Enable nullability in Controls.UnitTests (#20373)

src/Avalonia.Controls/HotkeyManager.cs

@@ -159,7 +159,7 @@ namespace Avalonia.Controls
                 new Manager(control).Init();
             });
         }
-        public static void SetHotKey(AvaloniaObject target, KeyGesture value) => target.SetValue(HotKeyProperty, value);
+        public static void SetHotKey(AvaloniaObject target, KeyGesture? value) => target.SetValue(HotKeyProperty, value);
         public static KeyGesture? GetHotKey(AvaloniaObject target) => target.GetValue(HotKeyProperty);
     }
 }

tests/Avalonia.Controls.UnitTests/ApplicationTests.cs

@@ -12,7 +12,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
-                Assert.Throws<ArgumentNullException>(() => { Application.Current.Run(null); });
+                Assert.Throws<ArgumentNullException>(() => { Application.Current!.Run(null!); });
             }
         }
 
@@ -22,7 +22,7 @@ namespace Avalonia.Controls.UnitTests
             // Test for #1765.
             using (UnitTestApplication.Start())
             {
-                var resources = Application.Current.Resources;
+                var resources = Application.Current!.Resources;
                 var raised = false;
 
                 Application.Current.ResourcesChanged += (s, e) => raised = true;
@@ -37,13 +37,13 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start())
             {
-                var application = Application.Current;
+                var application = Application.Current!;
 
                 application.DataContext = "Test";
 
                 application.Bind(Application.NameProperty, new Binding("."));
 
-                Assert.Equal("Test", Application.Current.Name);
+                Assert.Equal("Test", application.Name);
             }
         }
     }

tests/Avalonia.Controls.UnitTests/AutoCompleteBoxTests.cs

@@ -21,60 +21,60 @@ namespace Avalonia.Controls.UnitTests
         [Fact]
         public void Search_Filters()
         {
-            Assert.True(GetFilter(AutoCompleteFilterMode.Contains)("am", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.Contains)("am", "name"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.Contains)("AME", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.Contains)("AME", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.Contains)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.Contains)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("na", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("AME", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("AME", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.ContainsCaseSensitive)("hello", "name"));
 
             Assert.Null(GetFilter(AutoCompleteFilterMode.Custom));
             Assert.Null(GetFilter(AutoCompleteFilterMode.None));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.Equals)("na", "na"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.Equals)("na", "na"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.Equals)("na", "NA"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.Equals)("na", "NA"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.Equals)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.Equals)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("na", "na"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("na", "na"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("na", "NA"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("na", "NA"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.EqualsCaseSensitive)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWith)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWith)("na", "name"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWith)("NAM", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWith)("NAM", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWith)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWith)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("na", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("NAM", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("NAM", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWithCaseSensitive)("hello", "name"));
         }
 
         [Fact]
         public void Ordinal_Search_Filters()
         {
-            Assert.True(GetFilter(AutoCompleteFilterMode.ContainsOrdinal)("am", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinal)("am", "name"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.ContainsOrdinal)("AME", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinal)("AME", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.ContainsOrdinal)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinal)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("na", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("AME", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("AME", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.ContainsOrdinalCaseSensitive)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.EqualsOrdinal)("na", "na"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinal)("na", "na"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.EqualsOrdinal)("na", "NA"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinal)("na", "NA"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.EqualsOrdinal)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinal)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("na", "na"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("na", "na"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("na", "NA"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("na", "NA"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.EqualsOrdinalCaseSensitive)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWithOrdinal)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinal)("na", "name"));
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWithOrdinal)("NAM", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinal)("NAM", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWithOrdinal)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinal)("hello", "name"));
 
-            Assert.True(GetFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("na", "name"));
+            Assert.True(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("na", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("NAM", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("NAM", "name"));
-            Assert.False(GetFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("hello", "name"));
+            Assert.False(GetNotNullFilter(AutoCompleteFilterMode.StartsWithOrdinalCaseSensitive)("hello", "name"));
         }
 
         [Fact]
@@ -261,7 +261,7 @@ namespace Avalonia.Controls.UnitTests
                 control.Populated += (s, e) =>
                 {
                     populated = true;
-                    ReadOnlyCollection<object> collection = e.Data as ReadOnlyCollection<object>;
+                    var collection = e.Data as ReadOnlyCollection<object>;
                     populatedOk = collection != null && collection.Count == 1;
                 };
 
@@ -328,14 +328,10 @@ namespace Avalonia.Controls.UnitTests
             RunTest((control, textbox) =>
             {
                 control.FilterMode = AutoCompleteFilterMode.Custom;
-                control.ItemFilter = (search, item) =>
+                control.ItemFilter = (_, item) => item is string;
-                {
-                    string s = item as string;
-                    return s == null ? false : true;
-                };
 
                 // Just set to null briefly to exercise that code path
-                AutoCompleteFilterPredicate<object> filter = control.ItemFilter;
+                var filter = control.ItemFilter;
                 Assert.NotNull(filter);
                 control.ItemFilter = null;
                 Assert.Null(control.ItemFilter);
@@ -377,6 +373,8 @@ namespace Avalonia.Controls.UnitTests
         {
             RunTest((control, textbox) =>
             {
+                Assert.NotNull(control.ItemsSource);
+
                 object selectedItem = control.ItemsSource.Cast<object>().First();
                 string input = "42";
 
@@ -394,6 +392,8 @@ namespace Avalonia.Controls.UnitTests
         {
             RunTest((control, textbox) =>
             {
+                Assert.NotNull(control.ItemsSource);
+
                 object selectedItem = control.ItemsSource.Cast<object>().First();
                 string input = "42";
 
@@ -416,8 +416,8 @@ namespace Avalonia.Controls.UnitTests
                 control.Bind(AutoCompleteBox.TextProperty, textObservable);
                 Dispatcher.UIThread.RunJobs();
 
-                Assert.Equal(DataValidationErrors.GetHasErrors(control), true);
+                Assert.True(DataValidationErrors.GetHasErrors(control));
-                Assert.Equal(DataValidationErrors.GetErrors(control).SequenceEqual(new[] { exception }), true);
+                Assert.Equal([exception], DataValidationErrors.GetErrors(control));
             });
         }
         
@@ -453,8 +453,8 @@ namespace Avalonia.Controls.UnitTests
                 control.Bind(AutoCompleteBox.SelectedItemProperty, itemObservable);
                 Dispatcher.UIThread.RunJobs();
 
-                Assert.Equal(DataValidationErrors.GetHasErrors(control), true);
+                Assert.True(DataValidationErrors.GetHasErrors(control));
-                Assert.Equal(DataValidationErrors.GetErrors(control).SequenceEqual(new[] { exception }), true);
+                Assert.Equal([exception], DataValidationErrors.GetErrors(control));
             });
         }
 
@@ -587,12 +587,19 @@ namespace Avalonia.Controls.UnitTests
         /// </summary>
         /// <param name="mode">The FilterMode of interest.</param>
         /// <returns>Returns the predicate instance.</returns>
-        private static AutoCompleteFilterPredicate<string> GetFilter(AutoCompleteFilterMode mode)
+        private static AutoCompleteFilterPredicate<string?>? GetFilter(AutoCompleteFilterMode mode)
         {
             return new AutoCompleteBox { FilterMode = mode }
                 .TextFilter;
         }
 
+        private static AutoCompleteFilterPredicate<string?> GetNotNullFilter(AutoCompleteFilterMode mode)
+        {
+            var filter = GetFilter(mode);
+            Assert.NotNull(filter);
+            return filter;
+        }
+
         /// <summary>
         /// Creates a large list of strings for AutoCompleteBox testing.
         /// </summary>
@@ -1200,7 +1207,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window {Content = control};
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 Dispatcher.UIThread.RunJobs();
                 test.Invoke(control, textBox);
             }

tests/Avalonia.Controls.UnitTests/Avalonia.Controls.UnitTests.csproj

@@ -13,6 +13,7 @@
   <Import Project="..\..\build\Base.props" />
   <Import Project="..\..\build\SharedVersion.props" />
   <Import Project="..\..\build\HarfBuzzSharp.props" />
+  <Import Project="..\..\build\NullableEnable.props" />
   <ItemGroup>
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup.Xaml.Loader\Avalonia.Markup.Xaml.Loader.csproj" />
     <ProjectReference Include="..\..\src\Markup\Avalonia.Markup.Xaml\Avalonia.Markup.Xaml.csproj" />

tests/Avalonia.Controls.UnitTests/ButtonTests.cs

@@ -212,7 +212,7 @@ namespace Avalonia.Controls.UnitTests
             var pt = new Point(150, 50);
             renderer.Setup(r => r.HitTest(It.IsAny<Point>(), It.IsAny<Visual>(), It.IsAny<Func<Visual, bool>>()))
                 .Returns<Point, Visual, Func<Visual, bool>>((p, r, f) =>
-                    r.Bounds.Contains(p.Transform(r.RenderTransform.Value.Invert())) ?
+                    r.Bounds.Contains(p.Transform(r.RenderTransform!.Value.Invert())) ?
                     new Visual[] { r } : new Visual[0]);
 
             using var _ = UnitTestApplication.Start(TestServices.StyledWindow);
@@ -336,9 +336,9 @@ namespace Avalonia.Controls.UnitTests
             };
 
             root.ApplyTemplate();
-            root.Presenter.UpdateChild();
+            root.Presenter!.UpdateChild();
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
             kd.SetFocusedElement(target, NavigationMethod.Unspecified, KeyModifiers.None);
 
             Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
@@ -535,7 +535,7 @@ namespace Avalonia.Controls.UnitTests
         public void Button_CommandParameter_Does_Not_Change_While_Execution()
         {
             var target = new Button();
-            object lastParamenter = "A";
+            object? lastParamenter = "A";
             var generator = new Random();
             var onlyOnce = false;
             var command = new TestCommand(parameter =>
@@ -582,12 +582,12 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
-        private KeyEventArgs CreateKeyDownEvent(Key key, Interactive source = null)
+        private KeyEventArgs CreateKeyDownEvent(Key key, Interactive? source = null)
         {
             return new KeyEventArgs { RoutedEvent = InputElement.KeyDownEvent, Key = key, Source = source };
         }
 
-        private KeyEventArgs CreateKeyUpEvent(Key key, Interactive source = null)
+        private KeyEventArgs CreateKeyUpEvent(Key key, Interactive? source = null)
         {
             return new KeyEventArgs { RoutedEvent = InputElement.KeyUpEvent, Key = key, Source = source };
         }
@@ -682,6 +682,7 @@ namespace Avalonia.Controls.UnitTests
 
                 button.ApplyTemplate();
                 var presenter = button.Presenter;
+                Assert.NotNull(presenter);
 
                 button.LetterSpacing = 5.0;
 
@@ -758,7 +759,7 @@ namespace Avalonia.Controls.UnitTests
             private readonly ILayoutManager _layoutManager;
             public bool IsClosed { get; private set; }
 
-            public TestTopLevel(ITopLevelImpl impl, ILayoutManager layoutManager = null)
+            public TestTopLevel(ITopLevelImpl impl, ILayoutManager? layoutManager = null)
                 : base(impl)
             {
                 _layoutManager = layoutManager ?? new LayoutManager(this);

tests/Avalonia.Controls.UnitTests/CalendarDatePickerTests.cs

@@ -44,6 +44,7 @@ namespace Avalonia.Controls.UnitTests
             using (UnitTestApplication.Start(Services))
             {
                 CalendarDatePicker datePicker = CreateControl();
+                Assert.NotNull(datePicker.BlackoutDates);
                 datePicker.BlackoutDates.AddDatesInPast();
 
                 DateTime goodValue = DateTime.Today.AddDays(1);
@@ -65,7 +66,7 @@ namespace Avalonia.Controls.UnitTests
                 datePicker.SelectedDate = DateTime.Today.AddDays(5);
 
                 Assert.ThrowsAny<ArgumentOutOfRangeException>(
-                    () => datePicker.BlackoutDates.Add(new CalendarDateRange(DateTime.Today, DateTime.Today.AddDays(10))));
+                    () => datePicker.BlackoutDates!.Add(new CalendarDateRange(DateTime.Today, DateTime.Today.AddDays(10))));
             }
         }
 
@@ -86,7 +87,7 @@ namespace Avalonia.Controls.UnitTests
                 RaiseKeyEvent(tb, Key.Enter, KeyModifiers.None);
 
                 Assert.Equal("17.10.2024", datePicker.Text);
-                Assert.True(CompareDates(datePicker.SelectedDate.Value, new DateTime(2024, 10, 17)));
+                Assert.True(CompareDates(datePicker.SelectedDate!.Value, new DateTime(2024, 10, 17)));
 
                 tb.Clear();
                 RaiseTextEvent(tb, "12.10.2024");

tests/Avalonia.Controls.UnitTests/CalendarTests.cs

@@ -143,12 +143,12 @@ namespace Avalonia.Controls.UnitTests
         /// <summary>
         /// The days added to the SelectedDates collection.
         /// </summary>
-        private IList<object> _selectedDatesChangedAddedDays;
+        private IList<object>? _selectedDatesChangedAddedDays;
 
         /// <summary>
         /// The days removed from the SelectedDates collection.
         /// </summary>
-        private IList<object> _selectedDateChangedRemovedDays;
+        private IList<object>? _selectedDatesChangedRemovedDays;
 
