-                    {
-                        daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek) % NumberOfDaysPerWeek];
-                    }
+                    daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)Owner.FirstDayOfWeek) % NumberOfDaysPerWeek];
+                }
+                else
+                {
+                    daytitle.DataContext = DateTimeHelper.GetCurrentDateFormat().ShortestDayNames[(childIndex + (int)DateTimeHelper.GetCurrentDateFormat().FirstDayOfWeek) % NumberOfDaysPerWeek];
                 }
             }
         }
@@ -527,7 +524,7 @@ namespace Avalonia.Controls.Primitives
                 childButton.Content = dateToAdd.Day.ToString(DateTimeHelper.GetCurrentDateFormat());
                 childButton.DataContext = dateToAdd;
 
-                if (DateTime.Compare((DateTime)DateTimeHelper.DiscardTime(DateTime.MaxValue), dateToAdd) > 0)
+                if (DateTime.Compare(DateTimeHelper.DiscardTime(DateTime.MaxValue), dateToAdd) > 0)
                 {
                     // Since we are sure DisplayDate is not equal to
                     // DateTime.MaxValue, it is safe to use AddDays 
@@ -587,7 +584,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                _currentMonth = (DateTime)Owner.SelectedMonth;
+                _currentMonth = Owner.SelectedMonth;
             }
             else
             {
@@ -676,7 +673,7 @@ namespace Avalonia.Controls.Primitives
             if (Owner != null)
             {
                 selectedYear = Owner.SelectedYear;
-                _currentMonth = (DateTime)Owner.SelectedMonth;
+                _currentMonth = Owner.SelectedMonth;
             }
             else
             {
@@ -696,9 +693,9 @@ namespace Avalonia.Controls.Primitives
                 SetYearButtons(decade, decadeEnd);
             }
         }
-        internal void UpdateYearViewSelection(CalendarButton calendarButton)
+        internal void UpdateYearViewSelection(CalendarButton? calendarButton)
         {
-            if (Owner != null && calendarButton != null && calendarButton.DataContext != null)
+            if (Owner != null && calendarButton?.DataContext is DateTime selectedDate)
             {
                 Owner.FocusCalendarButton!.IsCalendarButtonFocused = false;
                 Owner.FocusCalendarButton = calendarButton;
@@ -706,11 +703,11 @@ namespace Avalonia.Controls.Primitives
 
                 if (Owner.DisplayMode == CalendarMode.Year)
                 {
-                    Owner.SelectedMonth = (DateTime)calendarButton.DataContext;
+                    Owner.SelectedMonth = selectedDate;
                 }
                 else
                 {
-                    Owner.SelectedYear = (DateTime)calendarButton.DataContext;
+                    Owner.SelectedYear = selectedDate;
                 }
             }
         }
@@ -719,7 +716,7 @@ namespace Avalonia.Controls.Primitives
         {
             int year;
             int count = -1;
-            foreach (object child in YearView!.Children)
+            foreach (var child in YearView!.Children)
             {
                 CalendarButton childButton = (CalendarButton)child;
                 year = decade + count;
@@ -859,7 +856,8 @@ namespace Avalonia.Controls.Primitives
         {
             if (Owner != null)
             {
-                if (_isMouseLeftButtonDown && sender is CalendarDayButton b && b.IsEnabled && !b.IsBlackout)
+                if (_isMouseLeftButtonDown
+                    && sender is CalendarDayButton { IsEnabled: true, IsBlackout: false, DataContext: DateTime selectedDate } b)
                 {
                     // Update the states of all buttons to be selected starting
                     // from HoverStart to b
@@ -867,7 +865,6 @@ namespace Avalonia.Controls.Primitives
                     {
                         case CalendarSelectionMode.SingleDate:
                             {
-                                DateTime selectedDate = (DateTime)b.DataContext!;
                                 Owner.CalendarDatePickerDisplayDateFlag = true;
                                 if (Owner.SelectedDates.Count == 0)
                                 {
@@ -882,10 +879,9 @@ namespace Avalonia.Controls.Primitives
                         case CalendarSelectionMode.SingleRange:
                         case CalendarSelectionMode.MultipleRange:
                             {
-                                Debug.Assert(b.DataContext != null, "The DataContext should not be null!");
                                 Owner.UnHighlightDays();
                                 Owner.HoverEndIndex = b.Index;
-                                Owner.HoverEnd = (DateTime?)b.DataContext;
+                                Owner.HoverEnd = selectedDate;
                                 // Update the States of the buttons
                                 Owner.HighlightDays();
                                 return;
@@ -904,22 +900,14 @@ namespace Avalonia.Controls.Primitives
                     Owner.Focus();
                 }
 
-                bool ctrl, shift;
-                CalendarExtensions.GetMetaKeyState(e.KeyModifiers, out ctrl, out shift);
-                CalendarDayButton b = (CalendarDayButton)sender!;
+                CalendarExtensions.GetMetaKeyState(e.KeyModifiers, out var ctrl, out var shift);
 
-                if (b != null)
+                if (sender is CalendarDayButton b)
                 {
                     _isControlPressed = ctrl;
-                    if (b.IsEnabled && !b.IsBlackout)
+                    if (b.IsEnabled && !b.IsBlackout && b.DataContext is DateTime selectedDate)
                     {
-                        DateTime selectedDate = (DateTime)b.DataContext!;
                         _isMouseLeftButtonDown = true;
-                        // null check is added for unit tests
-                        if (e != null)
-                        {
-                            _downEventArg = e;
-                        }
 
                         switch (Owner.SelectionMode)
                         {
@@ -1010,12 +998,12 @@ namespace Avalonia.Controls.Primitives
                 }
             }
         }
-        private void AddSelection(CalendarDayButton b)
+        private void AddSelection(CalendarDayButton b, DateTime selectedDate)
         {
             if (Owner != null)
             {
                 Owner.HoverEndIndex = b.Index;
-                Owner.HoverEnd = (DateTime)b.DataContext!;
+                Owner.HoverEnd = selectedDate;
 
                 if (Owner.HoverEnd != null && Owner.HoverStart != null)
                 {
@@ -1025,7 +1013,7 @@ namespace Avalonia.Controls.Primitives
                     // SelectionMode
                     Owner.IsMouseSelection = true;
                     Owner.SelectedDates.AddRange(Owner.HoverStart.Value, Owner.HoverEnd.Value);
-                    Owner.OnDayClick((DateTime)b.DataContext);
+                    Owner.OnDayClick(selectedDate);
                 }
             }
         }
@@ -1039,11 +1027,11 @@ namespace Avalonia.Controls.Primitives
                     Owner.OnDayButtonMouseUp(e);
                 }
                 _isMouseLeftButtonDown = false;
-                if (b != null && b.DataContext != null)
+                if (b != null && b.DataContext is DateTime selectedDate)
                 {
                     if (Owner.SelectionMode == CalendarSelectionMode.None || Owner.SelectionMode == CalendarSelectionMode.SingleDate)
                     {
-                        Owner.OnDayClick((DateTime)b.DataContext);
+                        Owner.OnDayClick(selectedDate);
                         return;
                     }
                     if (Owner.HoverStart.HasValue)
@@ -1058,14 +1046,14 @@ namespace Avalonia.Controls.Primitives
                                         Owner.RemovedItems.Add(item);
                                     }
                                     Owner.SelectedDates.ClearInternal();
-                                    AddSelection(b);
+                                    AddSelection(b, selectedDate);
                                     return;
                                 }
                             case CalendarSelectionMode.MultipleRange:
                                 {
                                     // add the selection (either single day or
                                     // SingleRange day)
-                                    AddSelection(b);
+                                    AddSelection(b, selectedDate);
                                     return;
                                 }
                         }
@@ -1076,7 +1064,7 @@ namespace Avalonia.Controls.Primitives
                         // be able to switch months
                         if (b.IsInactive && b.IsBlackout)
                         {
-                            Owner.OnDayClick((DateTime)b.DataContext);
+                            Owner.OnDayClick(selectedDate);
                         }
                     }
                 }
@@ -1095,9 +1083,9 @@ namespace Avalonia.Controls.Primitives
                         Owner.HoverStart = null;
                         _isMouseLeftButtonDown = false;
                         b.IsSelected = false;
-                        if (b.DataContext != null)
+                        if (b.DataContext is DateTime selectedDate)
                         {
-                            Owner.SelectedDates.Remove((DateTime)b.DataContext);
+                            Owner.SelectedDates.Remove(selectedDate);
                         }
                     }
                 }
@@ -1107,35 +1095,26 @@ namespace Avalonia.Controls.Primitives
 
         private void Month_CalendarButtonMouseDown(object? sender, PointerPressedEventArgs e)
         {
-            CalendarButton b = (CalendarButton)sender!;
-
             _isMouseLeftButtonDownYearView = true;
 
-            if (e != null)
-            {
-                _downEventArgYearView = e;
-            }
-
-            UpdateYearViewSelection(b);
+            UpdateYearViewSelection(sender as CalendarButton);
         }
 
         internal void Month_CalendarButtonMouseUp(object? sender, PointerReleasedEventArgs e)
         {
             _isMouseLeftButtonDownYearView = false;
 
-            if (Owner != null)
+            if (Owner != null && (sender as CalendarButton)?.DataContext is DateTime newMonth)
             {
-                DateTime newmonth = (DateTime)((CalendarButton)sender!).DataContext!;
-
                 if (Owner.DisplayMode == CalendarMode.Year)
                 {
-                    Owner.DisplayDate = newmonth;
+                    Owner.DisplayDate = newMonth;
                     Owner.DisplayMode = CalendarMode.Month;
                 }
                 else
                 {
                     Debug.Assert(Owner.DisplayMode == CalendarMode.Decade, "The owning Calendar should be in decade mode!");
-                    Owner.SelectedMonth = newmonth;
+                    Owner.SelectedMonth = newMonth;
                     Owner.DisplayMode = CalendarMode.Year;
                 }
             }
@@ -1145,8 +1124,7 @@ namespace Avalonia.Controls.Primitives
         {
             if (_isMouseLeftButtonDownYearView)
             {
-                CalendarButton b = (CalendarButton)sender!;
-                UpdateYearViewSelection(b);
+                UpdateYearViewSelection(sender as CalendarButton);
             }
         }
 

src/Avalonia.Controls/Calendar/DateTimeHelper.cs

@@ -53,7 +53,7 @@ namespace Avalonia.Controls
 
         public static int CompareDays(DateTime dt1, DateTime dt2)
         {
-            return DateTime.Compare(DiscardTime(dt1).Value, DiscardTime(dt2).Value);
+            return DateTime.Compare(DiscardTime(dt1), DiscardTime(dt2));
         }
 
         public static int CompareYearMonth(DateTime dt1, DateTime dt2)
@@ -71,14 +71,9 @@ namespace Avalonia.Controls
             return new DateTime(d.Year, d.Month, 1, 0, 0, 0);
         }
 
-        [return: NotNullIfNotNull("d")]
-        public static DateTime? DiscardTime(DateTime? d)
+        public static DateTime DiscardTime(DateTime d)
         {
-            if (d == null)
-            {
-                return null;
-            }
-            return d.Value.Date;
+            return d.Date;
         }
 
         public static int EndOfDecade(DateTime date)
@@ -127,28 +122,14 @@ namespace Avalonia.Controls
 
         public static string ToYearMonthPatternString(DateTime date)
         {
-            string result = string.Empty;
-            DateTimeFormatInfo format = GetCurrentDateFormat();
-
-            if (format != null)
-            {
-                result = date.ToString(format.YearMonthPattern, format);
-            }
-
-            return result;
+            var format = GetCurrentDateFormat();
+            return date.ToString(format.YearMonthPattern, format);
         }
 
         public static string ToYearString(DateTime date)
         {
-            string result = string.Empty;
-            DateTimeFormatInfo format = GetCurrentDateFormat();
-
-            if (format != null)
-            {
-                result = date.Year.ToString(format);
-            }
-
-            return result;
+            var format = GetCurrentDateFormat();
+            return date.Year.ToString(format);
         }
     }
 }

src/Avalonia.Controls/CalendarDatePicker/CalendarDatePicker.cs

@@ -51,11 +51,11 @@ namespace Avalonia.Controls
         private bool _isDropDownOpen;
         private DateTime? _selectedDate;
         private string? _text;
-        private bool _suspendTextChangeHandler = false;
-        private bool _isPopupClosing = false;
-        private bool _ignoreButtonClick = false;
-        private bool _isFlyoutOpen = false;
-        private bool _isPressed = false;
+        private bool _suspendTextChangeHandler;
+        private bool _isPopupClosing;
+        private bool _ignoreButtonClick;
+        private bool _isFlyoutOpen;
+        private bool _isPressed;
 
         /// <summary>
         /// Occurs when the drop-down
@@ -185,7 +185,7 @@ namespace Avalonia.Controls
             {
                 _textBox.KeyDown += TextBox_KeyDown;
                 _textBox.GotFocus += TextBox_GotFocus;
-                _textBoxTextChangedSubscription = _textBox.GetObservable(TextBox.TextProperty).Subscribe(txt => TextBox_TextChanged());
+                _textBoxTextChangedSubscription = _textBox.GetObservable(TextBox.TextProperty).Subscribe(_ => TextBox_TextChanged());
 
                 if(SelectedDate.HasValue)
                 {
@@ -292,7 +292,7 @@ namespace Avalonia.Controls
             // Text
             else if (change.Property == TextProperty)
             {
-                var (oldValue, newValue) = change.GetOldAndNewValue<string>();
+                var (_, newValue) = change.GetOldAndNewValue<string?>();
 
                 if (!_suspendTextChangeHandler)
                 {
@@ -595,9 +595,9 @@ namespace Avalonia.Controls
 
         private void Calendar_KeyDown(object? sender, KeyEventArgs e)
         {
-            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)
+            if (!e.Handled
+                && sender is Calendar { DisplayMode: CalendarMode.Month }
+                && (e.Key == Key.Enter || e.Key == Key.Space || e.Key == Key.Escape))
             {
                 Focus();
                 IsDropDownOpen = false;

src/Avalonia.Controls/Chrome/TitleBar.cs

@@ -17,28 +17,26 @@ namespace Avalonia.Controls.Chrome
 
         private void UpdateSize(Window window)
         {
-            if (window != null)
+            Margin = new Thickness(
+                window.OffScreenMargin.Left,
+                window.OffScreenMargin.Top,
+                window.OffScreenMargin.Right,
+                window.OffScreenMargin.Bottom);
+
+            if (window.WindowState != WindowState.FullScreen)
             {
-                Margin = new Thickness(
-                    window.OffScreenMargin.Left,
-                    window.OffScreenMargin.Top,
-                    window.OffScreenMargin.Right,
-                    window.OffScreenMargin.Bottom);
+                Height = window.WindowDecorationMargin.Top;
 
-                if (window.WindowState != WindowState.FullScreen)
+                if (_captionButtons != null)
                 {
-                    Height = window.WindowDecorationMargin.Top;
-
-                    if (_captionButtons != null)
-                    {
-                        _captionButtons.Height = Height;
-                    }
+                    _captionButtons.Height = Height;
                 }
-
-                IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
             }
+
+            IsVisible = window.PlatformImpl?.NeedsManagedDecorations ?? false;
         }
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
@@ -55,6 +53,7 @@ namespace Avalonia.Controls.Chrome
             }
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnAttachedToVisualTree(e);
@@ -64,13 +63,13 @@ namespace Avalonia.Controls.Chrome
                 _disposables = new CompositeDisposable(6)
                 {
                     window.GetObservable(Window.WindowDecorationMarginProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.ExtendClientAreaTitleBarHeightHintProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.OffScreenMarginProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.ExtendClientAreaChromeHintsProperty)
-                        .Subscribe(x => UpdateSize(window)),
+                        .Subscribe(_ => UpdateSize(window)),
                     window.GetObservable(Window.WindowStateProperty)
                         .Subscribe(x =>
                         {
@@ -80,11 +79,12 @@ namespace Avalonia.Controls.Chrome
                             PseudoClasses.Set(":fullscreen", x == WindowState.FullScreen);
                         }),
                     window.GetObservable(Window.IsExtendedIntoWindowDecorationsProperty)
-                        .Subscribe(x => UpdateSize(window))
+                        .Subscribe(_ => UpdateSize(window))
                 };
             }
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnDetachedFromVisualTree(e);

src/Avalonia.Controls/Converters/MenuScrollingVisibilityConverter.cs

@@ -14,7 +14,6 @@ namespace Avalonia.Controls.Converters
         public object? Convert(IList<object?> values, Type targetType, object? parameter, CultureInfo culture)
         {
             if (parameter == null ||
-                values == null ||
                 values.Count != 4 ||
                 !(values[0] is ScrollBarVisibility visibility) ||
                 !(values[1] is double offset) ||

src/Avalonia.Controls/DefinitionBase.cs

@@ -21,9 +21,9 @@ namespace Avalonia.Controls
         /// <summary>
         /// SharedSizeGroup property.
         /// </summary>
-        public string SharedSizeGroup
+        public string? SharedSizeGroup
         {
-            get { return (string)GetValue(SharedSizeGroupProperty); }
+            get { return GetValue(SharedSizeGroupProperty); }
             set { SetValue(SharedSizeGroupProperty, value); }
         }
 
@@ -32,20 +32,15 @@ namespace Avalonia.Controls
         /// </summary>
         internal void OnEnterParentTree()
         {
-            this.InheritanceParent = Parent;
+            InheritanceParent = Parent;
             if (_sharedState == null)
             {
                 //  start with getting SharedSizeGroup value. 
                 //  this property is NOT inherited which should result in better overall perf.
-                string sharedSizeGroupId = SharedSizeGroup;
-                if (sharedSizeGroupId != null)
+                if (SharedSizeGroup is { } sharedSizeGroupId && PrivateSharedSizeScope is { } privateSharedSizeScope)
                 {
-                    SharedSizeScope? privateSharedSizeScope = PrivateSharedSizeScope;
-                    if (privateSharedSizeScope != null)
-                    {
-                        _sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
-                        _sharedState.AddMember(this);
-                    }
+                    _sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
+                    _sharedState.AddMember(this);
                 }
             }
 
@@ -321,13 +316,12 @@ namespace Avalonia.Controls
             return ((_flags & flags) == flags);
         }
 
-        private static void OnSharedSizeGroupPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
+        private static void OnSharedSizeGroupPropertyChanged(DefinitionBase definition,
+            AvaloniaPropertyChangedEventArgs<string?> e)
         {
-            DefinitionBase definition = (DefinitionBase)d;
-
             if (definition.Parent != null)
             {
-                string sharedSizeGroupId = (string)e.NewValue!;
+                string? sharedSizeGroupId = e.NewValue.Value;
 
                 if (definition._sharedState != null)
                 {
@@ -337,16 +331,14 @@ namespace Avalonia.Controls
                     definition._sharedState = null;
                 }
 
-                if ((definition._sharedState == null) && (sharedSizeGroupId != null))
+                if (definition._sharedState == null
+                    && sharedSizeGroupId != null
+                    && definition.PrivateSharedSizeScope is { } privateSharedSizeScope)
                 {
-                    SharedSizeScope? privateSharedSizeScope = definition.PrivateSharedSizeScope;
-                    if (privateSharedSizeScope != null)
-                    {
-                        //  if definition is not registered and both: shared size group id AND private shared scope 
-                        //  are available, then register definition.
-                        definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
-                        definition._sharedState.AddMember(definition);
-                    }
+                    //  if definition is not registered and both: shared size group id AND private shared scope
+                    //  are available, then register definition.
+                    definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroupId);
+                    definition._sharedState.AddMember(definition);
                 }
             }
         }
@@ -357,17 +349,15 @@ namespace Avalonia.Controls
         /// b) contains only letters, digits and underscore ('_').
         /// c) does not start with a digit.
         /// </remarks>
-        private static bool SharedSizeGroupPropertyValueValid(string value)
+        private static bool SharedSizeGroupPropertyValueValid(string? id)
         {
             //  null is default value
-            if (value == null)
+            if (id == null)
             {
                 return true;
             }
 
-            string id = (string)value;
-
-            if (!string.IsNullOrEmpty(id))
+            if (id.Length > 0)
             {
                 int i = -1;
                 while (++i < id.Length)
@@ -397,14 +387,11 @@ namespace Avalonia.Controls
         /// existing scope just left. In both cases if the DefinitionBase object is already registered
         /// in SharedSizeState, it should un-register and register itself in a new one.
         /// </remark>
-        private static void OnPrivateSharedSizeScopePropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
+        private static void OnPrivateSharedSizeScopePropertyChanged(DefinitionBase definition,
+            AvaloniaPropertyChangedEventArgs<SharedSizeScope?> e)
         {
-            DefinitionBase definition = (DefinitionBase)d;
-
             if (definition.Parent != null)
             {
-                SharedSizeScope privateSharedSizeScope = (SharedSizeScope)e.NewValue!;
-
                 if (definition._sharedState != null)
                 {
                     //  if definition is already registered And shared size scope is changing,
@@ -413,16 +400,14 @@ namespace Avalonia.Controls
                     definition._sharedState = null;
                 }
 
-                if ((definition._sharedState == null) && (privateSharedSizeScope != null))
+                if (definition._sharedState == null
+                    && e.NewValue.Value is { } privateSharedSizeScope
+                    && definition.SharedSizeGroup is { } sharedSizeGroup)
                 {
-                    string sharedSizeGroup = definition.SharedSizeGroup;
-                    if (sharedSizeGroup != null)
-                    {
-                        //  if definition is not registered and both: shared size group id AND private shared scope 
-                        //  are available, then register definition.
-                        definition._sharedState = privateSharedSizeScope.EnsureSharedState(definition.SharedSizeGroup);
-                        definition._sharedState.AddMember(definition);
-                    }
+                    //  if definition is not registered and both: shared size group id AND private shared scope
+                    //  are available, then register definition.
+                    definition._sharedState = privateSharedSizeScope.EnsureSharedState(sharedSizeGroup);
+                    definition._sharedState.AddMember(definition);
                 }
             }
         }
@@ -432,7 +417,7 @@ namespace Avalonia.Controls
         /// </summary>
         private SharedSizeScope? PrivateSharedSizeScope
         {
-            get { return (SharedSizeScope?)GetValue(PrivateSharedSizeScopeProperty); }
+            get { return GetValue(PrivateSharedSizeScopeProperty); }
         }
 
         /// <summary>
@@ -465,7 +450,7 @@ namespace Avalonia.Controls
 
         private SharedSizeState? _sharedState;           //  reference to shared state object this instance is registered with
 
-        [System.Flags]
+        [Flags]
         private enum Flags : byte
         {
             //
@@ -520,11 +505,10 @@ namespace Avalonia.Controls
             /// </summary>
             internal SharedSizeState(SharedSizeScope sharedSizeScope, string sharedSizeGroupId)
             {
-                Debug.Assert(sharedSizeScope != null && sharedSizeGroupId != null);
                 _sharedSizeScope = sharedSizeScope;
                 _sharedSizeGroupId = sharedSizeGroupId;
                 _registry = new List<DefinitionBase>();
-                _layoutUpdated = new EventHandler(OnLayoutUpdated);
+                _layoutUpdated = OnLayoutUpdated;
                 _broadcastInvalidation = true;
             }
 
@@ -568,7 +552,7 @@ namespace Avalonia.Controls
                 {
                     for (int i = 0, count = _registry.Count; i < count; ++i)
                     {
-                        Grid parentGrid = (Grid)(_registry[i].Parent!);
+                        Grid parentGrid = _registry[i].Parent!;
                         parentGrid.Invalidate();
                     }
                     _broadcastInvalidation = false;
@@ -703,7 +687,7 @@ namespace Avalonia.Controls
                         // measure is invalid - it used the old shared size,
                         // which is larger than d's (possibly changed) minSize
                         measureIsValid = (definitionBase.LayoutWasUpdated &&
-                                        MathUtilities.GreaterThanOrClose(definitionBase._minSize, this.MinSize));
+                                        MathUtilities.GreaterThanOrClose(definitionBase._minSize, MinSize));
                     }
 
                     if(!measureIsValid)
@@ -786,8 +770,8 @@ namespace Avalonia.Controls
         /// </description></item>
         /// </list>
         /// </remarks> 
-        public static readonly AttachedProperty<string> SharedSizeGroupProperty =
-            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, string>(
+        public static readonly AttachedProperty<string?> SharedSizeGroupProperty =
+            AvaloniaProperty.RegisterAttached<DefinitionBase, Control, string?>(
                 "SharedSizeGroup",
                 validate: SharedSizeGroupPropertyValueValid);
 
@@ -796,8 +780,8 @@ namespace Avalonia.Controls
         /// </summary>
         static DefinitionBase()
         {
-            SharedSizeGroupProperty.Changed.AddClassHandler<DefinitionBase>(OnSharedSizeGroupPropertyChanged);
-            PrivateSharedSizeScopeProperty.Changed.AddClassHandler<DefinitionBase>(OnPrivateSharedSizeScopePropertyChanged);
+            SharedSizeGroupProperty.Changed.AddClassHandler<DefinitionBase, string?>(OnSharedSizeGroupPropertyChanged);
+            PrivateSharedSizeScopeProperty.Changed.AddClassHandler<DefinitionBase, SharedSizeScope?>(OnPrivateSharedSizeScopePropertyChanged);
         }
 
         /// <summary>

src/Avalonia.Controls/DockPanel.cs

@@ -101,9 +101,6 @@ namespace Avalonia.Controls
                 Size childConstraint;             // Contains the suggested input constraint for this child.
                 Size childDesiredSize;            // Contains the return size from child measure.
 