         /// <summary>
         /// The number of times the SelectedDatesChanged event has been fired.
@@ -160,13 +160,13 @@ namespace Avalonia.Controls.UnitTests
         /// </summary>
         /// <param name="sender">The calendar.</param>
         /// <param name="e">Event arguments.</param>
-        private void OnSelectedDatesChanged(object sender, SelectionChangedEventArgs e)
+        private void OnSelectedDatesChanged(object? sender, SelectionChangedEventArgs e)
         {
             _selectedDatesChangedAddedDays =
                 e.AddedItems
                  .Cast<object>()
                  .ToList();
-            _selectedDateChangedRemovedDays = 
+            _selectedDatesChangedRemovedDays =
                 e.RemovedItems
                  .Cast<object>()
                  .ToList();
@@ -183,9 +183,9 @@ namespace Avalonia.Controls.UnitTests
                 _selectedDatesChangedAddedDays.Clear();
             }
 
-            if (_selectedDateChangedRemovedDays != null)
+            if (_selectedDatesChangedRemovedDays != null)
             {
-                _selectedDateChangedRemovedDays.Clear();
+                _selectedDatesChangedRemovedDays.Clear();
             }
 
             _selectedDatesChangedCount = 0;
@@ -203,8 +203,10 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 1);
             Assert.True(CompareDates(calendar.SelectedDates[0], DateTime.Today));
             Assert.True(_selectedDatesChangedCount == 1);
+            Assert.NotNull(_selectedDatesChangedAddedDays);
             Assert.True(_selectedDatesChangedAddedDays.Count == 1);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 0);
+            Assert.NotNull(_selectedDatesChangedRemovedDays);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 0);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDate = DateTime.Today;
@@ -241,8 +243,10 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 1);
             Assert.True(CompareDates(calendar.SelectedDates[0], DateTime.Today));
             Assert.True(_selectedDatesChangedCount == 1);
+            Assert.NotNull(_selectedDatesChangedAddedDays);
             Assert.True(_selectedDatesChangedAddedDays.Count == 1);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 0);
+            Assert.NotNull(_selectedDatesChangedRemovedDays);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 0);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDates.Clear();
@@ -264,7 +268,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(calendar.SelectedDates.Count == 21);
             Assert.True(_selectedDatesChangedCount == 1);
             Assert.True(_selectedDatesChangedAddedDays.Count == 21);
-            Assert.True(_selectedDateChangedRemovedDays.Count == 11);
+            Assert.True(_selectedDatesChangedRemovedDays.Count == 11);
             ResetSelectedDatesChanged();
 
             calendar.SelectedDates.Add(DateTime.Today.AddDays(100));

tests/Avalonia.Controls.UnitTests/CarouselTests.cs

@@ -257,7 +257,7 @@ namespace Avalonia.Controls.UnitTests
                 target.Bind(ComboBox.SelectedItemProperty, textObservable);
 
                 Assert.True(DataValidationErrors.GetHasErrors(target));
-                Assert.True(DataValidationErrors.GetErrors(target).SequenceEqual(new[] { exception }));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(target));
             }
         }
 
@@ -336,7 +336,7 @@ namespace Avalonia.Controls.UnitTests
 
         private static void Layout(Carousel target)
         {
-            ((ILayoutRoot)target.GetVisualRoot()).LayoutManager.ExecuteLayoutPass();
+            ((ILayoutRoot)target.GetVisualRoot()!).LayoutManager.ExecuteLayoutPass();
         }
 
         private static IControlTemplate CarouselTemplate()
@@ -371,7 +371,7 @@ namespace Avalonia.Controls.UnitTests
                 });
         }
 
-        private static TextBlock GetContainerTextBlock(object control)
+        private static TextBlock GetContainerTextBlock(object? control)
         {
             var contentPresenter = Assert.IsType<ContentPresenter>(control);
             return Assert.IsType<TextBlock>(contentPresenter.Child);

tests/Avalonia.Controls.UnitTests/ComboBoxTests.cs

@@ -81,7 +81,7 @@ namespace Avalonia.Controls.UnitTests
                 };
                 var root = new TestRoot(target);
                 target.ApplyTemplate();
-                ((Control)target.Presenter).ApplyTemplate();
+                target.Presenter!.ApplyTemplate();
                 target.Focus();
                 Assert.Equal(target.SelectedIndex, -1);
                 Assert.True(target.IsFocused);
@@ -117,7 +117,7 @@ namespace Avalonia.Controls.UnitTests
                 };
                 var root = new TestRoot(target);
                 target.ApplyTemplate();
-                ((Control)target.Presenter).ApplyTemplate();
+                target.Presenter!.ApplyTemplate();
                 target.Focus();
                 Assert.Equal(target.SelectedIndex, -1);
                 Assert.True(target.IsFocused);
@@ -166,9 +166,10 @@ namespace Avalonia.Controls.UnitTests
 
             var root = new TestRoot { Child = target };
             target.ApplyTemplate();
-            ((Control)target.Presenter).ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
 
             var rectangle = target.GetValue(ComboBox.SelectionBoxItemProperty) as Rectangle;
+            Assert.NotNull(rectangle);
             Assert.True(((ILogical)target).IsAttachedToLogicalTree);
             Assert.True(((ILogical)rectangle).IsAttachedToLogicalTree);
 
@@ -202,7 +203,7 @@ namespace Avalonia.Controls.UnitTests
                                 Content = new ItemsPresenter
                                 {
                                     Name = "PART_ItemsPresenter",
-                                    ItemsPanel = new FuncTemplate<Panel>(() => new VirtualizingStackPanel()),
+                                    ItemsPanel = new FuncTemplate<Panel?>(() => new VirtualizingStackPanel()),
                                 }.RegisterInNameScope(scope)
                             }.RegisterInNameScope(scope)
                         }.RegisterInNameScope(scope),
@@ -234,7 +235,7 @@ namespace Avalonia.Controls.UnitTests
                 var root = new TestRoot { Child = panel = new StackPanel { Children = { target, other } } };
 
                 target.ApplyTemplate();
-                target.Presenter.ApplyTemplate();
+                target.Presenter!.ApplyTemplate();
 
                 other.Focus();
 
@@ -379,14 +380,14 @@ namespace Avalonia.Controls.UnitTests
                 };
 
                 target.ApplyTemplate();
-                target.Presenter.ApplyTemplate();
+                target.Presenter!.ApplyTemplate();
                 
                 var exception = new System.InvalidCastException("failed validation");
                 var textObservable = new BehaviorSubject<BindingNotification>(new BindingNotification(exception, BindingErrorType.DataValidationError));
                 target.Bind(ComboBox.SelectedItemProperty, textObservable);
 
                 Assert.True(DataValidationErrors.GetHasErrors(target));
-                Assert.True(DataValidationErrors.GetErrors(target).SequenceEqual(new[] { exception }));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(target));
             }
             
         }
@@ -473,7 +474,7 @@ namespace Avalonia.Controls.UnitTests
             target.SelectedIndex = 0;
 
             var rectangle = target.GetValue(ComboBox.SelectionBoxItemProperty) as Rectangle;
-
+            Assert.NotNull(rectangle);
             Assert.Equal(FlowDirection.LeftToRight, rectangle.FlowDirection);
         }
 
@@ -503,6 +504,7 @@ namespace Avalonia.Controls.UnitTests
             target.SelectedIndex = 0;
 
             var rectangle = target.GetValue(ComboBox.SelectionBoxItemProperty) as Rectangle;
+            Assert.NotNull(rectangle);
             Assert.Equal(FlowDirection.LeftToRight, rectangle.FlowDirection);
 
             parentContent.FlowDirection = FlowDirection.RightToLeft;
@@ -539,6 +541,7 @@ namespace Avalonia.Controls.UnitTests
                 target.SelectedIndex = 0;
 
                 var rectangle = target.GetValue(ComboBox.SelectionBoxItemProperty) as Rectangle;
+                Assert.NotNull(rectangle);
                 Assert.Equal(FlowDirection.LeftToRight, rectangle.FlowDirection);
 
                 parentContent.FlowDirection = FlowDirection.RightToLeft;
@@ -807,7 +810,7 @@ namespace Avalonia.Controls.UnitTests
             keyboardNavHandler.SetOwner(root);
 
             target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
 
             var containerPanel = target.GetTemplateChildren().OfType<Panel>().FirstOrDefault(x => x.Name == "container");
             var editableTextBox = containerPanel?.GetVisualDescendants().OfType<TextBox>().FirstOrDefault(x => x.Name == "PART_EditableTextBox");
@@ -815,6 +818,9 @@ namespace Avalonia.Controls.UnitTests
             var popupScrollViewer = popup?.Child as ScrollViewer;
             var scrollViewerItemsPresenter = popupScrollViewer?.Content as ItemsPresenter;
             var popupVirtualizingStackPanel = scrollViewerItemsPresenter?.GetVisualDescendants().OfType<VirtualizingStackPanel>().FirstOrDefault();
+
+            Assert.NotNull(editableTextBox);
+            Assert.NotNull(scrollViewerItemsPresenter);
             Assert.NotNull(popupVirtualizingStackPanel);
 
             //force the popup to render the ComboBoxItem(s) as they are what get set as "focused" if this test fails

tests/Avalonia.Controls.UnitTests/ContentControlTests.cs

@@ -24,7 +24,7 @@ namespace Avalonia.Controls.UnitTests
             target.Content = "Foo";
             target.Template = GetTemplate();
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             var child = target.VisualChildren.Single();
             Assert.IsType<Border>(child);
@@ -55,7 +55,7 @@ namespace Avalonia.Controls.UnitTests
             root.Child = target;
 
             target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
 
             foreach (Control child in target.GetTemplateChildren())
                 Assert.Equal("foo", child.Tag);
@@ -70,9 +70,10 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = child;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             var contentPresenter = child.GetVisualParent<ContentPresenter>();
+            Assert.NotNull(contentPresenter);
             Assert.Equal(target, contentPresenter.TemplatedParent);
         }
 
@@ -85,7 +86,7 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = child;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             Assert.Null(child.TemplatedParent);
         }
@@ -117,7 +118,7 @@ namespace Avalonia.Controls.UnitTests
             var child = new Control();
             target.Content = child;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             Assert.Equal(child.Parent, target);
             Assert.Equal(child.GetLogicalParent(), target);
@@ -135,7 +136,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.Content = "Foo";
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             var child = target.Presenter.Child;
 
@@ -152,7 +153,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.Content = "Foo";
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             var child = target.Presenter.Child;
 
@@ -192,7 +193,7 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = child;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             Assert.True(called);
         }
@@ -207,7 +208,7 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = child;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             ((ILogical)target).LogicalChildren.CollectionChanged += (s, e) => called = true;
 
@@ -228,7 +229,7 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = child1;
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             ((ILogical)target).LogicalChildren.CollectionChanged += (s, e) => called = true;
 
@@ -245,14 +246,14 @@ namespace Avalonia.Controls.UnitTests
 
             target.Template = GetTemplate();
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
             target.Content = "Foo";
             target.Presenter.UpdateChild();
-            Assert.Equal("Foo", ((TextBlock)target.Presenter.Child).Text);
+            Assert.Equal("Foo", ((TextBlock)target.Presenter.Child!).Text);
             target.Content = "Bar";
             target.Presenter.UpdateChild();
-            Assert.Equal("Bar", ((TextBlock)target.Presenter.Child).Text);
+            Assert.Equal("Bar", ((TextBlock)target.Presenter.Child!).Text);
         }
 
         [Fact]
@@ -263,9 +264,9 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = "Foo";
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
-            Assert.Equal("Foo", target.Presenter.Child.DataContext);
+            Assert.Equal("Foo", target.Presenter.Child!.DataContext);
         }
 
         [Fact]
@@ -276,16 +277,16 @@ namespace Avalonia.Controls.UnitTests
             target.Template = GetTemplate();
             target.Content = new TextBlock();
             target.ApplyTemplate();
-            target.Presenter.UpdateChild();
+            target.Presenter!.UpdateChild();
 
-            Assert.Null(target.Presenter.Child.DataContext);
+            Assert.Null(target.Presenter.Child!.DataContext);
         }
 
         [Fact]
         public void Binding_ContentTemplate_After_Content_Does_Not_Leave_Orpaned_TextBlock()
         {
             // Test for #1271.
-            var children = new List<Control>();
+            var children = new List<Control?>();
             var presenter = new ContentPresenter();
 
             // The content and then the content template property need to be bound with delayed bindings
@@ -328,7 +329,7 @@ namespace Avalonia.Controls.UnitTests
                 x => Assert.IsType<TextBlock>(x),
                 x => Assert.IsType<Canvas>(x));
 
-            var textBlock = (TextBlock)children[1];
+            var textBlock = (TextBlock)children[1]!;
 
             // The leak in #1271 was caused by the TextBlock's logical parent not being cleared when
             // it is replaced by the Canvas.
@@ -356,9 +357,9 @@ namespace Avalonia.Controls.UnitTests
 
             target.Content = "Foo";
             target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
 
-            Assert.Equal(target, target.Presenter.Child.GetLogicalParent());
+            Assert.Equal(target, target.Presenter!.Child!.GetLogicalParent());
             Assert.Equal(new[] { target.Presenter.Child }, target.LogicalChildren);
 
             root.Child = null;
@@ -370,7 +371,7 @@ namespace Avalonia.Controls.UnitTests
             root.Child = target;
             target.Content = "Bar";
 
-            Assert.Equal(target, target.Presenter.Child.GetLogicalParent());
+            Assert.Equal(target, target.Presenter.Child!.GetLogicalParent());
             Assert.Equal(new[] { target.Presenter.Child }, target.LogicalChildren);
         }
 

tests/Avalonia.Controls.UnitTests/ContextMenuTests.cs

@@ -1,11 +1,9 @@
 using System;
-
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
 using Avalonia.Markup.Xaml;
 using Avalonia.Platform;
-using Avalonia.Rendering;
-using Avalonia.Rendering.Composition;
 using Avalonia.UnitTests;
 
 using Moq;
@@ -15,7 +13,7 @@ namespace Avalonia.Controls.UnitTests
 {
     public class ContextMenuTests : ScopedTestBase
     {
-        private Mock<IPopupImpl> popupImpl;
+        private Mock<IPopupImpl>? _popupImpl;
         private MouseTestHelper _mouse = new MouseTestHelper();
 