-                if (child == null)
-                { continue; }
-
                 // Child constraint is the remaining size; this is total size minus size consumed by previous children.
                 childConstraint = new Size(Math.Max(0.0, constraint.Width - accumulatedWidth),
                                            Math.Max(0.0, constraint.Height - accumulatedHeight));
@@ -122,7 +119,7 @@ namespace Avalonia.Controls
                 // will deal with computing our minimum size (parentSize) due to that accumulation.
                 // Therefore, we only need to compute our minimum size (parentSize) in dimensions that this child does
                 //   not accumulate: Width for Top/Bottom, Height for Left/Right.
-                switch (DockPanel.GetDock((Control)child))
+                switch (GetDock(child))
                 {
                     case Dock.Left:
                     case Dock.Right:
@@ -164,8 +161,6 @@ namespace Avalonia.Controls
             for (int i = 0; i < totalChildrenCount; ++i)
             {
                 var child = children[i];
-                if (child == null)
-                { continue; }
 
                 Size childDesiredSize = child.DesiredSize;
                 Rect rcChild = new Rect(
@@ -176,7 +171,7 @@ namespace Avalonia.Controls
 
                 if (i < nonFillChildrenCount)
                 {
-                    switch (DockPanel.GetDock((Control)child))
+                    switch (GetDock(child))
                     {
                         case Dock.Left:
                             accumulatedLeft += childDesiredSize.Width;

src/Avalonia.Controls/Documents/Inline.cs

@@ -13,8 +13,8 @@ namespace Avalonia.Controls.Documents
         /// <summary>
         /// AvaloniaProperty for <see cref="TextDecorations" /> property.
         /// </summary>
-        public static readonly StyledProperty<TextDecorationCollection> TextDecorationsProperty =
-            AvaloniaProperty.Register<Inline, TextDecorationCollection>(
+        public static readonly StyledProperty<TextDecorationCollection?> TextDecorationsProperty =
+            AvaloniaProperty.Register<Inline, TextDecorationCollection?>(
                 nameof(TextDecorations));
 
         /// <summary>
@@ -28,7 +28,7 @@ namespace Avalonia.Controls.Documents
         /// <summary>
         /// The TextDecorations property specifies decorations that are added to the text of an element.
         /// </summary>
-        public TextDecorationCollection TextDecorations
+        public TextDecorationCollection? TextDecorations
         {
             get { return GetValue(TextDecorationsProperty); }
             set { SetValue(TextDecorationsProperty, value); }
@@ -83,7 +83,8 @@ namespace Avalonia.Controls.Documents
             return new GenericTextRunProperties(new Typeface(FontFamily, fontStyle, fontWeight), FontSize,
                 textDecorations, Foreground, background, BaselineAlignment);
         }
-        
+
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);

src/Avalonia.Controls/Documents/Span.cs

@@ -1,3 +1,4 @@
+using System;
 using System.Collections.Generic;
 using System.Text;
 using Avalonia.Media.TextFormatting;
@@ -51,6 +52,7 @@ namespace Avalonia.Controls.Documents
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -68,26 +70,26 @@ namespace Avalonia.Controls.Documents
         {
             base.OnInlineHostChanged(oldValue, newValue);
 
-            if (Inlines is not null)
-            {
-                Inlines.InlineHost = newValue;
-            }
+            Inlines.InlineHost = newValue;
         }
 
         private void OnInlinesChanged(InlineCollection? oldValue, InlineCollection? newValue)
         {
+            void OnInlinesInvalidated(object? sender, EventArgs e)
+                => InlineHost?.Invalidate();
+
             if (oldValue is not null)
             {
                 oldValue.LogicalChildren = null;
                 oldValue.InlineHost = null;
-                oldValue.Invalidated -= (s, e) => InlineHost?.Invalidate();
+                oldValue.Invalidated -= OnInlinesInvalidated;
             }
 
             if (newValue is not null)
             {
                 newValue.LogicalChildren = LogicalChildren;
                 newValue.InlineHost = InlineHost;
-                newValue.Invalidated += (s, e) => InlineHost?.Invalidate();
+                newValue.Invalidated += OnInlinesInvalidated;
             }
         }
     }

src/Avalonia.Controls/Grid.cs

@@ -164,20 +164,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Returns a ColumnDefinitions of column definitions.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         public ColumnDefinitions ColumnDefinitions
         {
             get
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                if (_data.ColumnDefinitions == null) { _data.ColumnDefinitions = new ColumnDefinitions() { Parent = this }; }
+                if (_extData == null) { _extData = new ExtendedData(); }
+                if (_extData.ColumnDefinitions == null) { _extData.ColumnDefinitions = new ColumnDefinitions() { Parent = this }; }
 
-                return (_data.ColumnDefinitions);
+                return (_extData.ColumnDefinitions);
             }
             set
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                _data.ColumnDefinitions = value;
-                _data.ColumnDefinitions.Parent = this;
+                if (_extData == null) { _extData = new ExtendedData(); }
+                _extData.ColumnDefinitions = value;
+                _extData.ColumnDefinitions.Parent = this;
                 InvalidateMeasure();
             }
         }
@@ -185,20 +186,21 @@ namespace Avalonia.Controls
         /// <summary>
         /// Returns a RowDefinitions of row definitions.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         public RowDefinitions RowDefinitions
         {
             get
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                if (_data.RowDefinitions == null) { _data.RowDefinitions = new RowDefinitions() { Parent = this }; }
+                if (_extData == null) { _extData = new ExtendedData(); }
+                if (_extData.RowDefinitions == null) { _extData.RowDefinitions = new RowDefinitions() { Parent = this }; }
 
-                return (_data.RowDefinitions);
+                return (_extData.RowDefinitions);
             }
             set
             {
-                if (_data == null) { _data = new ExtendedData(); }
-                _data.RowDefinitions = value;
-                _data.RowDefinitions.Parent = this;
+                if (_extData == null) { _extData = new ExtendedData(); }
+                _extData.RowDefinitions = value;
+                _extData.RowDefinitions.Parent = this;
                 InvalidateMeasure();
             }
         }
@@ -211,7 +213,7 @@ namespace Avalonia.Controls
         protected override Size MeasureOverride(Size constraint)
         {
             Size gridDesiredSize;
-            ExtendedData extData = ExtData;
+            var extData = _extData;
 
             try
             {
@@ -221,17 +223,14 @@ namespace Avalonia.Controls
                 if (extData == null)
                 {
                     gridDesiredSize = new Size();
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int i = 0, count = children.Count; i < count; ++i)
                     {
                         var child = children[i];
-                        if (child != null)
-                        {
-                            child.Measure(constraint);
-                            gridDesiredSize = new Size(Math.Max(gridDesiredSize.Width, child.DesiredSize.Width),
-                                                       Math.Max(gridDesiredSize.Height, child.DesiredSize.Height));
-                        }
+                        child.Measure(constraint);
+                        gridDesiredSize = new Size(Math.Max(gridDesiredSize.Width, child.DesiredSize.Width),
+                                                   Math.Max(gridDesiredSize.Height, child.DesiredSize.Height));
                     }
                 }
                 else
@@ -512,17 +511,14 @@ namespace Avalonia.Controls
             {
                 ArrangeOverrideInProgress = true;
 
-                if (_data == null)
+                if (_extData is null)
                 {
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int i = 0, count = children.Count; i < count; ++i)
                     {
                         var child = children[i];
-                        if (child != null)
-                        {
-                            child.Arrange(new Rect(arrangeSize));
-                        }
+                        child.Arrange(new Rect(arrangeSize));
                     }
                 }
                 else
@@ -532,15 +528,11 @@ namespace Avalonia.Controls
                     SetFinalSize(DefinitionsU, arrangeSize.Width, true);
                     SetFinalSize(DefinitionsV, arrangeSize.Height, false);
 
-                    var children = this.Children;
+                    var children = Children;
 
                     for (int currentCell = 0; currentCell < PrivateCells.Length; ++currentCell)
                     {
                         var cell = children[currentCell];
-                        if (cell == null)
-                        {
-                            continue;
-                        }
 
                         int columnIndex = PrivateCells[currentCell].ColumnIndex;
                         int rowIndex = PrivateCells[currentCell].RowIndex;
@@ -599,7 +591,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Debug.Assert(_data != null);
+            Debug.Assert(_extData != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!ColumnDefinitionsDirty)
@@ -621,7 +613,7 @@ namespace Avalonia.Controls
         {
             double value = 0.0;
 
-            Debug.Assert(_data != null);
+            Debug.Assert(_extData != null);
 
             //  actual value calculations require structure to be up-to-date
             if (!RowDefinitionsDirty)
@@ -654,18 +646,20 @@ namespace Avalonia.Controls
         /// <summary>
         /// Convenience accessor to ValidDefinitionsUStructure bit flag.
         /// </summary> 
+        [MemberNotNull(nameof(_extData))]
         internal bool ColumnDefinitionsDirty
         {
-            get => ColumnDefinitions?.IsDirty ?? false;
+            get => ColumnDefinitions.IsDirty;
             set => ColumnDefinitions.IsDirty = value;
         }
 
         /// <summary>
         /// Convenience accessor to ValidDefinitionsVStructure bit flag.
         /// </summary>
+        [MemberNotNull(nameof(_extData))]
         internal bool RowDefinitionsDirty
         {
-            get => RowDefinitions?.IsDirty ?? false;
+            get => RowDefinitions.IsDirty;
             set => RowDefinitions.IsDirty = value;
         }
 
@@ -686,8 +680,10 @@ namespace Avalonia.Controls
         /// </summary>
         private void ValidateCellsCore()
         {
-            var children = this.Children;
-            ExtendedData extData = ExtData;
+            Debug.Assert(_extData is not null);
+
+            var children = Children;
+            var extData = _extData!;
 
             extData.CellCachesCollection = new CellCache[children.Count];
             extData.CellGroup1 = int.MaxValue;
@@ -702,10 +698,6 @@ namespace Avalonia.Controls
             for (int i = PrivateCells.Length - 1; i >= 0; --i)
             {
                 var child = children[i];
-                if (child == null)
-                {
-                    continue;
-                }
 
                 CellCache cell = new CellCache();
 
@@ -713,19 +705,19 @@ namespace Avalonia.Controls
                 //  Read indices from the corresponding properties:
                 //      clamp to value < number_of_columns
                 //      column >= 0 is guaranteed by property value validation callback
-                cell.ColumnIndex = Math.Min(GetColumn((Control)child), DefinitionsU.Count - 1);
+                cell.ColumnIndex = Math.Min(GetColumn(child), DefinitionsU.Count - 1);
                 //      clamp to value < number_of_rows
                 //      row >= 0 is guaranteed by property value validation callback
-                cell.RowIndex = Math.Min(GetRow((Control)child), DefinitionsV.Count - 1);
+                cell.RowIndex = Math.Min(GetRow(child), DefinitionsV.Count - 1);
 
                 //  Read span properties:
                 //      clamp to not exceed beyond right side of the grid
                 //      column_span > 0 is guaranteed by property value validation callback
-                cell.ColumnSpan = Math.Min(GetColumnSpan((Control)child), DefinitionsU.Count - cell.ColumnIndex);
+                cell.ColumnSpan = Math.Min(GetColumnSpan(child), DefinitionsU.Count - cell.ColumnIndex);
 
                 //      clamp to not exceed beyond bottom side of the grid
                 //      row_span > 0 is guaranteed by property value validation callback
-                cell.RowSpan = Math.Min(GetRowSpan((Control)child), DefinitionsV.Count - cell.RowIndex);
+                cell.RowSpan = Math.Min(GetRowSpan(child), DefinitionsV.Count - cell.RowIndex);
 
                 Debug.Assert(0 <= cell.ColumnIndex && cell.ColumnIndex < DefinitionsU.Count);
                 Debug.Assert(0 <= cell.RowIndex && cell.RowIndex < DefinitionsV.Count);
@@ -792,7 +784,7 @@ namespace Avalonia.Controls
         {
             if (ColumnDefinitionsDirty)
             {
-                ExtendedData extData = ExtData;
+                var extData = _extData;
 
                 if (extData.ColumnDefinitions == null)
                 {
@@ -818,7 +810,7 @@ namespace Avalonia.Controls
                 ColumnDefinitionsDirty = false;
             }
 
-            Debug.Assert(ExtData.DefinitionsU != null && ExtData.DefinitionsU.Count > 0);
+            Debug.Assert(_extData is { DefinitionsU.Count: > 0 });
         }
 
         /// <summary>
@@ -833,7 +825,7 @@ namespace Avalonia.Controls
         {
             if (RowDefinitionsDirty)
             {
-                ExtendedData extData = ExtData;
+                var extData = _extData;
 
                 if (extData.RowDefinitions == null)
                 {
@@ -859,7 +851,7 @@ namespace Avalonia.Controls
                 RowDefinitionsDirty = false;
             }
 
-            Debug.Assert(ExtData.DefinitionsV != null && ExtData.DefinitionsV.Count > 0);
+            Debug.Assert(_extData is { DefinitionsV.Count: > 0 });
         }
 
         /// <summary>
@@ -965,8 +957,7 @@ namespace Avalonia.Controls
             bool ignoreDesiredSizeU,
             bool forceInfinityV)
         {
-            bool unusedHasDesiredSizeUChanged;
-            MeasureCellsGroup(cellsHead, referenceSize, ignoreDesiredSizeU, forceInfinityV, out unusedHasDesiredSizeUChanged);
+            MeasureCellsGroup(cellsHead, referenceSize, ignoreDesiredSizeU, forceInfinityV, out _);
         }
 
         /// <summary>
@@ -994,7 +985,7 @@ namespace Avalonia.Controls
                 return;
             }
 
-            var children = this.Children;
+            var children = Children;
             Hashtable? spanStore = null;
             bool ignoreDesiredSizeV = forceInfinityV;
 
@@ -1101,8 +1092,6 @@ namespace Avalonia.Controls
             int cell,
             bool forceInfinityV)
         {
-
-
             double cellMeasureWidth;
             double cellMeasureHeight;
 
@@ -1144,15 +1133,9 @@ namespace Avalonia.Controls
             }
 
 
-            var child = this.Children[cell];
-            if (child != null)
-            {
-                Size childConstraint = new Size(cellMeasureWidth, cellMeasureHeight);
-                child.Measure(childConstraint);
-            }
-
-
-
+            var child = Children[cell];
+            Size childConstraint = new Size(cellMeasureWidth, cellMeasureHeight);
+            child.Measure(childConstraint);
         }
 
         /// <summary>
@@ -1230,7 +1213,7 @@ namespace Avalonia.Controls
             //  avoid processing when asked to distribute "0"
             if (!MathUtilities.IsZero(requestedSize))
             {
-                DefinitionBase[] tempDefinitions = TempDefinitions; //  temp array used to remember definitions for sorting
+                DefinitionBase?[] tempDefinitions = TempDefinitions; //  temp array used to remember definitions for sorting
                 int end = start + count;
                 int autoDefinitionsCount = 0;
                 double rangeMinSize = 0;
@@ -1288,20 +1271,24 @@ namespace Avalonia.Controls
                         Array.Sort(tempDefinitions, 0, count, s_spanPreferredDistributionOrderComparer);
                         for (i = 0, sizeToDistribute = requestedSize; i < autoDefinitionsCount; ++i)
                         {
+                            var tempDefinition = tempDefinitions[i]!;
+
                             //  sanity check: only auto definitions allowed in this loop
-                            Debug.Assert(tempDefinitions[i].UserSize.IsAuto);
+                            Debug.Assert(tempDefinition.UserSize.IsAuto);
 
                             //  adjust sizeToDistribute value by subtracting auto definition min size
-                            sizeToDistribute -= (tempDefinitions[i].MinSize);
+                            sizeToDistribute -= (tempDefinition.MinSize);
                         }
 
                         for (; i < count; ++i)
                         {
+                            var tempDefinition = tempDefinitions[i]!;
+
                             //  sanity check: no auto definitions allowed in this loop
-                            Debug.Assert(!tempDefinitions[i].UserSize.IsAuto);
+                            Debug.Assert(!tempDefinition.UserSize.IsAuto);
 
-                            double newMinSize = Math.Min(sizeToDistribute / (count - i), tempDefinitions[i].PreferredSize);
-                            if (newMinSize > tempDefinitions[i].MinSize) { tempDefinitions[i].UpdateMinSize(newMinSize); }
+                            double newMinSize = Math.Min(sizeToDistribute / (count - i), tempDefinition.PreferredSize);
+                            if (newMinSize > tempDefinition.MinSize) { tempDefinition.UpdateMinSize(newMinSize); }
                             sizeToDistribute -= newMinSize;
                         }
 
@@ -1325,24 +1312,28 @@ namespace Avalonia.Controls
                         Array.Sort(tempDefinitions, 0, count, s_spanMaxDistributionOrderComparer);
                         for (i = 0, sizeToDistribute = requestedSize - rangePreferredSize; i < count - autoDefinitionsCount; ++i)
                         {
+                            var tempDefinition = tempDefinitions[i]!;
+
                             //  sanity check: no auto definitions allowed in this loop
-                            Debug.Assert(!tempDefinitions[i].UserSize.IsAuto);
+                            Debug.Assert(!tempDefinition.UserSize.IsAuto);
 
-                            double preferredSize = tempDefinitions[i].PreferredSize;
+                            double preferredSize = tempDefinition.PreferredSize;
                             double newMinSize = preferredSize + sizeToDistribute / (count - autoDefinitionsCount - i);
-                            tempDefinitions[i].UpdateMinSize(Math.Min(newMinSize, tempDefinitions[i].SizeCache));
-                            sizeToDistribute -= (tempDefinitions[i].MinSize - preferredSize);
+                            tempDefinition.UpdateMinSize(Math.Min(newMinSize, tempDefinition.SizeCache));
+                            sizeToDistribute -= (tempDefinition.MinSize - preferredSize);
                         }
 
                         for (; i < count; ++i)
                         {
+                            var tempDefinition = tempDefinitions[i]!;
+
                             //  sanity check: only auto definitions allowed in this loop
-                            Debug.Assert(tempDefinitions[i].UserSize.IsAuto);
+                            Debug.Assert(tempDefinition.UserSize.IsAuto);
 
-                            double preferredSize = tempDefinitions[i].MinSize;
+                            double preferredSize = tempDefinition.MinSize;
                             double newMinSize = preferredSize + sizeToDistribute / (count - i);
-                            tempDefinitions[i].UpdateMinSize(Math.Min(newMinSize, tempDefinitions[i].SizeCache));
-                            sizeToDistribute -= (tempDefinitions[i].MinSize - preferredSize);
+                            tempDefinition.UpdateMinSize(Math.Min(newMinSize, tempDefinition.SizeCache));
+                            sizeToDistribute -= (tempDefinition.MinSize - preferredSize);
                         }
 
                         //  sanity check: requested size must all be distributed
@@ -1376,8 +1367,10 @@ namespace Avalonia.Controls
 
                             for (int i = 0; i < count; ++i)
                             {
-                                double deltaSize = (maxMaxSize - tempDefinitions[i].SizeCache) * sizeToDistribute / totalRemainingSize;
-                                tempDefinitions[i].UpdateMinSize(tempDefinitions[i].SizeCache + deltaSize);
+                                var tempDefinition = tempDefinitions[i]!;
+
+                                double deltaSize = (maxMaxSize - tempDefinition.SizeCache) * sizeToDistribute / totalRemainingSize;
+                                tempDefinition.UpdateMinSize(tempDefinition.SizeCache + deltaSize);
                             }
                         }
                         else
@@ -1388,7 +1381,7 @@ namespace Avalonia.Controls
                             //
                             for (int i = 0; i < count; ++i)
                             {
-                                tempDefinitions[i].UpdateMinSize(equalSize);
+                                tempDefinitions[i]!.UpdateMinSize(equalSize);
                             }
                         }
                     }
@@ -1429,7 +1422,7 @@ namespace Avalonia.Controls
             double availableSize)
         {
             int defCount = definitions.Count;
-            DefinitionBase[] tempDefinitions = TempDefinitions;
+            DefinitionBase?[] tempDefinitions = TempDefinitions;
             int minCount = 0, maxCount = 0;
             double takenSize = 0;
             double totalStarWeight = 0.0;
@@ -1560,8 +1553,8 @@ namespace Avalonia.Controls
                         remainingStarWeight = totalStarWeight - takenStarWeight;
                     }
 
-                    double minRatio = (minCount > 0) ? tempDefinitions[minCount - 1].MeasureSize : Double.PositiveInfinity;
-                    double maxRatio = (maxCount > 0) ? tempDefinitions[defCount + maxCount - 1].SizeCache : -1.0;
+                    double minRatio = (minCount > 0) ? tempDefinitions[minCount - 1]!.MeasureSize : Double.PositiveInfinity;
+                    double maxRatio = (maxCount > 0) ? tempDefinitions[defCount + maxCount - 1]!.SizeCache : -1.0;
 
                     // choose the def with larger ratio to the current proportion ("max discrepancy")
                     double proportion = remainingStarWeight / remainingAvailableSize;
@@ -1579,13 +1572,13 @@ namespace Avalonia.Controls
                     double resolvedSize;
                     if (chooseMin == true)
                     {
-                        resolvedDef = tempDefinitions[minCount - 1];
+                        resolvedDef = tempDefinitions[minCount - 1]!;
                         resolvedSize = resolvedDef.MinSize;
                         --minCount;
                     }
                     else
                     {
-                        resolvedDef = tempDefinitions[defCount + maxCount - 1];
+                        resolvedDef = tempDefinitions[defCount + maxCount - 1]!;
                         resolvedSize = Math.Max(resolvedDef.MinSize, resolvedDef.UserMaxSize);
                         --maxCount;
                     }
@@ -1603,12 +1596,12 @@ namespace Avalonia.Controls
 
                     // advance to the next candidate defs, removing ones that have been resolved.
                     // Both counts are advanced, as a def might appear in both lists.
-                    while (minCount > 0 && tempDefinitions[minCount - 1].MeasureSize < 0.0)
+                    while (minCount > 0 && tempDefinitions[minCount - 1]!.MeasureSize < 0.0)
                     {
                         --minCount;
                         tempDefinitions[minCount] = null!;
                     }
-                    while (maxCount > 0 && tempDefinitions[defCount + maxCount - 1].MeasureSize < 0.0)
+                    while (maxCount > 0 && tempDefinitions[defCount + maxCount - 1]!.MeasureSize < 0.0)
                     {
                         --maxCount;
                         tempDefinitions[defCount + maxCount] = null!;
@@ -1637,8 +1630,7 @@ namespace Avalonia.Controls
                     // resolved as 'min'.   Their allocation can be increased to make up the gap.
                     for (int i = minCount; i < minCountPhase2; ++i)
                     {
-                        DefinitionBase def = tempDefinitions[i];
-                        if (def != null)
+                        if (tempDefinitions[i] is { } def)
                         {
                             def.MeasureSize = 1.0;      // mark as 'not yet resolved'
                             ++starCount;
@@ -1653,8 +1645,7 @@ namespace Avalonia.Controls
                     // resolved as 'max'.   Their allocation can be decreased to make up the gap.
                     for (int i = maxCount; i < maxCountPhase2; ++i)
                     {
-                        DefinitionBase def = tempDefinitions[defCount + i];
-                        if (def != null)
+                        if (tempDefinitions[defCount + i] is { } def)
                         {
                             def.MeasureSize = 1.0;      // mark as 'not yet resolved'
                             ++starCount;
@@ -1695,7 +1686,7 @@ namespace Avalonia.Controls
                 totalStarWeight = 0.0;
                 for (int i = 0; i < starCount; ++i)
                 {
-                    DefinitionBase def = tempDefinitions[i];
+                    DefinitionBase def = tempDefinitions[i]!;
                     totalStarWeight += def.MeasureSize;
                     def.SizeCache = totalStarWeight;
                 }
@@ -1703,7 +1694,7 @@ namespace Avalonia.Controls
                 // resolve the defs, in decreasing order of weight
                 for (int i = starCount - 1; i >= 0; --i)
                 {
-                    DefinitionBase def = tempDefinitions[i];
+                    DefinitionBase def = tempDefinitions[i]!;
                     double resolvedSize = (def.MeasureSize > 0.0) ? Math.Max(availableSize - takenSize, 0.0) * (def.MeasureSize / def.SizeCache) : 0.0;
 
                     // min and max should have no effect by now, but just in case...
@@ -2095,7 +2086,7 @@ namespace Avalonia.Controls
             {
                 // DpiScale dpiScale = GetDpi();
                 // double dpi = columns ? dpiScale.DpiScaleX : dpiScale.DpiScaleY;
-                var dpi = (VisualRoot as Layout.ILayoutRoot)?.LayoutScaling ?? 1.0;
+                var dpi = (VisualRoot as ILayoutRoot)?.LayoutScaling ?? 1.0;
                 double[] roundingErrors = RoundingErrors;
                 double roundedTakenSize = 0.0;
 
@@ -2302,8 +2293,7 @@ namespace Avalonia.Controls
         /// </summary>
         private void SetValid()
         {
-            ExtendedData extData = ExtData;
-            if (extData != null)
+            if (_extData is { } extData)
             {
                 //                for (int i = 0; i < PrivateColumnCount; ++i) DefinitionsU[i].SetValid ();
                 //                for (int i = 0; i < PrivateRowCount; ++i) DefinitionsV[i].SetValid ();
@@ -2330,12 +2320,12 @@ namespace Avalonia.Controls
             if (ShowGridLines && (_gridLinesRenderer == null))
             {
                 _gridLinesRenderer = new GridLinesRenderer();
-                this.VisualChildren.Add(_gridLinesRenderer);
+                VisualChildren.Add(_gridLinesRenderer);
             }
 
             if ((!ShowGridLines) && (_gridLinesRenderer != null))
             {
-                this.VisualChildren.Add(_gridLinesRenderer);
+                VisualChildren.Add(_gridLinesRenderer);
                 _gridLinesRenderer = null;
             }
 
@@ -2364,7 +2354,7 @@ namespace Avalonia.Controls
         {
             Grid grid = (Grid)d;
 
-            if (grid.ExtData != null    // trivial grid is 1 by 1. there is no grid lines anyway
+            if (grid._extData != null    // trivial grid is 1 by 1. there is no grid lines anyway
                 && grid.ListenToNotifications)
             {
                 grid.InvalidateVisual();
@@ -2375,13 +2365,11 @@ namespace Avalonia.Controls
 
         private static void OnCellAttachedPropertyChanged(AvaloniaObject d, AvaloniaPropertyChangedEventArgs e)
         {
-            Visual? child = d as Visual;
-
-            if (child != null)
+            if (d is Visual child)
             {
                 Grid? grid = child.GetVisualParent<Grid>();
                 if (grid != null
-                    && grid.ExtData != null
+                    && grid._extData != null
                     && grid.ListenToNotifications)
                 {
                     grid.CellsStructureDirty = true;
@@ -2427,7 +2415,7 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsU
         {
-            get { return (ExtData.DefinitionsU!); }
+            get { return _extData!.DefinitionsU!; }
         }
 
         /// <summary>
@@ -2435,17 +2423,19 @@ namespace Avalonia.Controls
         /// </summary>
         private IReadOnlyList<DefinitionBase> DefinitionsV
         {
-            get { return (ExtData.DefinitionsV!); }
+            get { return _extData!.DefinitionsV!; }
         }
 
         /// <summary>
         /// Helper accessor to layout time array of definitions.
         /// </summary>
-        private DefinitionBase[] TempDefinitions
+        private DefinitionBase?[] TempDefinitions
         {
             get
             {
-                ExtendedData extData = ExtData;
+                Debug.Assert(_extData is not null);
+
+                var extData = _extData!;
                 int requiredLength = Math.Max(DefinitionsU.Count, DefinitionsV.Count) * 2;
 
                 if (extData.TempDefinitions == null
@@ -2516,7 +2506,7 @@ namespace Avalonia.Controls
         /// </summary>
         private CellCache[] PrivateCells
         {
-            get { return (ExtData.CellCachesCollection!); }
+            get { return _extData!.CellCachesCollection!; }
         }
 