         [Fact]
@@ -32,7 +30,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 int openedCount = 0;
 
@@ -65,7 +63,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 target.RaiseEvent(new ContextRequestedEventArgs());
 
@@ -135,7 +133,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 int openedCount = 0;
 
@@ -164,7 +162,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 bool opened = false;
 
@@ -193,7 +191,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 target.ContextMenu = null;
 
@@ -215,7 +213,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = target };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 sut.Open(target);
 
@@ -237,8 +235,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 var window = PreparedWindow();
                 window.Width = 100;
@@ -268,14 +266,15 @@ namespace Avalonia.Controls.UnitTests
                 c.Open(button);
 
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 _mouse.Down(overlay, MouseButton.Left, new Point(90, 90));
                 _mouse.Up(button, MouseButton.Left, new Point(90, 90));
 
                 Assert.Equal(1, tracker);
                 Assert.True(c.IsOpen);
 
-                popupImpl.Verify(x => x.Hide(), Times.Never);
+                _popupImpl.Verify(x => x.Hide(), Times.Never);
-                popupImpl.Verify(x => x.Show(true, false), Times.Exactly(1));
+                _popupImpl.Verify(x => x.Show(true, false), Times.Exactly(1));
             }
         }
 
@@ -284,8 +283,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 var window = PreparedWindow();
                 window.Width = 100;
@@ -308,12 +307,13 @@ namespace Avalonia.Controls.UnitTests
                 c.Open(button);
 
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 _mouse.Down(overlay, MouseButton.Left, new Point(90, 90));
                 _mouse.Up(button, MouseButton.Left, new Point(90, 90));
 
                 Assert.False(c.IsOpen);
-                popupImpl.Verify(x => x.Hide(), Times.Exactly(1));
+                _popupImpl.Verify(x => x.Hide(), Times.Exactly(1));
-                popupImpl.Verify(x => x.Show(true, false), Times.Exactly(1));
+                _popupImpl.Verify(x => x.Show(true, false), Times.Exactly(1));
             }
         }
 
@@ -322,8 +322,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 var sut = new ContextMenu();
                 var target = new Panel
@@ -334,6 +334,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = PreparedWindow(target);
                 window.Show();
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
 
                 _mouse.Click(target, MouseButton.Right);
 
@@ -343,8 +344,8 @@ namespace Avalonia.Controls.UnitTests
                 _mouse.Up(target);
 
                 Assert.False(sut.IsOpen);
-                popupImpl.Verify(x => x.Show(true, false), Times.Once);
+                _popupImpl.Verify(x => x.Show(true, false), Times.Once);
-                popupImpl.Verify(x => x.Hide(), Times.Once);
+                _popupImpl.Verify(x => x.Hide(), Times.Once);
             }
         }
 
@@ -353,8 +354,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 var sut = new ContextMenu();
                 var target = new Panel
@@ -366,6 +367,7 @@ namespace Avalonia.Controls.UnitTests
                 window.Show();
 
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
 
                 _mouse.Click(target, MouseButton.Right);
                 Assert.True(sut.IsOpen);
@@ -374,8 +376,8 @@ namespace Avalonia.Controls.UnitTests
                 _mouse.Up(target, MouseButton.Right);
 
                 Assert.True(sut.IsOpen);
-                popupImpl.Verify(x => x.Hide(), Times.Once);
+                _popupImpl.Verify(x => x.Hide(), Times.Once);
-                popupImpl.Verify(x => x.Show(true, false), Times.Exactly(2));
+                _popupImpl.Verify(x => x.Show(true, false), Times.Exactly(2));
             }
         }
         
@@ -400,7 +402,7 @@ namespace Avalonia.Controls.UnitTests
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                ((Control)window.Presenter).ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 _mouse.Click(target1, MouseButton.Right);
 
@@ -421,7 +423,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
 
                 bool eventCalled = false;
                 var sut = new ContextMenu();
@@ -437,7 +439,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Assert.True(eventCalled);
                 Assert.False(sut.IsOpen);
-                popupImpl.Verify(x => x.Show(true, false), Times.Never);
+                _popupImpl.Verify(x => x.Show(true, false), Times.Never);
             }
         }
 
@@ -496,8 +498,8 @@ namespace Avalonia.Controls.UnitTests
 </Window>";
 
                 var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
-                var target1 = window.Find<TextBlock>("target1");
+                var target1 = window.Get<TextBlock>("target1");
-                var target2 = window.Find<TextBlock>("target2");
+                var target2 = window.Get<TextBlock>("target2");
                 var mouse = new MouseTestHelper();
 
                 Assert.NotNull(target1.ContextMenu);
@@ -539,8 +541,8 @@ namespace Avalonia.Controls.UnitTests
 </Window>";
 
                 var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
-                var target1 = window.Find<TextBlock>("target1");
+                var target1 = window.Get<TextBlock>("target1");
-                var target2 = window.Find<TextBlock>("target2");
+                var target2 = window.Get<TextBlock>("target2");
                 var mouse = new MouseTestHelper();
 
                 Assert.NotNull(target1.ContextMenu);
@@ -562,8 +564,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 bool eventCalled = false;
                 var sut = new ContextMenu();
@@ -574,6 +576,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var window = PreparedWindow(target);
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
 
                 sut.Closing += (c, e) => { eventCalled = true; e.Cancel = true; };
 
@@ -589,8 +592,8 @@ namespace Avalonia.Controls.UnitTests
                 Assert.True(eventCalled);
                 Assert.True(sut.IsOpen);
 
-                popupImpl.Verify(x => x.Show(true, false), Times.Once());
+                _popupImpl.Verify(x => x.Show(true, false), Times.Once());
-                popupImpl.Verify(x => x.Hide(), Times.Never);
+                _popupImpl.Verify(x => x.Hide(), Times.Never);
             }
         }
 
@@ -599,8 +602,8 @@ namespace Avalonia.Controls.UnitTests
         {
             using (Application())
             {
-                popupImpl.Setup(x => x.Show(true, false)).Verifiable();
+                _popupImpl.Setup(x => x.Show(true, false)).Verifiable();
-                popupImpl.Setup(x => x.Hide()).Verifiable();
+                _popupImpl.Setup(x => x.Hide()).Verifiable();
 
                 var item = new MenuItem();
                 var sut = new ContextMenu
@@ -643,7 +646,7 @@ namespace Avalonia.Controls.UnitTests
             }
         }
 
-        private static Window PreparedWindow(object content = null)
+        private static Window PreparedWindow(object? content = null)
         {
             
             var platform = AvaloniaLocator.Current.GetRequiredService<IWindowingPlatform>();
@@ -653,10 +656,11 @@ namespace Avalonia.Controls.UnitTests
             var w = new Window(windowImpl.Object) { Content = content };
             w.ApplyStyling();
             w.ApplyTemplate();
-            ((Control)w.Presenter).ApplyTemplate();
+            w.Presenter!.ApplyTemplate();
             return w;
         }
 
+        [MemberNotNull(nameof(_popupImpl))]
         private IDisposable Application()
         {
             var screen = new PixelRect(new PixelPoint(), new PixelSize(100, 100));
@@ -665,9 +669,9 @@ namespace Avalonia.Controls.UnitTests
             screenImpl.Setup(X => X.AllScreens).Returns( new[] { new Screen(1, screen, screen, true) });
 
             var windowImpl = MockWindowingPlatform.CreateWindowMock();
-            popupImpl = MockWindowingPlatform.CreatePopupMock(windowImpl.Object);
+            _popupImpl = MockWindowingPlatform.CreatePopupMock(windowImpl.Object);
-            popupImpl.SetupGet(x => x.RenderScaling).Returns(1);
+            _popupImpl.SetupGet(x => x.RenderScaling).Returns(1);
-            windowImpl.Setup(x => x.CreatePopup()).Returns(popupImpl.Object);
+            windowImpl.Setup(x => x.CreatePopup()).Returns(_popupImpl.Object);
 
             windowImpl.Setup(x => x.TryGetFeature(It.Is<Type>(t => t == typeof(IScreenImpl)))).Returns(screenImpl.Object);
 
@@ -675,7 +679,7 @@ namespace Avalonia.Controls.UnitTests
                                         keyboardDevice: () => new KeyboardDevice(),
                                         inputManager: new InputManager(),
                                         windowImpl: windowImpl.Object,
-                                        windowingPlatform: new MockWindowingPlatform(() => windowImpl.Object, x => popupImpl.Object));
+                                        windowingPlatform: new MockWindowingPlatform(() => windowImpl.Object, x => _popupImpl.Object));
 
             return UnitTestApplication.Start(services);
         }

tests/Avalonia.Controls.UnitTests/DatePickerTests.cs

@@ -50,13 +50,10 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = datePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                TextBlock dayText = null;
+                TextBlock? dayText = null;
-                Grid container = null;
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
-                container = (desc.ElementAt(1) as Button).Content as Grid;
-                Assert.True(container != null);
 
                 for(int i = 0; i < container.Children.Count; i++)
                 {
@@ -67,9 +64,9 @@ namespace Avalonia.Controls.UnitTests
                     }
                 }
 
-                Assert.True(dayText != null);
+                Assert.NotNull(dayText);
-                Assert.True(!dayText.IsVisible);
+                Assert.False(dayText.IsVisible);
-                Assert.True(container.ColumnDefinitions.Count == 3);
+                Assert.Equal(3, container.ColumnDefinitions.Count);
             }
         }
 
@@ -88,13 +85,10 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = datePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                TextBlock monthText = null;
+                TextBlock? monthText = null;
-                Grid container = null;
-
-                Assert.True(desc.ElementAt(1) is Button);
 
-                container = (desc.ElementAt(1) as Button).Content as Grid;
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-                Assert.True(container != null);
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
 
                 for (int i = 0; i < container.Children.Count; i++)
                 {
@@ -105,9 +99,9 @@ namespace Avalonia.Controls.UnitTests
                     }
                 }
 
-                Assert.True(monthText != null);
+                Assert.NotNull(monthText);
-                Assert.True(!monthText.IsVisible);
+                Assert.False(monthText.IsVisible);
-                Assert.True(container.ColumnDefinitions.Count == 3);
+                Assert.Equal(3, container.ColumnDefinitions.Count);
             }
         }
 
@@ -126,13 +120,10 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = datePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                TextBlock yearText = null;
+                TextBlock? yearText = null;
-                Grid container = null;
-
-                Assert.True(desc.ElementAt(1) is Button);
 
-                container = (desc.ElementAt(1) as Button).Content as Grid;
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-                Assert.True(container != null);
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
 
                 for (int i = 0; i < container.Children.Count; i++)
                 {
@@ -143,9 +134,9 @@ namespace Avalonia.Controls.UnitTests
                     }
                 }
 
-                Assert.True(yearText != null);
+                Assert.NotNull(yearText);
-                Assert.True(!yearText.IsVisible);
+                Assert.False(yearText.IsVisible);
-                Assert.True(container.ColumnDefinitions.Count == 3);
+                Assert.Equal(3, container.ColumnDefinitions.Count);
             }
         }
 
@@ -164,15 +155,12 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = datePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                TextBlock yearText = null;
+                TextBlock? yearText = null;
-                TextBlock monthText = null;
+                TextBlock? monthText = null;
-                TextBlock dayText = null;
+                TextBlock? dayText = null;
-                Grid container = null;
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
-                container = (desc.ElementAt(1) as Button).Content as Grid;
-                Assert.True(container != null);
 
                 for (int i = 0; i < container.Children.Count; i++)
                 {
@@ -190,6 +178,10 @@ namespace Avalonia.Controls.UnitTests
                     }
                 }
 
+                Assert.NotNull(dayText);
+                Assert.NotNull(monthText);
+                Assert.NotNull(yearText);
+
                 DateTimeOffset value = new DateTimeOffset(2000, 10, 10, 0, 0, 0, TimeSpan.Zero);
                 datePicker.SelectedDate = value;
 

tests/Avalonia.Controls.UnitTests/DesktopStyleApplicationLifetimeTests.cs

@@ -317,7 +317,7 @@ namespace Avalonia.Controls.UnitTests
 
                 window.Show();
                 Assert.Equal(1, lifetime.Windows.Count);
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
 
                 Assert.Empty(lifetime.Windows);
             }
@@ -443,7 +443,7 @@ namespace Avalonia.Controls.UnitTests
                 lifetime.Exit += (_, _) => Assert.Fail("lifetime.Exit was called.");
                 Dispatcher.UIThread.ShutdownStarted += UiThreadOnShutdownStarted;
 
-                static void UiThreadOnShutdownStarted(object sender, EventArgs e)
+                static void UiThreadOnShutdownStarted(object? sender, EventArgs e)
                 {
                     Assert.Fail("Dispatcher.UIThread.ShutdownStarted was called.");
                 }

tests/Avalonia.Controls.UnitTests/FlyoutTests.cs

@@ -185,6 +185,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var e = CreatePointerPressedEventArgs(window, new Point(90, 90));
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 overlay.RaiseEvent(e);
 
                 Assert.Equal(1, tracker);
@@ -218,6 +219,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var e = CreatePointerPressedEventArgs(window, new Point(90, 90));
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 overlay.RaiseEvent(e);
 
                 Assert.False(f.IsOpen);
@@ -259,6 +261,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var e = CreatePointerPressedEventArgs(window, new Point(90, 90));
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 overlay.RaiseEvent(e);
 
                 Assert.False(f.IsOpen);
@@ -301,6 +304,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var e = CreatePointerPressedEventArgs(window, new Point(90, 90));
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 overlay.RaiseEvent(e);
 