         /// <summary>
@@ -2582,18 +2572,10 @@ namespace Avalonia.Controls
             set { SetFlags(value, Flags.HasGroup3CellsInAutoRows); }
         }
 
-        /// <summary>
-        /// Returns reference to extended data bag.
-        /// </summary>
-        private ExtendedData ExtData
-        {
-            get { return (_data!); }
-        }
-
         /// <summary>
         /// Returns *-weight, adjusted for scale computed during Phase 1
         /// </summary>
-        static double StarWeight(DefinitionBase def, double scale)
+        private static double StarWeight(DefinitionBase def, double scale)
         {
             if (scale < 0.0)
             {
@@ -2609,17 +2591,17 @@ namespace Avalonia.Controls
         }
 
         // Extended data instantiated on demand, for non-trivial case handling only
-        private ExtendedData? _data;
+        private ExtendedData? _extData;
 
         // Grid validity / property caches dirtiness flags
         private Flags _flags;
         private GridLinesRenderer? _gridLinesRenderer;
 
         // Keeps track of definition indices.
-        int[]? _definitionIndices;
+        private int[]? _definitionIndices;
 
         // Stores unrounded values and rounding errors during layout rounding.
-        double[]? _roundingErrors;
+        private double[]? _roundingErrors;
 
         // 5 is an arbitrary constant chosen to end the measure loop
         private const int c_layoutLoopMaxCount = 5;
@@ -2645,14 +2627,14 @@ namespace Avalonia.Controls
             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)
         }
 
         /// <summary>
         /// Grid validity / property caches dirtiness flags
         /// </summary>
-        [System.Flags]
+        [Flags]
         private enum Flags
         {
             //
@@ -2768,7 +2750,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// LayoutTimeSizeType is used internally and reflects layout-time size type.
         /// </summary>
-        [System.Flags]
+        [Flags]
         internal enum LayoutTimeSizeType : byte
         {
             None = 0x00,
@@ -3317,7 +3299,7 @@ namespace Avalonia.Controls
             internal void UpdateRenderBounds(Size arrangeSize)
             {
                 _lastArrangeSize = arrangeSize;
-                this.InvalidateVisual();
+                InvalidateVisual();
             }
 
             private static Size _lastArrangeSize;

src/Avalonia.Controls/Image.cs

@@ -14,8 +14,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Source"/> property.
         /// </summary>
-        public static readonly StyledProperty<IImage> SourceProperty =
-            AvaloniaProperty.Register<Image, IImage>(nameof(Source));
+        public static readonly StyledProperty<IImage?> SourceProperty =
+            AvaloniaProperty.Register<Image, IImage?>(nameof(Source));
 
         /// <summary>
         /// Defines the <see cref="Stretch"/> property.
@@ -42,7 +42,7 @@ namespace Avalonia.Controls
         /// Gets or sets the image that will be displayed.
         /// </summary>
         [Content]
-        public IImage Source
+        public IImage? Source
         {
             get { return GetValue(SourceProperty); }
             set { SetValue(SourceProperty, value); }
@@ -66,6 +66,7 @@ namespace Avalonia.Controls
             set { SetValue(StretchDirectionProperty, value); }
         }
 
+        /// <inheritdoc />
         protected override bool BypassFlowDirectionPolicies => true;
         
         /// <summary>

src/Avalonia.Controls/ItemsControl.cs

@@ -2,7 +2,6 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.Diagnostics.CodeAnalysis;
 using Avalonia.Automation.Peers;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
@@ -17,7 +16,6 @@ using Avalonia.Layout;
 using Avalonia.LogicalTree;
 using Avalonia.Metadata;
 using Avalonia.Styling;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls
 {
@@ -106,7 +104,6 @@ namespace Avalonia.Controls
         private Tuple<int, Control>? _containerBeingPrepared;
         private ScrollViewer? _scrollViewer;
         private ItemsPresenter? _itemsPresenter;
-        private IScrollSnapPointsInfo? _scrolSnapPointInfo;
 
         /// <summary>
         /// Initializes a new instance of the <see cref="ItemsControl"/> class.
@@ -221,6 +218,7 @@ namespace Avalonia.Controls
         }
 
 
+        /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> HorizontalSnapPointsChanged
         {
             add
@@ -240,6 +238,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         public event EventHandler<RoutedEventArgs> VerticalSnapPointsChanged
         {
             add
@@ -424,13 +423,12 @@ namespace Avalonia.Controls
         /// <returns>true if the item is (or is eligible to be) its own container; otherwise, false.</returns>
         protected internal virtual bool IsItemItsOwnContainerOverride(Control item) => true;
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
             _scrollViewer = e.NameScope.Find<ScrollViewer>("PART_ScrollViewer");
             _itemsPresenter = e.NameScope.Find<ItemsPresenter>("PART_ItemsPresenter");
-
-            _scrolSnapPointInfo = _itemsPresenter as IScrollSnapPointsInfo;
         }
 
         /// <summary>
@@ -477,11 +475,13 @@ namespace Avalonia.Controls
             base.OnKeyDown(e);
         }
 
+        /// <inheritdoc />
         protected override AutomationPeer OnCreateAutomationPeer()
         {
             return new ItemsControlAutomationPeer(this);
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -748,11 +748,13 @@ namespace Avalonia.Controls
             return true;
         }
 
+        /// <inheritdoc />
         public IReadOnlyList<double> GetIrregularSnapPoints(Orientation orientation, SnapPointsAlignment snapPointsAlignment)
         {
             return _itemsPresenter?.GetIrregularSnapPoints(orientation, snapPointsAlignment) ?? new List<double>();
         }
 
+        /// <inheritdoc />
         public double GetRegularSnapPoints(Orientation orientation, SnapPointsAlignment snapPointsAlignment, out double offset)
         {
             offset = 0;

src/Avalonia.Controls/LayoutTransformControl.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Controls
                 .AddClassHandler<LayoutTransformControl>((x, e) => x.OnLayoutTransformChanged(e));
 
             ChildProperty.Changed
-                .AddClassHandler<LayoutTransformControl>((x, e) => x.OnChildChanged(e));
+                .AddClassHandler<LayoutTransformControl>((x, _) => x.OnChildChanged());
 
             UseRenderTransformProperty.Changed
                 .AddClassHandler<LayoutTransformControl>((x, e) => x.OnUseRenderTransformPropertyChanged(e));
@@ -146,7 +146,7 @@ namespace Avalonia.Controls
             return transformedDesiredSize;
         }
 
-        IDisposable? _renderTransformChangedEvent;
+        private IDisposable? _renderTransformChangedEvent;
 
         private void OnUseRenderTransformPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
@@ -167,8 +167,7 @@ namespace Avalonia.Controls
                             .Subscribe(
                                 (x) =>
                                 {
-                                    var target2 = x.Sender as LayoutTransformControl;
-                                    if (target2 != null)
+                                    if (x.Sender is LayoutTransformControl target2)
                                     {
                                         target2.LayoutTransform = target2.RenderTransform;
                                     }
@@ -182,7 +181,7 @@ namespace Avalonia.Controls
             }
         }
 
-        private void OnChildChanged(AvaloniaPropertyChangedEventArgs e)
+        private void OnChildChanged()
         {
             if (null != TransformRoot)
             {
@@ -206,18 +205,18 @@ namespace Avalonia.Controls
         /// <summary>
         /// Actual DesiredSize of Child element (the value it returned from its MeasureOverride method).
         /// </summary>
-        private Size _childActualSize = default;
+        private Size _childActualSize;
 
         /// <summary>
         /// RenderTransform/MatrixTransform applied to TransformRoot.
         /// </summary>
-        private MatrixTransform _matrixTransform = new MatrixTransform();
+        private readonly MatrixTransform _matrixTransform = new();
 
         /// <summary>
         /// Transformation matrix corresponding to _matrixTransform.
         /// </summary>
         private Matrix _transformation;
-        private IDisposable? _transformChangedEvent = null;
+        private IDisposable? _transformChangedEvent;
 
         /// <summary>
         /// Returns true if Size a is smaller than Size b in either dimension.
@@ -263,10 +262,7 @@ namespace Avalonia.Controls
             // Get the transform matrix and apply it
             _transformation = RoundMatrix(LayoutTransform.Value, DecimalsAfterRound);
 
-            if (null != _matrixTransform)
-            {
-                _matrixTransform.Matrix = _transformation;
-            }
+            _matrixTransform.Matrix = _transformation;
 
             // New transform means re-layout is necessary
             InvalidateMeasure();

src/Avalonia.Controls/MaskedTextBox.cs

@@ -178,12 +178,11 @@ namespace Avalonia.Controls
                 }
 
             }
-
-
         }
 
         Type IStyleable.StyleKey => typeof(TextBox);
 
+        /// <inheritdoc />
         protected override void OnGotFocus(GotFocusEventArgs e)
         {
             if (HidePromptOnLeave == true && MaskProvider != null)
@@ -193,6 +192,7 @@ namespace Avalonia.Controls
             base.OnGotFocus(e);
         }
 
+        /// <inheritdoc />
         protected override async void OnKeyDown(KeyEventArgs e)
         {
             if (MaskProvider == null)
@@ -271,15 +271,17 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override void OnLostFocus(RoutedEventArgs e)
         {
-            if (HidePromptOnLeave == true && MaskProvider != null)
+            if (HidePromptOnLeave && MaskProvider != null)
             {
                 Text = MaskProvider.ToString(!HidePromptOnLeave, true);
             }
             base.OnLostFocus(e);
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             void UpdateMaskProvider()
@@ -357,6 +359,8 @@ namespace Avalonia.Controls
             }
             base.OnPropertyChanged(change);
         }
+
+        /// <inheritdoc />
         protected override void OnTextInput(TextInputEventArgs e)
         {
             _ignoreTextChanges = true;
@@ -423,7 +427,7 @@ namespace Avalonia.Controls
             return startPosition;
         }
 
-        private void RefreshText(MaskedTextProvider provider, int position)
+        private void RefreshText(MaskedTextProvider? provider, int position)
         {
             if (provider != null)
             {

src/Avalonia.Controls/NativeControlHost.cs

@@ -16,19 +16,17 @@ namespace Avalonia.Controls
         private IPlatformHandle? _nativeControlHandle;
         private bool _queuedForDestruction;
         private bool _queuedForMoveResize;
-        private readonly List<Visual> _propertyChangedSubscriptions = new List<Visual>();
+        private readonly List<Visual> _propertyChangedSubscriptions = new();
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             _currentRoot = e.Root as TopLevel;
             var visual = (Visual)this;
             while (visual != null)
             {
-                if (visual is Visual v)
-                {
-                    v.PropertyChanged += PropertyChangedHandler;
-                    _propertyChangedSubscriptions.Add(v);
-                }
+                visual.PropertyChanged += PropertyChangedHandler;
+                _propertyChangedSubscriptions.Add(visual);
 
                 visual = visual.GetVisualParent();
             }
@@ -42,15 +40,13 @@ namespace Avalonia.Controls
                 EnqueueForMoveResize();
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromVisualTree(VisualTreeAttachmentEventArgs e)
         {
             _currentRoot = null;
-            if (_propertyChangedSubscriptions != null)
-            {
-                foreach (var v in _propertyChangedSubscriptions)
-                    v.PropertyChanged -= PropertyChangedHandler;
-                _propertyChangedSubscriptions.Clear();
-            }
+            foreach (var v in _propertyChangedSubscriptions)
+                v.PropertyChanged -= PropertyChangedHandler;
+            _propertyChangedSubscriptions.Clear();
             UpdateHost();
         }
 
@@ -128,7 +124,7 @@ namespace Avalonia.Controls
             return new Rect(position.Value, bounds.Size);
         }
 
-        void EnqueueForMoveResize()
+        private void EnqueueForMoveResize()
         {
             if(_queuedForMoveResize)
                 return;

src/Avalonia.Controls/NativeMenu.Export.cs

@@ -12,10 +12,10 @@ namespace Avalonia.Controls
 
         public static bool GetIsNativeMenuExported(TopLevel tl) => tl.GetValue(IsNativeMenuExportedProperty);
         
-        private static readonly AttachedProperty<NativeMenuInfo> s_nativeMenuInfoProperty =
-            AvaloniaProperty.RegisterAttached<NativeMenu, TopLevel, NativeMenuInfo>("___NativeMenuInfo");
-        
-        class NativeMenuInfo
+        private static readonly AttachedProperty<NativeMenuInfo?> s_nativeMenuInfoProperty =
+            AvaloniaProperty.RegisterAttached<NativeMenu, TopLevel, NativeMenuInfo?>("___NativeMenuInfo");
+
+        private sealed class NativeMenuInfo
         {
             public bool ChangingIsExported { get; set; }
             public ITopLevelNativeMenuExporter? Exporter { get; }
@@ -33,7 +33,7 @@ namespace Avalonia.Controls
             }
         }
 
-        static NativeMenuInfo GetInfo(TopLevel target)
+        private static NativeMenuInfo GetInfo(TopLevel target)
         {
             var rv = target.GetValue(s_nativeMenuInfoProperty);
             if (rv == null)
@@ -45,18 +45,18 @@ namespace Avalonia.Controls
             return rv;
         }
 
-        static void SetIsNativeMenuExported(TopLevel tl, bool value)
+        private static void SetIsNativeMenuExported(TopLevel tl, bool value)
         {
             GetInfo(tl).ChangingIsExported = true;
             tl.SetValue(IsNativeMenuExportedProperty, value);
         }
 
-        public static readonly AttachedProperty<NativeMenu> MenuProperty
-            = AvaloniaProperty.RegisterAttached<NativeMenu, AvaloniaObject, NativeMenu>("Menu");
+        public static readonly AttachedProperty<NativeMenu?> MenuProperty
+            = AvaloniaProperty.RegisterAttached<NativeMenu, AvaloniaObject, NativeMenu?>("Menu");
 
-        public static void SetMenu(AvaloniaObject o, NativeMenu menu) => o.SetValue(MenuProperty, menu);
+        public static void SetMenu(AvaloniaObject o, NativeMenu? menu) => o.SetValue(MenuProperty, menu);
 
-        public static NativeMenu GetMenu(AvaloniaObject o) => o.GetValue(MenuProperty);
+        public static NativeMenu? GetMenu(AvaloniaObject o) => o.GetValue(MenuProperty);
         
         static NativeMenu()
         {

src/Avalonia.Controls/Primitives/AdornerLayer.cs

@@ -34,8 +34,8 @@ namespace Avalonia.Controls.Primitives
         public static readonly AttachedProperty<Control?> AdornerProperty =
             AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, Control?>("Adorner");
 
-        private static readonly AttachedProperty<AdornedElementInfo> s_adornedElementInfoProperty =
-            AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, AdornedElementInfo>("AdornedElementInfo");
+        private static readonly AttachedProperty<AdornedElementInfo?> s_adornedElementInfoProperty =
+            AvaloniaProperty.RegisterAttached<AdornerLayer, Visual, AdornedElementInfo?>("AdornedElementInfo");
 
         private static readonly AttachedProperty<AdornerLayer?> s_savedAdornerLayerProperty =
             AvaloniaProperty.RegisterAttached<Visual, Visual, AdornerLayer?>("SavedAdornerLayer");
@@ -159,8 +159,8 @@ namespace Avalonia.Controls.Primitives
                 return;
             }
 
-            AdornerLayer.SetAdornedElement(adorner, visual);
-            AdornerLayer.SetIsClipEnabled(adorner, false);
+            SetAdornedElement(adorner, visual);
+            SetIsClipEnabled(adorner, false);
 
             ((ISetLogicalParent) adorner).SetParent(visual);
             layer.Children.Add(adorner);
@@ -177,6 +177,7 @@ namespace Avalonia.Controls.Primitives
             ((ISetLogicalParent) adorner).SetParent(null);
         }
 
+        /// <inheritdoc />
         protected override Size MeasureOverride(Size availableSize)
         {
             foreach (var child in Children)
@@ -199,6 +200,7 @@ namespace Avalonia.Controls.Primitives
             return default;
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             foreach (var child in Children)
@@ -217,7 +219,7 @@ namespace Avalonia.Controls.Primitives
                     }
                     else
                     {
-                        ArrangeChild((Control) child, finalSize);
+                        ArrangeChild(child, finalSize);
                     }
                 }
             }

src/Avalonia.Controls/Primitives/Popup.cs

@@ -120,7 +120,7 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<bool> TopmostProperty =
             AvaloniaProperty.Register<Popup, bool>(nameof(Topmost));
 
-        private bool _isOpenRequested = false;
+        private bool _isOpenRequested;
         private bool _isOpen;
         private bool _ignoreIsOpenChanged;
         private PopupOpenState? _openState;
@@ -377,9 +377,9 @@ namespace Avalonia.Controls.Primitives
             popupHost.SetChild(Child);
             ((ISetLogicalParent)popupHost).SetParent(this);
 
-            if (InheritsTransform && placementTarget is Control c)
+            if (InheritsTransform)
             {
-                TransformTrackingHelper.Track(c, PlacementTargetTransformChanged)
+                TransformTrackingHelper.Track(placementTarget, PlacementTargetTransformChanged)
                     .DisposeWith(handlerCleanup);
             }
             else
@@ -518,6 +518,7 @@ namespace Avalonia.Controls.Primitives
             Close();
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -579,7 +580,7 @@ namespace Avalonia.Controls.Primitives
             var scaleX = 1.0;
             var scaleY = 1.0;
 
-            if (InheritsTransform && placementTarget.TransformToVisual(topLevel) is Matrix m)
+            if (InheritsTransform && placementTarget.TransformToVisual(topLevel) is { } m)
             {
                 scaleX = Math.Sqrt(m.M11 * m.M11 + m.M12 * m.M12);
                 scaleY = Math.Sqrt(m.M11 * m.M11 + m.M12 * m.M12);
@@ -623,6 +624,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override AutomationPeer OnCreateAutomationPeer()
         {
             return new PopupAutomationPeer(this);
@@ -850,7 +852,7 @@ namespace Avalonia.Controls.Primitives
 
             var popupHost = _openState.PopupHost;
 
-            return popupHost != null && ((Visual)popupHost).IsVisualAncestorOf(visual);
+            return ((Visual)popupHost).IsVisualAncestorOf(visual);
         }
 
         public bool IsPointerOverPopup => ((IInputElement?)_openState?.PopupHost)?.IsPointerOver ?? false;

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -3,16 +3,14 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-using System.Xml.Linq;
-using Avalonia.Controls.Generators;
 using Avalonia.Controls.Selection;
 using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
 using Avalonia.Interactivity;
 using Avalonia.Threading;
-using Avalonia.VisualTree;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -255,6 +253,7 @@ namespace Avalonia.Controls.Primitives
         /// <summary>
         /// Gets or sets the model that holds the current selection.
         /// </summary>
+        [AllowNull]
         protected ISelectionModel Selection
         {
             get
@@ -399,6 +398,7 @@ namespace Avalonia.Controls.Primitives
             return null;
         }
 
+        /// <inheritdoc />
         protected override void ItemsCollectionChanged(object? sender, NotifyCollectionChangedEventArgs e)
         {
             base.ItemsCollectionChanged(sender!, e);
@@ -409,12 +409,14 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToVisualTree(VisualTreeAttachmentEventArgs e)
         {
             base.OnAttachedToVisualTree(e);
             AutoScrollToSelectedItemIfNecessary();
         }
 
+        /// <inheritdoc />
         protected override void OnApplyTemplate(TemplateAppliedEventArgs e)
         {
             base.OnApplyTemplate(e);
@@ -431,6 +433,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected internal override void PrepareContainerForItemOverride(Control element, object? item, int index)
         {
             base.PrepareContainerForItemOverride(element, item, index);
@@ -447,12 +450,14 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void ContainerIndexChangedOverride(Control container, int oldIndex, int newIndex)
         {
             base.ContainerIndexChangedOverride(container, oldIndex, newIndex);
             MarkContainerSelected(container, Selection.IsSelected(newIndex));
         }
 
+        /// <inheritdoc />
         protected internal override void ClearContainerForItemOverride(Control element)
         {
             base.ClearContainerForItemOverride(element);
@@ -463,7 +468,7 @@ namespace Avalonia.Controls.Primitives
                 KeyboardNavigation.SetTabOnceActiveElement(panel, null);
             }
 
-            if (element is ISelectable selectable)
+            if (element is ISelectable)
                 MarkContainerSelected(element, false);
         }
 
@@ -498,7 +503,8 @@ namespace Avalonia.Controls.Primitives
                 DataValidationErrors.SetError(this, error);
             }
         }
-        
+
+        /// <inheritdoc />
         protected override void OnInitialized()
         {
             base.OnInitialized();
@@ -509,6 +515,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnTextInput(TextInputEventArgs e)
         {
             if (!e.Handled)
@@ -551,6 +558,7 @@ namespace Avalonia.Controls.Primitives
             base.OnTextInput(e);
         }
 
+        /// <inheritdoc />
         protected override void OnKeyDown(KeyEventArgs e)
         {
             base.OnKeyDown(e);
@@ -582,6 +590,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
@@ -592,7 +601,7 @@ namespace Avalonia.Controls.Primitives
             }
             if (change.Property == ItemsProperty && _updateState is null && _selection is object)
             {
-                var newValue = change.GetNewValue<IEnumerable>();
+                var newValue = change.GetNewValue<IEnumerable?>();
                 _selection.Source = newValue;
 
                 if (newValue is null)
@@ -695,7 +704,7 @@ namespace Avalonia.Controls.Primitives
             {
                 if (multi)
                 {
-                    if (Selection.IsSelected(index) == true)
+                    if (Selection.IsSelected(index))
                     {
                         Selection.Deselect(index);
                     }
@@ -716,12 +725,10 @@ namespace Avalonia.Controls.Primitives
                 Selection.Select(index);
             }
 
-            if (Presenter?.Panel != null)
+            if (Presenter?.Panel is { } panel)
             {
                 var container = ContainerFromIndex(index);
-                KeyboardNavigation.SetTabOnceActiveElement(
-                    (InputElement)Presenter.Panel,
-                    container);
+                KeyboardNavigation.SetTabOnceActiveElement(panel, container);
             }
         }
 
@@ -940,7 +947,7 @@ namespace Avalonia.Controls.Primitives
 
         private void UpdateContainerSelection()
         {
-            if (Presenter?.Panel is Panel panel)
+            if (Presenter?.Panel is { } panel)
             {
                 foreach (var container in panel.Children)
                 {

src/Avalonia.Controls/Primitives/TemplatedControl.cs

@@ -290,12 +290,6 @@ namespace Avalonia.Controls.Primitives
                     ApplyTemplatedParent(child, this);
                     ((ISetLogicalParent)child).SetParent(this);
                     VisualChildren.Add(child);
-                    
-                    // Existing code kinda expect to see a NameScope even if it's empty
-                    if (nameScope == null)
-                    {
-                        nameScope = new NameScope();
-                    }
 
                     var e = new TemplateAppliedEventArgs(nameScope);
                     OnApplyTemplate(e);
@@ -320,6 +314,7 @@ namespace Avalonia.Controls.Primitives
             return this;
         }
 
+        /// <inheritdoc />
         protected sealed override void NotifyChildResourcesChanged(ResourcesChangedEventArgs e)
         {
             var count = VisualChildren.Count;

src/Avalonia.Controls/Primitives/TextSearch.cs

@@ -11,15 +11,15 @@ namespace Avalonia.Controls.Primitives
         /// Defines the Text attached property.
         /// This text will be considered during text search in <see cref="SelectingItemsControl"/> (such as <see cref="ComboBox"/>)
         /// </summary>
-        public static readonly AttachedProperty<string> TextProperty
-            = AvaloniaProperty.RegisterAttached<Interactive, string>("Text", typeof(TextSearch));
+        public static readonly AttachedProperty<string?> TextProperty
+            = AvaloniaProperty.RegisterAttached<Interactive, string?>("Text", typeof(TextSearch));
 
         /// <summary>
         /// Sets the <see cref="TextProperty"/> for a control.
         /// </summary>
         /// <param name="control">The control</param>
         /// <param name="text">The search text to set</param>
-        public static void SetText(Control control, string text)
+        public static void SetText(Control control, string? text)
         {
             control.SetValue(TextProperty, text);
         }
@@ -29,7 +29,7 @@ namespace Avalonia.Controls.Primitives
         /// </summary>
         /// <param name="control">The control</param>
         /// <returns>The property value</returns>
-        public static string GetText(Control control)
+        public static string? GetText(Control control)
         {
             return control.GetValue(TextProperty);
         }

src/Avalonia.Controls/Primitives/Track.cs

@@ -5,7 +5,6 @@
 
 using System;
 using Avalonia.Controls.Metadata;
-using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Layout;
 using Avalonia.Metadata;
@@ -31,14 +30,14 @@ namespace Avalonia.Controls.Primitives
         public static readonly StyledProperty<Orientation> OrientationProperty =
             ScrollBar.OrientationProperty.AddOwner<Track>();
 
-        public static readonly StyledProperty<Thumb> ThumbProperty =
-            AvaloniaProperty.Register<Track, Thumb>(nameof(Thumb));
+        public static readonly StyledProperty<Thumb?> ThumbProperty =
+            AvaloniaProperty.Register<Track, Thumb?>(nameof(Thumb));
 
-        public static readonly StyledProperty<Button> IncreaseButtonProperty =
-            AvaloniaProperty.Register<Track, Button>(nameof(IncreaseButton));
+        public static readonly StyledProperty<Button?> IncreaseButtonProperty =
+            AvaloniaProperty.Register<Track, Button?>(nameof(IncreaseButton));
 
-        public static readonly StyledProperty<Button> DecreaseButtonProperty =
-            AvaloniaProperty.Register<Track, Button>(nameof(DecreaseButton));
+        public static readonly StyledProperty<Button?> DecreaseButtonProperty =
+            AvaloniaProperty.Register<Track, Button?>(nameof(DecreaseButton));
 
         public static readonly StyledProperty<bool> IsDirectionReversedProperty =
             AvaloniaProperty.Register<Track, bool>(nameof(IsDirectionReversed));
@@ -94,19 +93,19 @@ namespace Avalonia.Controls.Primitives
         }
 
         [Content]
-        public Thumb Thumb
+        public Thumb? Thumb
         {
             get { return GetValue(ThumbProperty); }
             set { SetValue(ThumbProperty, value); }
         }
 
-        public Button IncreaseButton
+        public Button? IncreaseButton
         {
             get { return GetValue(IncreaseButtonProperty); }
             set { SetValue(IncreaseButtonProperty, value); }
         }
 
-        public Button DecreaseButton
+        public Button? DecreaseButton
         {
             get { return GetValue(DecreaseButtonProperty); }
             set { SetValue(DecreaseButtonProperty, value); }

src/Avalonia.Controls/Primitives/VisualLayerManager.cs

@@ -1,6 +1,5 @@
 using System.Collections.Generic;
 using Avalonia.LogicalTree;
-using Avalonia.Media;
 
 namespace Avalonia.Controls.Primitives
 {
@@ -12,10 +11,10 @@ namespace Avalonia.Controls.Primitives
         private const int OverlayZIndex = int.MaxValue - 97;
 
         private ILogicalRoot? _logicalRoot;
-        private readonly List<Control> _layers = new List<Control>();
+        private readonly List<Control> _layers = new();
 