                 Assert.False(f.IsOpen);
@@ -410,9 +414,9 @@ namespace Avalonia.Controls.UnitTests
                 window.Show();
 
                 button.Focus();
-                Assert.True(window.FocusManager.GetFocusedElement() == button);
+                Assert.Same(button, window.FocusManager?.GetFocusedElement());
                 button.Flyout.ShowAt(button);
-                Assert.True(window.FocusManager.GetFocusedElement() == button);
+                Assert.Same(button, window.FocusManager?.GetFocusedElement());
             }
         }
 
@@ -586,8 +590,8 @@ namespace Avalonia.Controls.UnitTests
 </Window>";
 
                 var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
-                var target1 = window.Find<TextBlock>("target1");
+                var target1 = window.Get<TextBlock>("target1");
-                var target2 = window.Find<TextBlock>("target2");
+                var target2 = window.Get<TextBlock>("target2");
                 var mouse = new MouseTestHelper();
 
                 Assert.NotNull(target1.ContextFlyout);
@@ -632,13 +636,13 @@ namespace Avalonia.Controls.UnitTests
                 window.Content = button;
                 window.Show();
 
-                (button.Flyout as Flyout).FlyoutPresenterClasses.Add("TestClass");
+                ((Flyout)button.Flyout).FlyoutPresenterClasses.Add("TestClass");
 
                 button.Flyout.ShowAt(button);
 
                 var presenter = flyoutPanel.GetVisualAncestors().OfType<FlyoutPresenter>().FirstOrDefault();
                 Assert.NotNull(presenter);
-                Assert.True((presenter.Background as ISolidColorBrush).Color == Colors.Red);
+                Assert.Equal(Colors.Red, (presenter.Background as ISolidColorBrush)?.Color);
             }
         }
 
@@ -650,7 +654,7 @@ namespace Avalonia.Controls.UnitTests
                     keyboardDevice: () => new KeyboardDevice()));
         }
 
-        private static Window PreparedWindow(object content = null)
+        private static Window PreparedWindow(object? content = null)
         {
             var platform = AvaloniaLocator.Current.GetRequiredService<IWindowingPlatform>();
             var windowImpl = Mock.Get(platform.CreateWindow());

tests/Avalonia.Controls.UnitTests/GridTests.cs

@@ -16,19 +16,19 @@ namespace Avalonia.Controls.UnitTests
             this.output = output;
         }
 
-        private static Grid CreateGrid(params (string name, GridLength width)[] columns)
+        private static Grid CreateGrid(params (string? name, GridLength width)[] columns)
         {
             return CreateGrid(columns.Select(c =>
                 (c.name, c.width, ColumnDefinition.MinWidthProperty.GetDefaultValue(typeof(ColumnDefinition)))).ToArray());
         }
 
-        private static Grid CreateGrid(params (string name, GridLength width, double minWidth)[] columns)
+        private static Grid CreateGrid(params (string? name, GridLength width, double minWidth)[] columns)
         {
             return CreateGrid(columns.Select(c =>
                 (c.name, c.width, c.minWidth, ColumnDefinition.MaxWidthProperty.GetDefaultValue(typeof(ColumnDefinition)))).ToArray());
         }
 
-        private static Grid CreateGrid(params (string name, GridLength width, double minWidth, double maxWidth)[] columns)
+        private static Grid CreateGrid(params (string? name, GridLength width, double minWidth, double maxWidth)[] columns)
         {
             var grid = new Grid();
             foreach (var k in columns.Select(c => new ColumnDefinition

tests/Avalonia.Controls.UnitTests/HeaderedItemsControlTests .cs

@@ -35,7 +35,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.Header = "Foo";
             target.ApplyTemplate();
-            target.HeaderPresenter.UpdateChild();
+            target.HeaderPresenter!.UpdateChild();
 
             var child = target.HeaderPresenter.Child;
 

tests/Avalonia.Controls.UnitTests/HotKeyedControlsTests.cs

@@ -16,17 +16,17 @@ namespace Avalonia.Controls.UnitTests
         {
             private readonly Action _action;
             public DelegateCommand(Action action) => _action = action;
-            public event EventHandler CanExecuteChanged { add { } remove { } }
+            public event EventHandler? CanExecuteChanged { add { } remove { } }
-            public bool CanExecute(object parameter) => true;
+            public bool CanExecute(object? parameter) => true;
-            public void Execute(object parameter) => _action();
+            public void Execute(object? parameter) => _action();
         }
         
-        public static readonly StyledProperty<KeyGesture> HotKeyProperty =
+        public static readonly StyledProperty<KeyGesture?> HotKeyProperty =
             HotKeyManager.HotKeyProperty.AddOwner<HotKeyedTextBox>();
 
-        private KeyGesture _hotkey;
+        private KeyGesture? _hotkey;
 
-        public KeyGesture HotKey
+        public KeyGesture? HotKey
         {
             get => GetValue(HotKeyProperty);
             set => SetValue(HotKeyProperty, value);
@@ -61,7 +61,7 @@ namespace Avalonia.Controls.UnitTests
 
         public ICommand Command => _command;
 
-        public object CommandParameter => null;
+        public object? CommandParameter => null;
 
         private readonly DelegateCommand _command;
 
@@ -73,7 +73,7 @@ namespace Avalonia.Controls.UnitTests
 
     public class HotKeyedControlsTests : ScopedTestBase
     {
-        private static Window PreparedWindow(object content = null)
+        private static Window PreparedWindow(object? content = null)
         {
             var platform = AvaloniaLocator.Current.GetRequiredService<IWindowingPlatform>();
             var windowImpl = Mock.Get(platform.CreateWindow());

tests/Avalonia.Controls.UnitTests/InlineTests.cs

@@ -22,6 +22,7 @@ namespace Avalonia.Controls.UnitTests
             bold.BuildTextRun(textRuns, default);
 
             var runProperties = textRuns[0].Properties;
+            Assert.NotNull(runProperties);
             Assert.Equal(FontWeight.Bold, runProperties.Typeface.Weight);
         }
 
@@ -38,6 +39,7 @@ namespace Avalonia.Controls.UnitTests
             italic.BuildTextRun(textRuns, default);
 
             var runProperties = textRuns[0].Properties;
+            Assert.NotNull(runProperties);
             Assert.Equal(FontStyle.Italic, runProperties.Typeface.Style);
         }
 
@@ -55,6 +57,7 @@ namespace Avalonia.Controls.UnitTests
             span.BuildTextRun(textRuns, default);
 
             var runProperties = textRuns[0].Properties;
+            Assert.NotNull(runProperties);
             Assert.Equal(FontStretch.Condensed, runProperties.Typeface.Stretch);
         }
 
@@ -74,6 +77,7 @@ namespace Avalonia.Controls.UnitTests
             span.BuildTextRun(textRuns, default);
 
             var runProperties = textRuns[0].Properties;
+            Assert.NotNull(runProperties);
             Assert.Equal(backgroundBrush, runProperties.BackgroundBrush);
         }
     }

tests/Avalonia.Controls.UnitTests/ItemsSourceViewTests.cs

@@ -21,11 +21,12 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Null(debug.GetCollectionChangedSubscribers());
 
-            void Handler(object sender, NotifyCollectionChangedEventArgs e) { }
+            void Handler(object? sender, NotifyCollectionChangedEventArgs e) { }
             target.CollectionChanged += Handler;
 
-            Assert.NotNull(debug.GetCollectionChangedSubscribers());
+            var subscribers = debug.GetCollectionChangedSubscribers();
-            Assert.Equal(1, debug.GetCollectionChangedSubscribers().Length);
+            Assert.NotNull(subscribers);
+            Assert.Equal(1, subscribers.Length);
 
             target.CollectionChanged -= Handler;
 
@@ -48,7 +49,9 @@ namespace Avalonia.Controls.UnitTests
 
             target.CollectionChanged += (s, e) => { };
 
-            Assert.Equal(1, debug.GetCollectionChangedSubscribers().Length);
+            var subscribers = debug.GetCollectionChangedSubscribers();
+            Assert.NotNull(subscribers);
+            Assert.Equal(1, subscribers.Length);
 
             target.SetSource(new string[0]);
 
@@ -65,12 +68,14 @@ namespace Avalonia.Controls.UnitTests
             target.CollectionChanged += (s, e) => { };
             target.SetSource(source);
 
-            Assert.Equal(1, debug.GetCollectionChangedSubscribers().Length);
+            var subscribers = debug.GetCollectionChangedSubscribers();
+            Assert.NotNull(subscribers);
+            Assert.Equal(1, subscribers.Length);
         }
 
         private class InvalidCollection : INotifyCollectionChanged, IEnumerable<string>
         {
-            public event NotifyCollectionChangedEventHandler CollectionChanged { add { } remove { } }
+            public event NotifyCollectionChangedEventHandler? CollectionChanged { add { } remove { } }
 
             public IEnumerator<string> GetEnumerator()
             {

tests/Avalonia.Controls.UnitTests/LayoutTransformControlTests.cs

@@ -198,6 +198,7 @@ namespace Avalonia.Controls.UnitTests
                                         25,
                                         new RotateTransform() { Angle = 90 });
 
+            Assert.NotNull(lt.TransformRoot);
             Assert.NotNull(lt.TransformRoot.RenderTransform);
 
             Matrix m = lt.TransformRoot.RenderTransform.Value;
@@ -222,6 +223,7 @@ namespace Avalonia.Controls.UnitTests
                                         25,
                                         new RotateTransform() { Angle = -90 });
 
+            Assert.NotNull(lt.TransformRoot);
             Assert.NotNull(lt.TransformRoot.RenderTransform);
 
             var m = lt.TransformRoot.RenderTransform.Value;
@@ -246,6 +248,7 @@ namespace Avalonia.Controls.UnitTests
                                         50,
                                         new ScaleTransform() { ScaleX = 2, ScaleY = 2 });
 
+            Assert.NotNull(lt.TransformRoot);
             Assert.NotNull(lt.TransformRoot.RenderTransform);
 
             Matrix m = lt.TransformRoot.RenderTransform.Value;
@@ -269,6 +272,7 @@ namespace Avalonia.Controls.UnitTests
                                         100,
                                         new SkewTransform() { AngleX = 45, AngleY = 45 });
 
+            Assert.NotNull(lt.TransformRoot);
             Assert.NotNull(lt.TransformRoot.RenderTransform);
 
             Matrix m = lt.TransformRoot.RenderTransform.Value;
@@ -293,6 +297,7 @@ namespace Avalonia.Controls.UnitTests
                                         100,
                                         new SkewTransform() { AngleX = -45, AngleY = -45 });
 
+            Assert.NotNull(lt.TransformRoot);
             Assert.NotNull(lt.TransformRoot.RenderTransform);
 
             Matrix m = lt.TransformRoot.RenderTransform.Value;
@@ -325,6 +330,8 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = lt };
                 window.Show();
 
+                Assert.NotNull(lt.TransformRoot);
+                Assert.NotNull(lt.TransformRoot.RenderTransform);
                 Matrix actual = lt.TransformRoot.RenderTransform.Value;
                 Matrix expected = Matrix.CreateSkew(Matrix.ToRadians(45), Matrix.ToRadians(45));
                 Assert.Equal(expected.M11, actual.M11, 3);
@@ -357,6 +364,7 @@ namespace Avalonia.Controls.UnitTests
 
             LayoutTransformControl lt = CreateWithChildAndMeasureAndTransform(size.Width, size.Height, transform);
 
+            Assert.NotNull(lt.TransformRoot);
             Rect outBounds = lt.TransformRoot.Bounds;
 
             Assert.Equal(outBounds.X, expectedBounds.X);

tests/Avalonia.Controls.UnitTests/ListBoxTests.cs

@@ -37,8 +37,8 @@ namespace Avalonia.Controls.UnitTests
 
             Prepare(target);
 
-            var container = (ListBoxItem)target.Presenter.Panel.Children[0];
+            var container = (ListBoxItem)target.Presenter!.Panel!.Children[0];
-            Assert.IsType<Canvas>(container.Presenter.Child);
+            Assert.IsType<Canvas>(container.Presenter!.Child);
         }
 
         [Fact]
@@ -62,7 +62,7 @@ namespace Avalonia.Controls.UnitTests
                 Template = ListBoxTemplate(),
             };
 
-            ScrollViewer viewer = null;
+            ScrollViewer? viewer = null;
 
             target.TemplateApplied += (sender, e) =>
             {
@@ -88,9 +88,9 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                var text = target.Presenter.Panel.Children
+                var text = target.Presenter!.Panel!.Children
                     .OfType<ListBoxItem>()
-                    .Select(x => x.Presenter.Child)
+                    .Select(x => x.Presenter!.Child)
                     .OfType<TextBlock>()
                     .Select(x => x.Text)
                     .ToList();
@@ -115,7 +115,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                var container = (ListBoxItem)target.Presenter.Panel.Children[0];
+                var container = (ListBoxItem)target.Presenter!.Panel!.Children[0];
 
                 Assert.Same(container.Theme, theme);
             }
@@ -136,7 +136,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                var container = (ListBoxItem)target.Presenter.Panel.Children[0];
+                var container = (ListBoxItem)target.Presenter!.Panel!.Children[0];
 
                 Assert.Same(container.Theme, theme);
             }
@@ -187,7 +187,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                var dataContexts = target.Presenter.Panel.Children
+                var dataContexts = target.Presenter!.Panel!.Children
                     .Cast<Control>()
                     .Select(x => x.DataContext)
                     .ToList();
@@ -214,14 +214,14 @@ namespace Avalonia.Controls.UnitTests
                 Prepare(target);
 
                 // Make sure we're virtualized and first item is selected.
-                Assert.Equal(10, target.Presenter.Panel.Children.Count);
+                Assert.Equal(10, target.Presenter!.Panel!.Children.Count);
                 Assert.True(((ListBoxItem)target.Presenter.Panel.Children[0]).IsSelected);
 