-        public static readonly StyledProperty<ChromeOverlayLayer> ChromeOverlayLayerProperty =
-            AvaloniaProperty.Register<VisualLayerManager, ChromeOverlayLayer>(nameof(ChromeOverlayLayer));
+        public static readonly StyledProperty<ChromeOverlayLayer?> ChromeOverlayLayerProperty =
+            AvaloniaProperty.Register<VisualLayerManager, ChromeOverlayLayer?>(nameof(ChromeOverlayLayer));
 
         public bool IsPopup { get; set; }
 
@@ -81,7 +80,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        T? FindLayer<T>() where T : class
+        private T? FindLayer<T>() where T : class
         {
             foreach (var layer in _layers)
                 if (layer is T match)
@@ -89,7 +88,7 @@ namespace Avalonia.Controls.Primitives
             return null;
         }
 
-        void AddLayer(Control layer, int zindex)
+        private void AddLayer(Control layer, int zindex)
         {
             _layers.Add(layer);
             ((ISetLogicalParent)layer).SetParent(this);
@@ -101,6 +100,7 @@ namespace Avalonia.Controls.Primitives
             InvalidateArrange();
         }
 
+        /// <inheritdoc />
         protected override void NotifyChildResourcesChanged(ResourcesChangedEventArgs e)
         {
             foreach (var l in _layers)
@@ -109,6 +109,7 @@ namespace Avalonia.Controls.Primitives
             base.NotifyChildResourcesChanged(e);
         }
 
+        /// <inheritdoc />
         protected override void OnAttachedToLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
             base.OnAttachedToLogicalTree(e);
@@ -118,6 +119,7 @@ namespace Avalonia.Controls.Primitives
                 ((ILogical)l).NotifyAttachedToLogicalTree(e);
         }
 
+        /// <inheritdoc />
         protected override void OnDetachedFromLogicalTree(LogicalTreeAttachmentEventArgs e)
         {
             _logicalRoot = null;
@@ -126,6 +128,7 @@ namespace Avalonia.Controls.Primitives
                 ((ILogical)l).NotifyDetachedFromLogicalTree(e);
         }
 
+        /// <inheritdoc />
         protected override Size MeasureOverride(Size availableSize)
         {
             foreach (var l in _layers)
@@ -133,6 +136,7 @@ namespace Avalonia.Controls.Primitives
             return base.MeasureOverride(availableSize);
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             foreach (var l in _layers)

src/Avalonia.Controls/RelativePanel.cs

@@ -33,13 +33,8 @@ namespace Avalonia.Controls
         protected override Size MeasureOverride(Size availableSize)
         {
             _childGraph.Clear();
-            foreach (Layoutable child in Children)
+            foreach (var child in Children)
             {
-                if (child == null)
-                {
-                    continue;
-                }
-
                 var node = _childGraph.AddNode(child);
 
                 node.AlignLeftWithNode = _childGraph.AddLink(node, GetDependencyElement(AlignLeftWithProperty, child));

src/Avalonia.Controls/Remote/Server/RemoteServerTopLevelImpl.cs

@@ -14,8 +14,8 @@ using Avalonia.Remote.Protocol.Viewport;
 using Avalonia.Rendering;
 using Avalonia.Threading;
 using Key = Avalonia.Input.Key;
-using PixelFormat = Avalonia.Platform.PixelFormat;
 using ProtocolPixelFormat = Avalonia.Remote.Protocol.Viewport.PixelFormat;
+using ProtocolMouseButton = Avalonia.Remote.Protocol.Input.MouseButton;
 
 namespace Avalonia.Controls.Remote.Server
 {
@@ -24,7 +24,7 @@ namespace Avalonia.Controls.Remote.Server
     {
         private readonly IAvaloniaRemoteTransportConnection _transport;
         private LockedFramebuffer? _framebuffer;
-        private object _lock = new object();
+        private readonly object _lock = new();
         private long _lastSentFrame = -1;
         private long _lastReceivedFrame = -1;
         private long _nextFrameNumber = 1;
@@ -50,17 +50,17 @@ namespace Avalonia.Controls.Remote.Server
             return r;
         }
 
-        private static RawPointerEventType GetAvaloniaEventType (Avalonia.Remote.Protocol.Input.MouseButton button, bool pressed)
+        private static RawPointerEventType GetAvaloniaEventType(ProtocolMouseButton button, bool pressed)
         {
             switch (button)
             {
-                case Avalonia.Remote.Protocol.Input.MouseButton.Left:
+                case ProtocolMouseButton.Left:
                     return pressed ? RawPointerEventType.LeftButtonDown : RawPointerEventType.LeftButtonUp;
 
-                case Avalonia.Remote.Protocol.Input.MouseButton.Middle:
+                case ProtocolMouseButton.Middle:
                     return pressed ? RawPointerEventType.MiddleButtonDown : RawPointerEventType.MiddleButtonUp;
 
-                case Avalonia.Remote.Protocol.Input.MouseButton.Right:
+                case ProtocolMouseButton.Right:
                     return pressed ? RawPointerEventType.RightButtonDown : RawPointerEventType.RightButtonUp;
 
                 default:
@@ -68,11 +68,7 @@ namespace Avalonia.Controls.Remote.Server
             }
         }
 
-        private static RawInputModifiers GetAvaloniaRawInputModifiers(
-            Avalonia.Remote.Protocol.Input.InputModifiers[] modifiers)
-            => (RawInputModifiers)GetAvaloniaInputModifiers(modifiers);
-        
-        private static RawInputModifiers GetAvaloniaInputModifiers (Avalonia.Remote.Protocol.Input.InputModifiers[] modifiers)
+        private static RawInputModifiers GetAvaloniaRawInputModifiers(InputModifiers[]? modifiers)
         {
             var result = RawInputModifiers.None;
 
@@ -85,31 +81,31 @@ namespace Avalonia.Controls.Remote.Server
             {
                 switch (modifier)
                 {
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.Control:
+                    case InputModifiers.Control:
                         result |= RawInputModifiers.Control;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.Alt:
+                    case InputModifiers.Alt:
                         result |= RawInputModifiers.Alt;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.Shift:
+                    case InputModifiers.Shift:
                         result |= RawInputModifiers.Shift;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.Windows:
+                    case InputModifiers.Windows:
                         result |= RawInputModifiers.Meta;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.LeftMouseButton:
+                    case InputModifiers.LeftMouseButton:
                         result |= RawInputModifiers.LeftMouseButton;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.MiddleMouseButton:
+                    case InputModifiers.MiddleMouseButton:
                         result |= RawInputModifiers.MiddleMouseButton;
                         break;
 
-                    case Avalonia.Remote.Protocol.Input.InputModifiers.RightMouseButton:
+                    case InputModifiers.RightMouseButton:
                         result |= RawInputModifiers.RightMouseButton;
                         break;
                 }
@@ -187,7 +183,7 @@ namespace Avalonia.Controls.Remote.Server
                             InputRoot!, 
                             RawPointerEventType.Move, 
                             new Point(pointer.X, pointer.Y), 
-                            GetAvaloniaInputModifiers(pointer.Modifiers)));
+                            GetAvaloniaRawInputModifiers(pointer.Modifiers)));
                     }, DispatcherPriority.Input);
                 }
                 if(obj is PointerPressedEventMessage pressed)
@@ -200,7 +196,7 @@ namespace Avalonia.Controls.Remote.Server
                             InputRoot!,
                             GetAvaloniaEventType(pressed.Button, true),
                             new Point(pressed.X, pressed.Y),
-                            GetAvaloniaInputModifiers(pressed.Modifiers)));
+                            GetAvaloniaRawInputModifiers(pressed.Modifiers)));
                     }, DispatcherPriority.Input);
                 }
                 if (obj is PointerReleasedEventMessage released)
@@ -213,7 +209,7 @@ namespace Avalonia.Controls.Remote.Server
                             InputRoot!,
                             GetAvaloniaEventType(released.Button, false),
                             new Point(released.X, released.Y),
-                            GetAvaloniaInputModifiers(released.Modifiers)));
+                            GetAvaloniaRawInputModifiers(released.Modifiers)));
                     }, DispatcherPriority.Input);
                 }
                 if(obj is ScrollEventMessage scroll)
@@ -226,7 +222,7 @@ namespace Avalonia.Controls.Remote.Server
                             InputRoot!,
                             new Point(scroll.X, scroll.Y),
                             new Vector(scroll.DeltaX, scroll.DeltaY),
-                            GetAvaloniaInputModifiers(scroll.Modifiers)));
+                            GetAvaloniaRawInputModifiers(scroll.Modifiers)));
                     }, DispatcherPriority.Input);
                 }
                 if(obj is KeyEventMessage key)
@@ -274,8 +270,8 @@ namespace Avalonia.Controls.Remote.Server
         }
 
         public override IEnumerable<object> Surfaces => new[] { this };
-        
-        FrameMessage RenderFrame(int width, int height, ProtocolPixelFormat? format)
+
+        private FrameMessage RenderFrame(int width, int height, ProtocolPixelFormat? format)
         {
             var scalingX = _dpi.X / 96.0;
             var scalingY = _dpi.Y / 96.0;

src/Avalonia.Controls/Slider.cs

@@ -81,11 +81,11 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="TicksProperty"/> property.
         /// </summary>
-        public static readonly StyledProperty<AvaloniaList<double>> TicksProperty =
+        public static readonly StyledProperty<AvaloniaList<double>?> TicksProperty =
             TickBar.TicksProperty.AddOwner<Slider>();
 
         // Slider required parts
-        private bool _isDragging = false;
+        private bool _isDragging;
         private Track? _track;
         private Button? _decreaseButton;
         private Button? _increaseButton;
@@ -124,7 +124,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the ticks to be drawn on the tick bar.
         /// </summary>
-        public AvaloniaList<double> Ticks
+        public AvaloniaList<double>? Ticks
         {
             get => GetValue(TicksProperty);
             set => SetValue(TicksProperty, value);
@@ -215,6 +215,7 @@ namespace Avalonia.Controls
             _pointerMovedDispose = this.AddDisposableHandler(PointerMovedEvent, TrackMoved, RoutingStrategies.Tunnel);
         }
 
+        /// <inheritdoc />
         protected override void OnKeyDown(KeyEventArgs e)
         {
             base.OnKeyDown(e);
@@ -350,8 +351,8 @@ namespace Avalonia.Controls
 
             var orient = Orientation == Orientation.Horizontal;
             var thumbLength = (orient
-                ? _track.Thumb.Bounds.Width
-                : _track.Thumb.Bounds.Height) + double.Epsilon;
+                ? _track.Thumb?.Bounds.Width ?? 0.0
+                : _track.Thumb?.Bounds.Height ?? 0.0) + double.Epsilon;
             var trackLength = (orient
                 ? _track.Bounds.Width
                 : _track.Bounds.Height) - thumbLength;
@@ -367,6 +368,7 @@ namespace Avalonia.Controls
             Value = IsSnapToTickEnabled ? SnapToTick(finalValue) : finalValue;
         }
 
+        /// <inheritdoc />
         protected override void UpdateDataValidation(
             AvaloniaProperty property,
             BindingValueType state,
@@ -378,6 +380,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);

src/Avalonia.Controls/StackPanel.cs

@@ -261,9 +261,6 @@ namespace Avalonia.Controls
                 // Get next child.
                 var child = children[i];
 
-                if (child == null)
-                { continue; }
-
                 bool isVisible = child.IsVisible;
 
                 if (isVisible && !hasVisibleChild)
@@ -319,8 +316,10 @@ namespace Avalonia.Controls
             {
                 var child = children[i];
 
-                if (child == null || !child.IsVisible)
-                { continue; }
+                if (!child.IsVisible)
+                {
+                    continue;
+                }
 
                 if (fHorizontal)
                 {

src/Avalonia.Controls/TickBar.cs

@@ -51,20 +51,16 @@ namespace Avalonia.Controls
                                    TicksProperty);
         }
 
-        public TickBar() : base()
-        {
-        }
-
         /// <summary>
         /// Defines the <see cref="Fill"/> property.
         /// </summary>
-        public static readonly StyledProperty<IBrush> FillProperty =
-            AvaloniaProperty.Register<TickBar, IBrush>(nameof(Fill));
+        public static readonly StyledProperty<IBrush?> FillProperty =
+            AvaloniaProperty.Register<TickBar, IBrush?>(nameof(Fill));
 
         /// <summary>
         /// Brush used to fill the TickBar's Ticks.
         /// </summary>
-        public IBrush Fill
+        public IBrush? Fill
         {
             get { return GetValue(FillProperty); }
             set { SetValue(FillProperty, value); }
@@ -136,15 +132,15 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="Ticks"/> property.
         /// </summary>
-        public static readonly StyledProperty<AvaloniaList<double>> TicksProperty =
-            AvaloniaProperty.Register<TickBar, AvaloniaList<double>>(nameof(Ticks));
+        public static readonly StyledProperty<AvaloniaList<double>?> TicksProperty =
+            AvaloniaProperty.Register<TickBar, AvaloniaList<double>?>(nameof(Ticks));
 