                 // The selected item must not be the anchor, otherwise it won't get recycled.
                 target.Selection.AnchorIndex = -1;
 
                 // Scroll down a page.
-                target.Scroll.Offset = new Vector(0, 10);
+                target.Scroll!.Offset = new Vector(0, 10);
                 Layout(target);
 
                 // Make sure recycled item isn't now selected.
@@ -244,7 +244,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                Assert.Equal(new Size(100, 200), target.Scroll.Extent);
+                Assert.Equal(new Size(100, 200), target.Scroll!.Extent);
                 Assert.Equal(new Size(100, 100), target.Scroll.Viewport);
             }
         }
@@ -273,7 +273,7 @@ namespace Avalonia.Controls.UnitTests
                 items.Remove("Item 2");
                 Layout(target);
 
-                var actual = target.GetRealizedContainers().Cast<ListBoxItem>().Select(x => (string)x.Content).ToList();
+                var actual = target.GetRealizedContainers().Cast<ListBoxItem>().Select(x => (string?)x.Content).ToList();
                 Assert.Equal(items.OrderBy(x => x), actual.OrderBy(x => x));
             }
         }
@@ -324,7 +324,7 @@ namespace Avalonia.Controls.UnitTests
                 };
 
                 Prepare(target);
-                target.Scroll.Offset = new Vector(0, 1);
+                target.Scroll!.Offset = new Vector(0, 1);
 
                 items.RemoveRange(0, 11);
             }
@@ -393,10 +393,12 @@ namespace Avalonia.Controls.UnitTests
                 lm.ExecuteLayoutPass();
 
                 var flags = System.Reflection.BindingFlags.Instance | System.Reflection.BindingFlags.NonPublic;
-                var toMeasure = lm.GetType().GetField("_toMeasure", flags).GetValue(lm) as System.Collections.Generic.IEnumerable<Layout.Layoutable>;
+                var toMeasure = lm.GetType().GetField("_toMeasure", flags)!.GetValue(lm) as IEnumerable<Layoutable>;
-                var toArrange = lm.GetType().GetField("_toArrange", flags).GetValue(lm) as System.Collections.Generic.IEnumerable<Layout.Layoutable>;
+                var toArrange = lm.GetType().GetField("_toArrange", flags)!.GetValue(lm) as IEnumerable<Layoutable>;
 
+                Assert.NotNull(toMeasure);
                 Assert.Equal(0, toMeasure.Count());
+                Assert.NotNull(toArrange);
                 Assert.Equal(0, toArrange.Count());
             }
         }
@@ -435,11 +437,11 @@ namespace Avalonia.Controls.UnitTests
 
                 Threading.Dispatcher.UIThread.RunJobs();
 
-                Assert.Equal("30", target.ContainerFromIndex(items.Count - 1).DataContext);
+                Assert.Equal("30", target.ContainerFromIndex(items.Count - 1)!.DataContext);
-                Assert.Equal("29", target.ContainerFromIndex(items.Count - 2).DataContext);
+                Assert.Equal("29", target.ContainerFromIndex(items.Count - 2)!.DataContext);
-                Assert.Equal("28", target.ContainerFromIndex(items.Count - 3).DataContext);
+                Assert.Equal("28", target.ContainerFromIndex(items.Count - 3)!.DataContext);
-                Assert.Equal("27", target.ContainerFromIndex(items.Count - 4).DataContext);
+                Assert.Equal("27", target.ContainerFromIndex(items.Count - 4)!.DataContext);
-                Assert.Equal("26", target.ContainerFromIndex(items.Count - 5).DataContext);
+                Assert.Equal("26", target.ContainerFromIndex(items.Count - 5)!.DataContext);
             }
         }
 
@@ -463,7 +465,7 @@ namespace Avalonia.Controls.UnitTests
                 Threading.Dispatcher.UIThread.RunJobs();
 
                 // First an item that is not index 0 must be selected.
-                _mouse.Click(target.Presenter.Panel.Children[1]);
+                _mouse.Click(target.Presenter!.Panel!.Children[1]);
 
                 Threading.Dispatcher.UIThread.RunJobs();
 
@@ -516,7 +518,7 @@ namespace Avalonia.Controls.UnitTests
 
                 lm.ExecuteInitialLayoutPass();
 
-                var panel = target.Presenter.Panel;
+                var panel = target.Presenter!.Panel!;
 
                 items.Add("Item 1");
                 target.Selection.Select(0);
@@ -640,7 +642,7 @@ namespace Avalonia.Controls.UnitTests
 
         private static void Layout(Control c)
         {
-            ((ILayoutRoot)c.GetVisualRoot()).LayoutManager.ExecuteLayoutPass();
+            ((ILayoutRoot)c.GetVisualRoot()!).LayoutManager.ExecuteLayoutPass();
         }
 
         private class Item
@@ -671,7 +673,7 @@ namespace Avalonia.Controls.UnitTests
             target.Bind(ComboBox.SelectedItemProperty, textObservable);
 
             Assert.True(DataValidationErrors.GetHasErrors(target));
-            Assert.True(DataValidationErrors.GetErrors(target).SequenceEqual(new[] { exception }));
+            Assert.Equal([exception], DataValidationErrors.GetErrors(target));
         }
 
         [Fact]
@@ -689,7 +691,7 @@ namespace Avalonia.Controls.UnitTests
 
             var realized = target.GetRealizedContainers()
                 .Cast<ListBoxItem>()
-                .Select(x => (string)x.DataContext)
+                .Select(x => (string?)x.DataContext)
                 .ToList();
 
             Assert.Equal(Enumerable.Range(0, 10).Select(x => $"Item{x}"), realized);
@@ -699,7 +701,7 @@ namespace Avalonia.Controls.UnitTests
 
             realized = target.GetRealizedContainers()
                 .Cast<ListBoxItem>()
-                .Select(x => (string)x.DataContext)
+                .Select(x => (string?)x.DataContext)
                 .ToList();
 
             Assert.Equal(Enumerable.Range(0, 10).Select(x => $"Item{99 - x}"), realized);
@@ -722,7 +724,7 @@ namespace Avalonia.Controls.UnitTests
 
             var realized = target.GetRealizedContainers()
                 .Cast<ListBoxItem>()
-                .Select(x => (string)x.DataContext)
+                .Select(x => (string?)x.DataContext)
                 .ToList();
 
             Assert.Equal(Enumerable.Range(0, 10).Select(x => $"Item{x}"), realized);
@@ -734,7 +736,7 @@ namespace Avalonia.Controls.UnitTests
 
             realized = target.GetRealizedContainers()
                 .Cast<ListBoxItem>()
-                .Select(x => (string)x.DataContext)
+                .Select(x => (string?)x.DataContext)
                 .ToList();
 
             // "Item1" should remain selected, and now be at the bottom of the viewport.
@@ -775,7 +777,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 Template = ListBoxTemplate(),
                 ItemsSource = items,
-                ItemsPanel = new FuncTemplate<Panel>(() => new VirtualizingStackPanel 
+                ItemsPanel = new FuncTemplate<Panel?>(() => new VirtualizingStackPanel
                 {
                     Orientation = Orientation.Horizontal 
                 }),
@@ -811,13 +813,13 @@ namespace Avalonia.Controls.UnitTests
             Prepare(target);
 
             RaiseKeyEvent(target, Key.Down);
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Down);
-            Assert.True(target.ContainerFromIndex(2).IsFocused);
+            Assert.True(target.ContainerFromIndex(2)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Up);
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
         }
 
         [Fact]
@@ -878,7 +880,7 @@ namespace Avalonia.Controls.UnitTests
             RaiseKeyEvent(target, Key.Down, KeyModifiers.Control);
 
             Assert.Equal(0, target.SelectedIndex);
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
         }
 
         [Fact]
@@ -899,16 +901,16 @@ namespace Avalonia.Controls.UnitTests
             Prepare(target);
 
             target.ContainerFromIndex(0)!.Focus();
-            target.Scroll.Offset = new Vector(0, 100);
+            target.Scroll!.Offset = new Vector(0, 100);
             Layout(target);
 
-            var panel = (VirtualizingStackPanel)target.ItemsPanelRoot;
+            var panel = (VirtualizingStackPanel)target.ItemsPanelRoot!;
             Assert.Equal(10, panel.FirstRealizedIndex);
 
             RaiseKeyEvent(target, Key.Down);
 
             Assert.Equal(1, target.SelectedIndex);
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
             Assert.Equal(new Vector(0, 10), target.Scroll.Offset);
         }
 
@@ -1184,6 +1186,7 @@ namespace Avalonia.Controls.UnitTests
             RaiseKeyEvent(button, Key.Tab);
 
             var item = target.ContainerFromIndex(1);
+            Assert.NotNull(item);
             Assert.Same(item, root.FocusManager.GetFocusedElement());
 
             RaiseKeyEvent(item, Key.Tab);
@@ -1340,33 +1343,33 @@ namespace Avalonia.Controls.UnitTests
                     new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
             }
 
-            public event NotifyCollectionChangedEventHandler CollectionChanged;
+            public event NotifyCollectionChangedEventHandler? CollectionChanged;
         }
 
         private class DataVirtualizingList : IList
         {
-            private readonly List<string> _inner = new(Enumerable.Repeat<string>(null, 100));
+            private readonly List<string?> _inner = new(Enumerable.Repeat<string?>(null, 100));
 
-            public object this[int index] 
+            public object? this[int index]
             { 
                 get => _inner[index] = $"Item{index}"; 
                 set => throw new NotSupportedException();
             }
 
-            public IEnumerable<string> GetRealizedItems() => _inner.Where(x => x is not null);
+            public IEnumerable<string> GetRealizedItems() => _inner.Where(x => x is not null)!;
             public bool IsFixedSize => true;
             public bool IsReadOnly => true;
             public int Count => _inner.Count;
             public bool IsSynchronized => false;
             public object SyncRoot => this;
-            public int Add(object value) => throw new NotSupportedException();
+            public int Add(object? value) => throw new NotSupportedException();
             public void Clear() => throw new NotSupportedException();
-            public bool Contains(object value) => throw new NotImplementedException();
+            public bool Contains(object? value) => throw new NotImplementedException();
             public void CopyTo(Array array, int index) => throw new NotImplementedException();
             public IEnumerator GetEnumerator() => _inner.GetEnumerator();
-            public int IndexOf(object value) => throw new NotImplementedException();
+            public int IndexOf(object? value) => throw new NotImplementedException();
-            public void Insert(int index, object value) => throw new NotSupportedException();
+            public void Insert(int index, object? value) => throw new NotSupportedException();
-            public void Remove(object value) => throw new NotSupportedException();
+            public void Remove(object? value) => throw new NotSupportedException();
             public void RemoveAt(int index) => throw new NotSupportedException();
         }
     }

tests/Avalonia.Controls.UnitTests/ListBoxTests_Multiple.cs

@@ -36,7 +36,7 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Shift);
+                _helper.Click(target.Presenter!.Panel!.Children[2], modifiers: KeyModifiers.Shift);
 
                 Assert.Equal(new[] { "Foo", "Bar", "Baz" }, target.SelectedItems);
                 Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
@@ -63,7 +63,7 @@ namespace Avalonia.Controls.UnitTests
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
 
-                SelectionChangedEventArgs receivedArgs = null;
+                SelectionChangedEventArgs? receivedArgs = null;
 
                 target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -81,22 +81,23 @@ namespace Avalonia.Controls.UnitTests
                     Assert.Empty(receivedArgs.AddedItems);
                 }
 
-                _helper.Click(target.Presenter.Panel.Children[1]);
+                var panel = target.Presenter!.Panel!;
+                _helper.Click(panel.Children[1]);
 
                 VerifyAdded("Bar");
 
                 receivedArgs = null;
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Control);
 
                 VerifyAdded("Baz");
 
                 receivedArgs = null;
-                _helper.Click(target.Presenter.Panel.Children[3], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[3], modifiers: KeyModifiers.Control);
 
                 VerifyAdded("Qux");
 
                 receivedArgs = null;
-                _helper.Click(target.Presenter.Panel.Children[1], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[1], modifiers: KeyModifiers.Control);
 
                 VerifyRemoved("Bar");
             }
@@ -120,15 +121,16 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[1]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[1]);
-                _helper.Click(target.Presenter.Panel.Children[3], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[3], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(1, target.SelectedIndex);
                 Assert.Equal("Bar", target.SelectedItem);
                 Assert.Equal(new[] { "Bar", "Baz", "Qux" }, target.SelectedItems);
 
-                _helper.Click(target.Presenter.Panel.Children[1], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[1], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(2, target.SelectedIndex);
                 Assert.Equal("Baz", target.SelectedItem);
@@ -154,13 +156,14 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[1]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[1]);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(1, target.SelectedIndex);
                 Assert.Equal("Bar", target.SelectedItem);
 
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(1, target.SelectedIndex);
                 Assert.Equal("Bar", target.SelectedItem);
@@ -185,13 +188,14 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
+                var panel = target.Presenter!.Panel!;
                 // Select first range
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                _helper.Click(panel.Children[0]);
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Shift);
 
                 // Select second range
-                _helper.Click(target.Presenter.Panel.Children[4], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[4], modifiers: KeyModifiers.Control);
-                _helper.Click(target.Presenter.Panel.Children[6], modifiers: KeyModifiers.Control | KeyModifiers.Shift);
+                _helper.Click(panel.Children[6], modifiers: KeyModifiers.Control | KeyModifiers.Shift);
 
                 Assert.Equal(new[] { "Foo", "Bar", "Baz", "Boo", "Far", "Faz" }, target.SelectedItems);
             }
@@ -215,10 +219,9 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[3]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[4], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[3]);
-
+                _helper.Click(panel.Children[4], modifiers: KeyModifiers.Control);
-                var panel = target.Presenter.Panel;
 
                 Assert.Equal(new[] { "Foo", "Bar" }, target.SelectedItems);
                 Assert.Equal(new[] { 3, 4 }, SelectedContainers(target));
@@ -243,10 +246,9 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[3]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[5], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[3]);
-
+                _helper.Click(panel.Children[5], modifiers: KeyModifiers.Shift);
-                var panel = target.Presenter.Panel;
 
                 Assert.Equal(new[] { "Foo", "Bar", "Baz" }, target.SelectedItems);
                 Assert.Equal(new[] { 3, 4, 5 }, SelectedContainers(target));
@@ -271,10 +273,9 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[5], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[0]);
-
+                _helper.Click(panel.Children[5], modifiers: KeyModifiers.Shift);
-                var panel = target.Presenter.Panel;
 
                 Assert.Equal(new[] { "Foo", "Bar", "Baz", "Foo", "Bar", "Baz" }, target.SelectedItems);
                 Assert.Equal(new[] { 0, 1, 2, 3, 4, 5 }, SelectedContainers(target));
@@ -299,7 +300,7 @@ namespace Avalonia.Controls.UnitTests
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
 
-                SelectionChangedEventArgs receivedArgs = null;
+                SelectionChangedEventArgs? receivedArgs = null;
 
                 target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -317,17 +318,18 @@ namespace Avalonia.Controls.UnitTests
                     Assert.Empty(receivedArgs.AddedItems);
                 }
 
-                _helper.Click(target.Presenter.Panel.Children[1]);
+                var panel = target.Presenter!.Panel!;
+                _helper.Click(panel.Children[1]);
 
                 VerifyAdded("Bar");
 
                 receivedArgs = null;
-                _helper.Click(target.Presenter.Panel.Children[3], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[3], modifiers: KeyModifiers.Shift);
 
                 VerifyAdded("Baz", "Qux");
 
                 receivedArgs = null;
-                _helper.Click(target.Presenter.Panel.Children[2], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[2], modifiers: KeyModifiers.Shift);
 
                 VerifyRemoved("Qux");
             }
@@ -351,19 +353,20 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                var panel = target.Presenter!.Panel!;
+                _helper.Click(panel.Children[0]);
 
                 Assert.Equal(new[] { "Foo" }, target.SelectedItems);
 
-                _helper.Click(target.Presenter.Panel.Children[4], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[4], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(new[] { "Foo", "Bar" }, target.SelectedItems);
 
-                _helper.Click(target.Presenter.Panel.Children[3], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[3], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(new[] { "Foo", "Bar", "Foo" }, target.SelectedItems);
 
-                _helper.Click(target.Presenter.Panel.Children[1], modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[1], modifiers: KeyModifiers.Control);
 
                 Assert.Equal(new[] { "Foo", "Bar", "Foo", "Bar" }, target.SelectedItems);
             }
@@ -389,9 +392,9 @@ namespace Avalonia.Controls.UnitTests
 
                 target.SelectAll();
 
-                Assert.Equal(3, target.SelectedItems.Count);
+                Assert.Equal(3, target.SelectedItems!.Count);
 
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                _helper.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(1, target.SelectedItems.Count);
                 Assert.Equal(new[] { "Foo", }, target.SelectedItems);
@@ -419,9 +422,9 @@ namespace Avalonia.Controls.UnitTests
 
                 target.SelectAll();
 
-                Assert.Equal(3, target.SelectedItems.Count);
+                Assert.Equal(3, target.SelectedItems!.Count);
 
-                _helper.Click(target.Presenter.Panel.Children[0], MouseButton.Right);
+                _helper.Click(target.Presenter!.Panel!.Children[0], MouseButton.Right);
 
                 Assert.Equal(3, target.SelectedItems.Count);
             }
@@ -446,13 +449,16 @@ namespace Avalonia.Controls.UnitTests
                 root.LayoutManager.ExecuteInitialLayoutPass();
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[1], modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[0]);
+                _helper.Click(panel.Children[1], modifiers: KeyModifiers.Shift);
 
+                Assert.NotNull(target.SelectedItems);
                 Assert.Equal(2, target.SelectedItems.Count);
 
-                _helper.Click(target.Presenter.Panel.Children[2], MouseButton.Right);
+                _helper.Click(panel.Children[2], MouseButton.Right);
 
+                Assert.NotNull(target.SelectedItems);
                 Assert.Equal(1, target.SelectedItems.Count);
             }
         }
@@ -477,9 +483,11 @@ namespace Avalonia.Controls.UnitTests
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
 
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[2], MouseButton.Right, modifiers: KeyModifiers.Shift);
+                _helper.Click(panel.Children[0]);
+                _helper.Click(panel.Children[2], MouseButton.Right, modifiers: KeyModifiers.Shift);
 
+                Assert.NotNull(target.SelectedItems);
                 Assert.Equal(1, target.SelectedItems.Count);
             }
         }
@@ -504,9 +512,11 @@ namespace Avalonia.Controls.UnitTests
 
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
 
-                _helper.Click(target.Presenter.Panel.Children[0]);
+                var panel = target.Presenter!.Panel!;
-                _helper.Click(target.Presenter.Panel.Children[2], MouseButton.Right, modifiers: KeyModifiers.Control);
+                _helper.Click(panel.Children[0]);
+                _helper.Click(panel.Children[2], MouseButton.Right, modifiers: KeyModifiers.Control);
 
+                Assert.NotNull(target.SelectedItems);
                 Assert.Equal(1, target.SelectedItems.Count);
             }
         }
@@ -532,19 +542,19 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Equal(new[] { "Foo", "Bar", }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Down, KeyModifiers.Shift);
 
             Assert.Equal(new[] { "Foo", "Bar", "Baz", }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(2).IsFocused);
+            Assert.True(target.ContainerFromIndex(2)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Up, KeyModifiers.Shift);
 
             Assert.Equal(new[] { "Foo", "Bar", }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
         }
 
         [Fact]
@@ -569,7 +579,7 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Equal(new[] { "Foo", "Bar", "Baz" }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1, 2 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(2).IsFocused);
+            Assert.True(target.ContainerFromIndex(2)!.IsFocused);
         }
 
         [Fact]
@@ -593,19 +603,19 @@ namespace Avalonia.Controls.UnitTests
 
             Assert.Equal(new[] { "Foo", "Bar" }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(1).IsFocused);
+            Assert.True(target.ContainerFromIndex(1)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Down, KeyModifiers.Control);
 
             Assert.Equal(new[] { "Foo", "Bar" }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(2).IsFocused);
+            Assert.True(target.ContainerFromIndex(2)!.IsFocused);
 
             RaiseKeyEvent(target, Key.Down, KeyModifiers.Shift);
 
             Assert.Equal(new[] { "Foo", "Bar", "Baz", "Qux" }, target.SelectedItems);
             Assert.Equal(new[] { 0, 1, 2, 3 }, SelectedContainers(target));
-            Assert.True(target.ContainerFromIndex(3).IsFocused);
+            Assert.True(target.ContainerFromIndex(3)!.IsFocused);
         }
 
         [Fact]
@@ -700,7 +710,7 @@ namespace Avalonia.Controls.UnitTests
 
         private static IEnumerable<int> SelectedContainers(SelectingItemsControl target)
         {
-            return target.Presenter.Panel.Children
+            return target.Presenter!.Panel!.Children
                 .Select(x => x.Classes.Contains(":selected") ? target.IndexFromContainer(x) : -1)
                 .Where(x => x != -1);
         }

tests/Avalonia.Controls.UnitTests/ListBoxTests_Single.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Prepare(target);
 
-                target.Presenter.Panel.Children[0].RaiseEvent(new GotFocusEventArgs
+                target.Presenter!.Panel!.Children[0].RaiseEvent(new GotFocusEventArgs
                 {
                     NavigationMethod = NavigationMethod.Tab,
                 });
@@ -57,7 +57,7 @@ namespace Avalonia.Controls.UnitTests
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
 
-                target.Presenter.Panel.Children[0].RaiseEvent(new GotFocusEventArgs
+                target.Presenter!.Panel!.Children[0].RaiseEvent(new GotFocusEventArgs
                 {
                     NavigationMethod = NavigationMethod.Directional,
                     KeyModifiers = KeyModifiers.Control
@@ -86,7 +86,7 @@ namespace Avalonia.Controls.UnitTests
                 };
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -104,7 +104,7 @@ namespace Avalonia.Controls.UnitTests
                     ItemsSource = new[] { "Foo", "Bar", "Baz " }
                 };
                 Prepare(target);
-                _pen.Down(target.Presenter.Panel.Children[0], MouseButton.Right);
+                _pen.Down(target.Presenter!.Panel!.Children[0], MouseButton.Right);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -122,7 +122,7 @@ namespace Avalonia.Controls.UnitTests
                     ItemsSource = new[] { "Foo", "Bar", "Baz " }
                 };
                 Prepare(target);
-                _pen.Down(target.Presenter.Panel.Children[0]);
+                _pen.Down(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(-1, target.SelectedIndex);
             }
@@ -156,7 +156,7 @@ namespace Avalonia.Controls.UnitTests
                 });
 
                 var pointer = type == PointerType.Mouse ? _mouse : _pen;
-                pointer.Click(target.Presenter.Panel.Children[0], MouseButton.Right, position: new Point(5, 5));
+                pointer.Click(target.Presenter!.Panel!.Children[0], MouseButton.Right, position: new Point(5, 5));
 
                 Assert.True(contextRaised);
                 Assert.Equal(0, target.SelectedIndex);
@@ -177,7 +177,7 @@ namespace Avalonia.Controls.UnitTests
                 Prepare(target);
                 target.SelectedIndex = 0;
 
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -197,7 +197,7 @@ namespace Avalonia.Controls.UnitTests
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
 
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -219,7 +219,7 @@ namespace Avalonia.Controls.UnitTests
                 Prepare(target);
                 target.SelectedIndex = 0;
 
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(-1, target.SelectedIndex);
             }
@@ -240,7 +240,7 @@ namespace Avalonia.Controls.UnitTests
                 Prepare(target);
                 target.SelectedIndex = 0;
 
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -261,7 +261,7 @@ namespace Avalonia.Controls.UnitTests
                 Prepare(target);
                 target.SelectedIndex = 1;
 
-                _mouse.Click(target.Presenter.Panel.Children[0]);
+                _mouse.Click(target.Presenter!.Panel!.Children[0]);
 
                 Assert.Equal(0, target.SelectedIndex);
             }
@@ -292,7 +292,7 @@ namespace Avalonia.Controls.UnitTests
         {
             using (UnitTestApplication.Start(TestServices.MockPlatformRenderInterface))
             {
-                var viewModel = new TestStackOverflowViewModel() { Items = new List<string> { "foo", "bar", "baz" } };
+                var viewModel = new TestStackOverflowViewModel() { Items = ["foo", "bar", "baz"] };
 
                 var target = new ListBox
                 {
@@ -316,17 +316,17 @@ namespace Avalonia.Controls.UnitTests
 
         private class TestStackOverflowViewModel : INotifyPropertyChanged
         {
-            public List<string> Items { get; set; }
+            public List<string?> Items { get; set; } = [];
 
             public int SetterInvokedCount { get; private set; }
 
             public const int MaxInvokedCount = 1000;
 
-            private string _selectedItem;
+            private string? _selectedItem;
 
-            public event PropertyChangedEventHandler PropertyChanged;
+            public event PropertyChangedEventHandler? PropertyChanged;
 
-            public string SelectedItem
+            public string? SelectedItem
             {
                 get { return _selectedItem; }
                 set

tests/Avalonia.Controls.UnitTests/MaskedTextBoxTests.cs

@@ -977,7 +977,7 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
-        private static IDisposable Start(TestServices services = null)
+        private static IDisposable Start(TestServices? services = null)
         {
             CultureInfo.CurrentCulture = CultureInfo.CurrentUICulture = CultureInfo.GetCultureInfo("en-US");
             return UnitTestApplication.Start((services ?? Services).With(assetLoader: new StandardAssetLoader()));
@@ -986,7 +986,7 @@ namespace Avalonia.Controls.UnitTests
         private class Class1 : NotifyingBase
         {
             private int _foo;
-            private string _bar;
+            private string? _bar;
 
             public int Foo
             {
@@ -994,7 +994,7 @@ namespace Avalonia.Controls.UnitTests
                 set { _foo = value; RaisePropertyChanged(); }
             }
 
-            public string Bar
+            public string? Bar
             {
                 get { return _bar; }
                 set { _bar = value; RaisePropertyChanged(); }
@@ -1005,7 +1005,7 @@ namespace Avalonia.Controls.UnitTests
         {
             private readonly ILayoutManager _layoutManager;
 
-            public TestTopLevel(ITopLevelImpl impl, ILayoutManager layoutManager = null)
+            public TestTopLevel(ITopLevelImpl impl, ILayoutManager? layoutManager = null)
                 : base(impl)
             {
                 _layoutManager = layoutManager ?? new LayoutManager(this);

tests/Avalonia.Controls.UnitTests/MenuItemTests.cs

@@ -17,7 +17,7 @@ namespace Avalonia.Controls.UnitTests
 {
     public class MenuItemTests : ScopedTestBase
     {
-        private Mock<IPopupImpl> popupImpl;
+        private Mock<IPopupImpl>? _popupImpl;
 
         [Fact]
         public void Header_Of_Minus_Should_Apply_Separator_Pseudoclass()
@@ -238,7 +238,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = new Panel { ContextMenu = contextMenu } };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
 
                 Assert.True(target.IsEffectivelyEnabled);
@@ -280,7 +280,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = button };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
                 Assert.True(target.IsEffectivelyEnabled);
                 target.Command = command;
@@ -321,7 +321,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = new Panel { ContextMenu = contextMenu } };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 contextMenu.Open();
 