         /// <summary>
         /// The Ticks property contains collection of value of type Double which
         /// are the logical positions use to draw the ticks.
         /// The property value is a <see cref="AvaloniaList{T}" />.
         /// </summary>
-        public AvaloniaList<double> Ticks
+        public AvaloniaList<double>? Ticks
         {
             get { return GetValue(TicksProperty); }
             set { SetValue(TicksProperty, value); }
@@ -281,7 +277,7 @@ namespace Avalonia.Controls
                     endPoint = new Point(0d, halfReservedSpace);
                     logicalToPhysical = size.Height / range * -1;
                     break;
-            };
+            }
 
             tickLen2 = tickLen * 0.75;
 

src/Avalonia.Controls/TrayIcon.cs

@@ -100,8 +100,8 @@ namespace Avalonia.Controls
         /// <summary>
         /// Defines the <see cref="TrayIcons"/> attached property.
         /// </summary>
-        public static readonly AttachedProperty<TrayIcons> IconsProperty
-            = AvaloniaProperty.RegisterAttached<TrayIcon, Application, TrayIcons>("Icons");
+        public static readonly AttachedProperty<TrayIcons?> IconsProperty
+            = AvaloniaProperty.RegisterAttached<TrayIcon, Application, TrayIcons?>("Icons");
 
         /// <summary>
         /// Defines the <see cref="Menu"/> property.
@@ -127,9 +127,9 @@ namespace Avalonia.Controls
         public static readonly StyledProperty<bool> IsVisibleProperty =
             Visual.IsVisibleProperty.AddOwner<TrayIcon>();
 
-        public static void SetIcons(Application o, TrayIcons trayIcons) => o.SetValue(IconsProperty, trayIcons);
+        public static void SetIcons(Application o, TrayIcons? trayIcons) => o.SetValue(IconsProperty, trayIcons);
 
-        public static TrayIcons GetIcons(Application o) => o.GetValue(IconsProperty);
+        public static TrayIcons? GetIcons(Application o) => o.GetValue(IconsProperty);
         
         /// <summary>
         /// Gets or sets the <see cref="Command"/> property of a TrayIcon.
@@ -213,6 +213,7 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);

src/Avalonia.Controls/TreeView.cs

@@ -3,17 +3,13 @@ using System;
 using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
-using System.ComponentModel;
+using System.Diagnostics.CodeAnalysis;
 using System.Linq;
-using Avalonia.Reactive;
 using Avalonia.Collections;
 using Avalonia.Controls.Generators;
 using Avalonia.Controls.Primitives;
-using Avalonia.Controls.Utils;
-using Avalonia.Data;
 using Avalonia.Input;
 using Avalonia.Input.Platform;
-using Avalonia.Interactivity;
 using Avalonia.Threading;
 using Avalonia.VisualTree;
 
@@ -132,6 +128,7 @@ namespace Avalonia.Controls
         /// <summary>
         /// Gets or sets the selected items.
         /// </summary>
+        [AllowNull]
         public IList SelectedItems
         {
             get
@@ -144,7 +141,6 @@ namespace Avalonia.Controls
 
                 return _selectedItems;
             }
-
             set
             {
                 if (value?.IsFixedSize == true || value?.IsReadOnly == true)
@@ -167,9 +163,9 @@ namespace Avalonia.Controls
         {
             item.IsExpanded = true;
 
-            if (item.Presenter?.Panel != null)
+            if (item.Presenter?.Panel is { } panel)
             {
-                foreach (var child in item.Presenter.Panel.Children)
+                foreach (var child in panel.Children)
                 {
                     if (child is TreeViewItem treeViewItem)
                     {
@@ -589,7 +585,7 @@ namespace Avalonia.Controls
                 case NavigationDirection.Right:
                     if (from?.IsExpanded == true && intoChildren && from.ItemCount > 0)
                     {
-                        result = (TreeViewItem)from.ItemContainerGenerator.ContainerFromIndex(0)!;
+                        result = (TreeViewItem)from.ContainerFromIndex(0)!;
                     }
                     else if (index < parent?.ItemCount - 1)
                     {
@@ -865,7 +861,7 @@ namespace Avalonia.Controls
         /// </summary>
         /// <param name="container">The container.</param>
         /// <param name="selected">Whether the control is selected</param>
-        private void MarkContainerSelected(Control container, bool selected)
+        private void MarkContainerSelected(Control? container, bool selected)
         {
             if (container == null)
             {

src/Avalonia.Controls/Viewbox.cs

@@ -36,6 +36,9 @@ namespace Avalonia.Controls
             AffectsMeasure<Viewbox>(StretchProperty, StretchDirectionProperty);
         }
 
+        /// <summary>
+        /// Initializes a new instance of the <see cref="Viewbox"/> class.
+        /// </summary>
         public Viewbox()
         {
             // The Child control is hosted inside a ViewboxContainer control so that the transform
@@ -85,13 +88,14 @@ namespace Avalonia.Controls
             set => _containerVisual.RenderTransform = value;
         }
 
+        /// <inheritdoc />
         protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == ChildProperty)
             {
-                var (oldChild, newChild) = change.GetOldAndNewValue<Control>();
+                var (oldChild, newChild) = change.GetOldAndNewValue<Control?>();
 
                 if (oldChild is not null)
                 {
@@ -111,41 +115,33 @@ namespace Avalonia.Controls
             }
         }
 
+        /// <inheritdoc />
         protected override Size MeasureOverride(Size availableSize)
         {
             var child = _containerVisual;
 
-            if (child != null)
-            {
-                child.Measure(Size.Infinity);
-
-                var childSize = child.DesiredSize;
+            child.Measure(Size.Infinity);
 
-                var size = Stretch.CalculateSize(availableSize, childSize, StretchDirection);
+            var childSize = child.DesiredSize;
 
-                return size;
-            }
+            var size = Stretch.CalculateSize(availableSize, childSize, StretchDirection);
 
-            return new Size();
+            return size;
         }
 
+        /// <inheritdoc />
         protected override Size ArrangeOverride(Size finalSize)
         {
             var child = _containerVisual;
 
-            if (child != null)
-            {
-                var childSize = child.DesiredSize;
-                var scale = Stretch.CalculateScaling(finalSize, childSize, StretchDirection);
-
-                InternalTransform = new ImmutableTransform(Matrix.CreateScale(scale.X, scale.Y));
+            var childSize = child.DesiredSize;
+            var scale = Stretch.CalculateScaling(finalSize, childSize, StretchDirection);
 
-                child.Arrange(new Rect(childSize));
+            InternalTransform = new ImmutableTransform(Matrix.CreateScale(scale.X, scale.Y));
 
-                return childSize * scale;
-            }
+            child.Arrange(new Rect(childSize));
 
-            return finalSize;
+            return childSize * scale;
         }
 
         /// <summary>

src/Avalonia.Controls/WrapPanel.cs

@@ -144,35 +144,32 @@ namespace Avalonia.Controls
             for (int i = 0, count = children.Count; i < count; i++)
             {
                 var child = children[i];
-                if (child != null)
-                {
-                    // Flow passes its own constraint to children
-                    child.Measure(childConstraint);
+                // Flow passes its own constraint to children
+                child.Measure(childConstraint);
 
-                    // This is the size of the child in UV space
-                    var sz = new UVSize(orientation,
-                        itemWidthSet ? itemWidth : child.DesiredSize.Width,
-                        itemHeightSet ? itemHeight : child.DesiredSize.Height);
+                // This is the size of the child in UV space
+                var sz = new UVSize(orientation,
+                    itemWidthSet ? itemWidth : child.DesiredSize.Width,
+                    itemHeightSet ? itemHeight : child.DesiredSize.Height);
 
-                    if (MathUtilities.GreaterThan(curLineSize.U + sz.U, uvConstraint.U)) // Need to switch to another line
-                    {
-                        panelSize.U = Max(curLineSize.U, panelSize.U);
-                        panelSize.V += curLineSize.V;
-                        curLineSize = sz;
-
-                        if (MathUtilities.GreaterThan(sz.U, uvConstraint.U)) // The element is wider then the constraint - give it a separate line                    
-                        {
-                            panelSize.U = Max(sz.U, panelSize.U);
-                            panelSize.V += sz.V;
-                            curLineSize = new UVSize(orientation);
-                        }
-                    }
-                    else // Continue to accumulate a line
+                if (MathUtilities.GreaterThan(curLineSize.U + sz.U, uvConstraint.U)) // Need to switch to another line
+                {
+                    panelSize.U = Max(curLineSize.U, panelSize.U);
+                    panelSize.V += curLineSize.V;
+                    curLineSize = sz;
+
+                    if (MathUtilities.GreaterThan(sz.U, uvConstraint.U)) // The element is wider then the constraint - give it a separate line
                     {
-                        curLineSize.U += sz.U;
-                        curLineSize.V = Max(sz.V, curLineSize.V);
+                        panelSize.U = Max(sz.U, panelSize.U);
+                        panelSize.V += sz.V;
+                        curLineSize = new UVSize(orientation);
                     }
                 }
+                else // Continue to accumulate a line
+                {
+                    curLineSize.U += sz.U;
+                    curLineSize.V = Max(sz.V, curLineSize.V);
+                }
             }
 
             // The last line size, if any should be added
@@ -202,34 +199,31 @@ namespace Avalonia.Controls
             for (int i = 0; i < children.Count; i++)
             {
                 var child = children[i];
-                if (child != null)
-                {
-                    var sz = new UVSize(orientation,
-                        itemWidthSet ? itemWidth : child.DesiredSize.Width,
-                        itemHeightSet ? itemHeight : child.DesiredSize.Height);
+                var sz = new UVSize(orientation,
+                    itemWidthSet ? itemWidth : child.DesiredSize.Width,
+                    itemHeightSet ? itemHeight : child.DesiredSize.Height);
 
-                    if (MathUtilities.GreaterThan(curLineSize.U + sz.U, uvFinalSize.U)) // Need to switch to another line
-                    {
-                        ArrangeLine(accumulatedV, curLineSize.V, firstInLine, i, useItemU, itemU);
-
-                        accumulatedV += curLineSize.V;
-                        curLineSize = sz;
+                if (MathUtilities.GreaterThan(curLineSize.U + sz.U, uvFinalSize.U)) // Need to switch to another line
+                {
+                    ArrangeLine(accumulatedV, curLineSize.V, firstInLine, i, useItemU, itemU);
 
-                        if (MathUtilities.GreaterThan(sz.U, uvFinalSize.U)) // The element is wider then the constraint - give it a separate line                    
-                        {
-                            // Switch to next line which only contain one element
-                            ArrangeLine(accumulatedV, sz.V, i, ++i, useItemU, itemU);
+                    accumulatedV += curLineSize.V;
+                    curLineSize = sz;
 
-                            accumulatedV += sz.V;
-                            curLineSize = new UVSize(orientation);
-                        }
-                        firstInLine = i;
-                    }
-                    else // Continue to accumulate a line
+                    if (MathUtilities.GreaterThan(sz.U, uvFinalSize.U)) // The element is wider then the constraint - give it a separate line
                     {
-                        curLineSize.U += sz.U;
-                        curLineSize.V = Max(sz.V, curLineSize.V);
+                        // Switch to next line which only contain one element
+                        ArrangeLine(accumulatedV, sz.V, i, ++i, useItemU, itemU);
+
+                        accumulatedV += sz.V;
+                        curLineSize = new UVSize(orientation);
                     }
+                    firstInLine = i;
+                }
+                else // Continue to accumulate a line
+                {
+                    curLineSize.U += sz.U;
+                    curLineSize.V = Max(sz.V, curLineSize.V);
                 }
             }
 
@@ -252,17 +246,14 @@ namespace Avalonia.Controls
             for (int i = start; i < end; i++)
             {
                 var child = children[i];
-                if (child != null)
-                {
-                    var childSize = new UVSize(orientation, child.DesiredSize.Width, child.DesiredSize.Height);
-                    double layoutSlotU = useItemU ? itemU : childSize.U;
-                    child.Arrange(new Rect(
-                        isHorizontal ? u : v,
-                        isHorizontal ? v : u,
-                        isHorizontal ? layoutSlotU : lineV,
-                        isHorizontal ? lineV : layoutSlotU));
-                    u += layoutSlotU;
-                }
+                var childSize = new UVSize(orientation, child.DesiredSize.Width, child.DesiredSize.Height);
+                double layoutSlotU = useItemU ? itemU : childSize.U;
+                child.Arrange(new Rect(
+                    isHorizontal ? u : v,
+                    isHorizontal ? v : u,
+                    isHorizontal ? layoutSlotU : lineV,
+                    isHorizontal ? lineV : layoutSlotU));
+                u += layoutSlotU;
             }
         }
 

src/Browser/Avalonia.Browser/ClipboardImpl.cs

@@ -8,14 +8,14 @@ namespace Avalonia.Browser
 {
     internal class ClipboardImpl : IClipboard
     {
-        public Task<string> GetTextAsync()
+        public Task<string?> GetTextAsync()
         {
-            return InputHelper.ReadClipboardTextAsync();
+            return InputHelper.ReadClipboardTextAsync()!;
         }
 
-        public Task SetTextAsync(string text)
+        public Task SetTextAsync(string? text)
         {
-            return InputHelper.WriteClipboardTextAsync(text);
+            return InputHelper.WriteClipboardTextAsync(text ?? string.Empty);
         }
 
         public async Task ClearAsync() => await SetTextAsync("");
@@ -24,6 +24,6 @@ namespace Avalonia.Browser
 
         public Task<string[]> GetFormatsAsync() => Task.FromResult(Array.Empty<string>());
 
-        public Task<object> GetDataAsync(string format) => Task.FromResult<object>(new());
+        public Task<object?> GetDataAsync(string format) => Task.FromResult<object?>(new());
     }
 }