                 Dispatcher.UIThread.RunJobs(DispatcherPriority.Loaded);
@@ -421,7 +421,7 @@ namespace Avalonia.Controls.UnitTests
             window.Show();
             window.LayoutManager.ExecuteInitialLayoutPass();
 
-            var panel = Assert.IsType<StackPanel>(menu.Presenter.Panel);
+            var panel = Assert.IsType<StackPanel>(menu.Presenter!.Panel);
             Assert.Equal(2, panel.Children.Count);
 
             for (var i = 0; i < panel.Children.Count; i++)
@@ -819,7 +819,7 @@ namespace Avalonia.Controls.UnitTests
         public void MenuItem_CommandParameter_Does_Not_Change_While_Execution()
         {
             var target = new MenuItem();
-            object lastParamenter = "A";
+            object? lastParamenter = "A";
             var generator = new Random();
             var onlyOnce = false;
             var command = new TestCommand(parameter =>
@@ -851,16 +851,16 @@ namespace Avalonia.Controls.UnitTests
             screenImpl.Setup(X => X.AllScreens).Returns(new[] { new Screen(1, screen, screen, true) });
 
             var windowImpl = MockWindowingPlatform.CreateWindowMock();
-            popupImpl = MockWindowingPlatform.CreatePopupMock(windowImpl.Object);
+            _popupImpl = MockWindowingPlatform.CreatePopupMock(windowImpl.Object);
-            popupImpl.SetupGet(x => x.RenderScaling).Returns(1);
+            _popupImpl.SetupGet(x => x.RenderScaling).Returns(1);
-            windowImpl.Setup(x => x.CreatePopup()).Returns(popupImpl.Object);
+            windowImpl.Setup(x => x.CreatePopup()).Returns(_popupImpl.Object);
 
             windowImpl.Setup(x => x.TryGetFeature(It.Is<Type>(t => t == typeof(IScreenImpl)))).Returns(screenImpl.Object);
 
             var services = TestServices.StyledWindow.With(
                 inputManager: new InputManager(),
                 windowImpl: windowImpl.Object,
-                windowingPlatform: new MockWindowingPlatform(() => windowImpl.Object, x => popupImpl.Object));
+                windowingPlatform: new MockWindowingPlatform(() => windowImpl.Object, x => _popupImpl.Object));
 
             return UnitTestApplication.Start(services);
         }
@@ -868,7 +868,7 @@ namespace Avalonia.Controls.UnitTests
 
         private record MenuViewModel(string Header)
         {
-            public IList<MenuViewModel> Children { get; set; }
+            public IList<MenuViewModel> Children { get; set; } = [];
         }
     }
 }

tests/Avalonia.Controls.UnitTests/NameScopeTests.cs

@@ -56,7 +56,8 @@ namespace Avalonia.Controls.UnitTests
          that the object graph is completely ready to use after it's built
          rather than have pending continuations queued by SynchronizationContext.
         */
-        object _found = null;
+        private object? _found;
+
         async void FindAsync(INameScope scope, string name)
         {
             _found = await scope.FindAsync(name);

tests/Avalonia.Controls.UnitTests/NumericUpDownTests.cs

@@ -5,7 +5,6 @@ using System.Linq;
 using System.Reactive.Subjects;
 using Avalonia.Controls.Templates;
 using Avalonia.Data;
-using Avalonia.Interactivity;
 using Avalonia.Threading;
 using Avalonia.UnitTests;
 using Xunit;
@@ -27,7 +26,7 @@ namespace Avalonia.Controls.UnitTests
                 Dispatcher.UIThread.RunJobs();
 
                 Assert.True(DataValidationErrors.GetHasErrors(control));
-                Assert.True(DataValidationErrors.GetErrors(control).SequenceEqual(new[] { exception }));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(control));
             });
         }
 
@@ -42,7 +41,7 @@ namespace Avalonia.Controls.UnitTests
                 Dispatcher.UIThread.RunJobs();
 
                 Assert.True(DataValidationErrors.GetHasErrors(control));
-                Assert.True(DataValidationErrors.GetErrors(control).SequenceEqual(new[] { exception }));
+                Assert.Equal([exception], DataValidationErrors.GetErrors(control));
             });
         }
 
@@ -101,19 +100,19 @@ namespace Avalonia.Controls.UnitTests
             });
         }
 
-        public static IEnumerable<object[]> Increment_Decrement_TestData()
+        public static IEnumerable<object?[]> Increment_Decrement_TestData()
         {
             // if min and max are not defined and value was null, 0 should be ne new value after spin
-            yield return new object[] { decimal.MinValue, decimal.MaxValue, null, SpinDirection.Decrease, 0m };
+            yield return [decimal.MinValue, decimal.MaxValue, null, SpinDirection.Decrease, 0m];
-            yield return new object[] { decimal.MinValue, decimal.MaxValue, null, SpinDirection.Increase, 0m };
+            yield return [decimal.MinValue, decimal.MaxValue, null, SpinDirection.Increase, 0m];
             
             // if no value was defined, but Min or Max are defined, use these as the new value
-            yield return new object[] { -400m, -200m, null, SpinDirection.Decrease, -200m };
+            yield return [-400m, -200m, null, SpinDirection.Decrease, -200m];
-            yield return new object[] { 200m, 400m, null, SpinDirection.Increase, 200m };
+            yield return [200m, 400m, null, SpinDirection.Increase, 200m];
             
             // Value should be clamped to Min / Max after spinning
-            yield return new object[] { 200m, 400m, 5m, SpinDirection.Increase, 200m };
+            yield return [200m, 400m, 5m, SpinDirection.Increase, 200m];
-            yield return new object[] { 200m, 400m, 200m, SpinDirection.Decrease, 200m };
+            yield return [200m, 400m, 200m, SpinDirection.Decrease, 200m];
         }
         
         private void RunTest(Action<NumericUpDown, TextBox> test)
@@ -125,7 +124,7 @@ namespace Avalonia.Controls.UnitTests
                 var window = new Window { Content = control };
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 Dispatcher.UIThread.RunJobs();
                 test.Invoke(control, textBox);
             }

tests/Avalonia.Controls.UnitTests/Platform/DefaultMenuInteractionHandlerTests.cs

@@ -2,10 +2,8 @@
 using Avalonia.Controls.Platform;
 using Avalonia.Controls.Primitives;
 using Avalonia.Input;
-using Avalonia.Input.GestureRecognizers;
 using Avalonia.Interactivity;
 using Avalonia.UnitTests;
-using Avalonia.VisualTree;
 using Moq;
 using Xunit;
 
@@ -575,7 +573,7 @@ namespace Avalonia.Controls.UnitTests.Platform
             bool isTopLevel = false,
             bool hasSubMenu = false,
             bool isSubMenuOpen = false,
-            IMenuElement parent = null)
+            IMenuElement? parent = null)
         {
             var mock = new Mock<Control>();
             var item = mock.As<IMenuItem>();
@@ -589,17 +587,18 @@ namespace Avalonia.Controls.UnitTests.Platform
 
         private class TestTimer
         {
-            private Action _action;
+            private Action? _action;
 
             public bool ActionIsQueued => _action != null;
 
             public void Pulse()
             {
+                Assert.NotNull(_action);
                 _action();
                 _action = null;
             }
 
-            public void RunOnce(Action action, TimeSpan timeSpan)
+            public void RunOnce(Action? action, TimeSpan timeSpan)
             {
                 if (_action != null)
                 {
@@ -614,14 +613,14 @@ namespace Avalonia.Controls.UnitTests.Platform
         {
             public int Id { get; } = Pointer.GetNextFreeId();
 
-            public void Capture(IInputElement control)
+            public void Capture(IInputElement? control)
             {
                 Captured = control;
             }
 
-            public IInputElement Captured { get; set; }
+            public IInputElement? Captured { get; set; }
-            public PointerType Type { get; }
+            public PointerType Type => PointerType.Mouse;
-            public bool IsPrimary { get; } = true;
+            public bool IsPrimary => true;
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_InTemplate.cs

@@ -85,6 +85,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.Content = "Foo";
 
             var child = target.Child;
+            Assert.NotNull(child);
             Assert.Equal(parent, child.GetLogicalParent());
             Assert.Equal(new[] { child }, parent.GetLogicalChildren());
         }
@@ -97,6 +98,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.Content = "Foo";
 
             var child = target.Child;
+            Assert.NotNull(child);
             Assert.Equal(target, child.GetVisualParent());
             Assert.Equal(new[] { child }, target.GetVisualChildren());
         }
@@ -289,7 +291,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             var canvas = new Canvas();
             var (target, host) = CreateTarget();
             var viewModel = new TestViewModel { Content = "foo" };
-            var dataContexts = new List<object>();
+            var dataContexts = new List<object?>();
 
             target.Bind(ContentPresenter.ContentProperty, (IBinding)new TemplateBinding(ContentControl.ContentProperty));
             canvas.GetObservable(ContentPresenter.DataContextProperty).Subscribe(x => dataContexts.Add(x));
@@ -302,7 +304,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             viewModel.Content = 42;
 
-            Assert.Equal(new object[]
+            Assert.Equal(new object?[]
             {
                 null,
                 "foo",
@@ -391,19 +393,19 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             templatedParent.ApplyTemplate();
 
-            return (templatedParent.Presenter, templatedParent);
+            return (templatedParent.Presenter!, templatedParent);
         }
 
         private class TestContentControl : ContentControl, IContentPresenterHost
         {
-            public Control Child { get; set; }
+            public Control? Child { get; set; }
         }
 
         private class TestViewModel : INotifyPropertyChanged
         {
-            private object _content;
+            private object? _content;
 
-            public object Content
+            public object? Content
             {
                 get => _content;
                 set
@@ -416,7 +418,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
                 }
             }
 
-            public event PropertyChangedEventHandler PropertyChanged;
+            public event PropertyChangedEventHandler? PropertyChanged;
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Presenters/ContentPresenterTests_Standalone.cs

@@ -109,7 +109,8 @@ namespace Avalonia.Controls.UnitTests.Presenters
             (contentControl as ISetLogicalParent).SetParent(parentMock.Object);
 
             contentControl.ApplyTemplate();
-            var target = contentControl.Presenter as ContentPresenter;
+            var target = contentControl.Presenter;
+            Assert.NotNull(target);
 
             contentControl.Content = "foo";
 
@@ -199,8 +200,8 @@ namespace Avalonia.Controls.UnitTests.Presenters
 
             logicalChildren = target.GetLogicalChildren();
 
-            Assert.Single(logicalChildren);
+            var logicalChild = Assert.Single(logicalChildren);
-            Assert.NotEqual(foo, logicalChildren.First());
+            Assert.NotEqual(foo, logicalChild);
         }
 
         [Fact]

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests.cs

@@ -446,7 +446,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.Arrange(new Rect(0, 0, 100, 100));
 
             // Border20 is at position 0,600 with bottom at Y=620
-            var border20 = target.FindControl<Border>("Border20");
+            var border20 = target.GetControl<Border>("Border20");
             target.BringDescendantIntoView(border20, new Rect(border20.Bounds.Size));
 
             // With viewport Height of 100, border becomes fully visible when alligned from the bottom at Offset Y=520, i.e. 620-100
@@ -458,7 +458,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.Arrange(new Rect(0, 0, 100, 100));
 
             // Border20 is at position 0,800 with bottom at Y=820
-            var border40 = target.FindControl<Border>("Border40");
+            var border40 = target.GetControl<Border>("Border40");
             target.BringDescendantIntoView(border40, new Rect(border40.Bounds.Size));
 
             // With viewport Height of 100, border becomes fully visible when alligned from the bottom at Offset Y=720, i.e. 820-100
@@ -502,7 +502,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
             target.UpdateChild();
             target.Measure(Size.Infinity);
             target.Arrange(new Rect(0, 0, 100, 100));
-            var border3 = target.FindControl<Border>("Border3");
+            var border3 = target.GetControl<Border>("Border3");
             target.BringDescendantIntoView(border3, new Rect(border3.Bounds.Size));
 
             // Border3 is still in view, offset hasn't changed

tests/Avalonia.Controls.UnitTests/Presenters/ScrollContentPresenterTests_ILogicalScrollable.cs

@@ -323,7 +323,7 @@ namespace Avalonia.Controls.UnitTests
             private Size _extent;
             private Vector _offset;
             private Size _viewport;
-            private EventHandler _scrollInvalidated;
+            private EventHandler? _scrollInvalidated;
 
             public bool CanHorizontallyScroll { get; set; }
             public bool CanVerticallyScroll { get; set; }
@@ -332,7 +332,7 @@ namespace Avalonia.Controls.UnitTests
 
             public bool HasScrollInvalidatedSubscriber => _scrollInvalidated != null;
             
-            public event EventHandler ScrollInvalidated
+            public event EventHandler? ScrollInvalidated
             {
                 add => _scrollInvalidated += value;
                 remove => _scrollInvalidated -= value;
@@ -400,7 +400,7 @@ namespace Avalonia.Controls.UnitTests
                 return new Size(150, 150);
             }
 
-            public Control GetControlInDirection(NavigationDirection direction, Control from)
+            public Control GetControlInDirection(NavigationDirection direction, Control? from)
             {
                 throw new NotImplementedException();
             }

tests/Avalonia.Controls.UnitTests/Presenters/TextPresenter_Tests.cs

@@ -72,7 +72,7 @@ namespace Avalonia.Controls.UnitTests.Presenters
                 Assert.Equal(1, presenter.TextLayout.TextLines.Count);
                 Assert.Equal(1, presenter.TextLayout.TextLines[0].TextRuns.Count);
                 Assert.NotNull(presenter.TextLayout.TextLines[0].TextRuns[0].Properties);
-                Assert.Equal(fontStretch, presenter.TextLayout.TextLines[0].TextRuns[0].Properties.Typeface.Stretch);
+                Assert.Equal(fontStretch, presenter.TextLayout.TextLines[0].TextRuns[0].Properties!.Typeface.Stretch);
             }
         }
     }

tests/Avalonia.Controls.UnitTests/Primitives/PopupRootTests.cs

@@ -35,7 +35,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             {
                 var target = CreateTarget(new Window());
 
-                Assert.True(((ILogical)target.Presenter).IsAttachedToLogicalTree);
+                Assert.True(((ILogical)target.Presenter!).IsAttachedToLogicalTree);
             }
         }
 
@@ -53,11 +53,11 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 target.ApplyTemplate();
-                target.Popup.Open();
+                target.Popup!.Open();
 
-                Assert.Equal(target.Popup, ((IStyleHost)target.Popup.Host).StylingParent);
+                Assert.Equal(target.Popup, ((IStyleHost)target.Popup.Host!).StylingParent);
             }
         }
 
@@ -74,10 +74,11 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 window.ApplyTemplate();
                 target.Open();
 
-               
-                Assert.Single(((Visual)target.Host).GetVisualChildren());
 
-                var templatedChild = ((Visual)target.Host).GetVisualChildren().Single();
+                var host = Assert.IsAssignableFrom<Visual>(target.Host);
+                Assert.Single(host.GetVisualChildren());
+
+                var templatedChild = host.GetVisualChildren().Single();
                 
                 Assert.IsType<LayoutTransformControl>(templatedChild);
 
@@ -93,8 +94,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 Assert.IsType<ContentPresenter>(contentPresenter);
                 
                 
-                Assert.Equal((PopupRoot)target.Host, ((Control)templatedChild).TemplatedParent);
+                Assert.Equal((PopupRoot)host, ((Control)templatedChild).TemplatedParent);
-                Assert.Equal((PopupRoot)target.Host, ((Control)contentPresenter).TemplatedParent);
+                Assert.Equal((PopupRoot)host, ((Control)contentPresenter).TemplatedParent);
             }
         }
         
@@ -107,7 +108,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 target.Open();
 
-                Assert.Null(((Visual)target.Host).GetVisualParent());
+                Assert.Null(((Visual)target.Host!).GetVisualParent());
             }
         }
         
@@ -180,9 +181,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
                 target.ApplyTemplate();
-                target.Popup.Open();
+                target.Popup!.Open();
                 target.PopupContent = null;
             }
         }
@@ -194,7 +195,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             {
                 var child = new ChildControl();
                 var window = new Window();
-                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl);
+                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl!);
                 popupImpl.Setup(x => x.MaxAutoSizeHint).Returns(new Size(1200, 1000));
                 var target = CreateTarget(window, popupImpl.Object);
                 
@@ -251,7 +252,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
                 var window = new Window();
-                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl);
+                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl!);
 
                 var child = new Canvas
                 {
@@ -280,7 +281,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
                 var window = new Window();
-                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl);
+                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl!);
 
                 var target = CreateTarget(window, popupImpl.Object);
                 target.MinWidth = 400;
@@ -296,7 +297,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 Assert.Equal(new Rect(0, 0, 400, 800), target.Bounds);
                 Assert.Equal(new Size(400, 800), target.ClientSize);
-                Assert.Equal(new Size(400, 800), target.PlatformImpl.ClientSize);
+                Assert.Equal(new Size(400, 800), target.PlatformImpl!.ClientSize);
             }
         }
 
@@ -307,7 +308,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             using (UnitTestApplication.Start(TestServices.StyledWindow))
             {
                 var window = new Window();
-                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl);
+                var popupImpl = MockWindowingPlatform.CreatePopupMock(window.PlatformImpl!);
                 var positioner = new Mock<IPopupPositioner>();
                 popupImpl.Setup(x => x.PopupPositioner).Returns(positioner.Object);
 
@@ -330,9 +331,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
         }
 
-        private static PopupRoot CreateTarget(TopLevel popupParent, IPopupImpl impl = null)
+        private static PopupRoot CreateTarget(TopLevel popupParent, IPopupImpl? impl = null)
         {
-            impl ??= popupParent.PlatformImpl.CreatePopup();
+            impl ??= popupParent.PlatformImpl!.CreatePopup()!;
 
             var result = new PopupRoot(popupParent, impl)
             {
@@ -351,8 +352,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class TemplatedControlWithPopup : Avalonia.Controls.Primitives.TemplatedControl
         {
-            public static readonly StyledProperty<Control> PopupContentProperty =
+            public static readonly StyledProperty<Control?> PopupContentProperty =
-                AvaloniaProperty.Register<TemplatedControlWithPopup, Control>(nameof(PopupContent));
+                AvaloniaProperty.Register<TemplatedControlWithPopup, Control?>(nameof(PopupContent));
 
             public TemplatedControlWithPopup()
             {
@@ -364,9 +365,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
                     });
             }
 
-            public Popup Popup { get; private set; }
+            public Popup? Popup { get; private set; }
 
-            public Control PopupContent
+            public Control? PopupContent
             {
                 get => GetValue(PopupContentProperty);
                 set => SetValue(PopupContentProperty, value);

tests/Avalonia.Controls.UnitTests/Primitives/PopupTests.cs

@@ -185,7 +185,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             {
                 var target = new Popup();
 
-                Assert.Null(((Visual)target.Host));
+                Assert.Null((Visual)target.Host!);
             }
         }
 
@@ -198,7 +198,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 target.Open();
 
-                Assert.Equal(target, ((Visual)target.Host).Parent);
+                Assert.Equal(target, ((Visual)target.Host!).Parent);
                 Assert.Equal(target, ((Visual)target.Host).GetLogicalParent());
             }
         }
@@ -213,7 +213,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 target.Open();
 
-                var popupRoot = (ILogical)((Visual)target.Host);
+                var popupRoot = (ILogical)(Visual)target.Host!;
 
                 Assert.True(popupRoot.IsAttachedToLogicalTree);
                 root.Content = null;
@@ -332,7 +332,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var popup = (Popup)target.GetTemplateChildren().First(x => x.Name == "popup");
                 popup.Open();
 
-                var popupRoot = (Control)popup.Host;
+                var popupRoot = (Control)popup.Host!;
                 popupRoot.Measure(Size.Infinity);
                 popupRoot.Arrange(new Rect(popupRoot.DesiredSize));
 
@@ -375,7 +375,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 if (UsePopupHost)
                 {
                     Assert.Equal(
-                        new object[]
+                        new object?[]
                         {
                             popupRoot,
                             popupRoot,
@@ -388,7 +388,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 else
                 {
                     Assert.Equal(
-                        new object[]
+                        new object?[]
                         {
                             popupRoot,
                             popupRoot,
@@ -423,7 +423,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var popup = (Popup)target.GetTemplateChildren().First(x => x.Name == "popup");
                 popup.Open();
 
-                var popupRoot = (Control)popup.Host;
+                var popupRoot = (Control)popup.Host!;
                 popupRoot.Measure(Size.Infinity);
                 popupRoot.Arrange(new Rect(popupRoot.DesiredSize));
 
@@ -468,7 +468,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 if (UsePopupHost)
                 {
                     Assert.Equal(
-                        new object[]
+                        new object?[]
                         {
                             popupRoot,
                             popupRoot,
@@ -482,7 +482,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 else
                 {
                     Assert.Equal(
-                        new object[]
+                        new object?[]
                         {
                             popupRoot,
                             popupRoot,
@@ -518,7 +518,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var popup = (Popup)target.GetTemplateChildren().First(x => x.Name == "popup");
                 popup.Open();
 
-                var popupRoot = (Control)popup.Host;
+                var popupRoot = (Control)popup.Host!;
                 popupRoot.Measure(Size.Infinity);
                 popupRoot.Arrange(new Rect(popupRoot.DesiredSize));
 
@@ -621,6 +621,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
                 var e = CreatePointerPressedEventArgs(window, new Point(10, 15));
                 var overlay = LightDismissOverlayLayer.GetLightDismissOverlayLayer(window);
+                Assert.NotNull(overlay);
                 overlay.RaiseEvent(e);
 
                 Assert.Equal(1, raised);
@@ -747,6 +748,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 p.Close();
 
                 var focusManager = window.FocusManager;
+                Assert.NotNull(focusManager);
                 var focus = focusManager.GetFocusedElement();
                 Assert.Same(window, focus);
             }
@@ -787,6 +789,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 windowTB.Focus();
 
                 var focusManager = window.FocusManager;
+                Assert.NotNull(focusManager);
                 var focus = focusManager.GetFocusedElement();
 
                 Assert.True(focus == windowTB);
@@ -1167,7 +1170,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var ev = new PointerPressedEventArgs(
                     popupContent,
                     pointer,
-                    popupContent.VisualRoot as PopupRoot,
+                    (PopupRoot)popupContent.VisualRoot!,
                     new Point(50 , 50),
                     0,
                     new PointerPointProperties(RawInputModifiers.None, PointerUpdateKind.LeftButtonPressed),
@@ -1231,6 +1234,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var root = PreparedWindow(windowContent);
 
                 var adornerLayer = AdornerLayer.GetAdornerLayer(adorned);
+                Assert.NotNull(adornerLayer);
                 adornerLayer.Children.Add(adorner);
                 AdornerLayer.SetAdornedElement(adorner, adorned);
 
@@ -1286,9 +1290,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }
         }
 
-        private static PopupRoot CreateRoot(TopLevel popupParent, IPopupImpl impl = null)
+        private static PopupRoot CreateRoot(TopLevel popupParent, IPopupImpl? impl = null)
         {
-            impl ??= popupParent.PlatformImpl.CreatePopup();
+            impl ??= popupParent.PlatformImpl!.CreatePopup()!;
 
             var result = new PopupRoot(popupParent, impl)
             {
@@ -1382,7 +1386,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             }, null);
         }
 
-        private static Window PreparedWindow(object content = null)
+        private static Window PreparedWindow(object? content = null)
         {
             var w = new Window { Content = content };
             w.Show();
@@ -1424,9 +1428,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class TestControl : Decorator
         {
-            public event EventHandler DataContextBeginUpdate;
+            public event EventHandler? DataContextBeginUpdate;
 
-            public new AvaloniaObject InheritanceParent => base.InheritanceParent;
+            public new AvaloniaObject? InheritanceParent => base.InheritanceParent;
 
             protected override void OnDataContextBeginUpdate()
             {

tests/Avalonia.Controls.UnitTests/Primitives/RangeBaseTests.cs

@@ -116,7 +116,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 Value = 50
             };
 
-            Track track = null;
+            Track? track = null;
 
             var target = new TestRange()
             {
@@ -157,7 +157,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             target.Bind(TestRange.ValueProperty, new Binding("Value") { Mode = BindingMode.TwoWay });
 
             target.ApplyTemplate();
-            track.Measure(new Size(100, 0));
+            track!.Measure(new Size(100, 0));
             track.Arrange(new Rect(0, 0, 100, 0));
 
             Assert.Equal(1, viewModel.SetterInvokedCount);
@@ -218,7 +218,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             private double _value;
 
-            public event PropertyChangedEventHandler PropertyChanged;
+            public event PropertyChangedEventHandler? PropertyChanged;
 
             public double Value
             {

tests/Avalonia.Controls.UnitTests/Primitives/ScrollBarTests.cs

@@ -81,7 +81,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                         Vector = new Vector(0, 0)
                     };
 
-                    track.Thumb.RaiseEvent(ev);
+                    track.Thumb!.RaiseEvent(ev);
                 });
 
             Assert.Equal(ScrollEventType.ThumbTrack, raisedEvent.Arguments.ScrollEventType);
@@ -110,7 +110,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                         Vector = new Vector(0, 0)
                     };
 
-                    track.Thumb.RaiseEvent(ev);
+                    track.Thumb!.RaiseEvent(ev);
                 });
 
             Assert.Equal(ScrollEventType.EndScroll, raisedEvent.Arguments.ScrollEventType);

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests.cs

@@ -266,7 +266,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             foreach (var v in selected)
             {
-                listBox.SelectedItems.Add(v);
+                listBox.SelectedItems!.Add(v);
             }
 
             listBox.BeginInit();
@@ -289,7 +289,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             foreach (var v in new[] { "bar", "baz" })
             {
-                listBox.SelectedItems.Add(v);
+                listBox.SelectedItems!.Add(v);
             }
 
             listBox.BeginInit();
@@ -629,7 +629,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(items[1], target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
 
-            SelectionChangedEventArgs receivedArgs = null;
+            SelectionChangedEventArgs? receivedArgs = null;
 
             target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -669,7 +669,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(items[1], target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
 
-            SelectionChangedEventArgs receivedArgs = null;
+            SelectionChangedEventArgs? receivedArgs = null;
 
             target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -704,7 +704,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(items[0], target.SelectedItem);
             Assert.Equal(0, target.SelectedIndex);
 
-            SelectionChangedEventArgs receivedArgs = null;
+            SelectionChangedEventArgs? receivedArgs = null;
 
             target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -741,7 +741,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(items[1], target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
 
-            SelectionChangedEventArgs receivedArgs = null;
+            SelectionChangedEventArgs? receivedArgs = null;
 
             target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -773,7 +773,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             Assert.Equal(items[1], target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
 
-            SelectionChangedEventArgs receivedArgs = null;
+            SelectionChangedEventArgs? receivedArgs = null;
 
             target.SelectionChanged += (_, args) => receivedArgs = args;
 
@@ -1295,9 +1295,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
             };
 
             root.ApplyTemplate();
-            root.Presenter.ApplyTemplate();
+            root.Presenter!.ApplyTemplate();
             nested.ApplyTemplate();
-            nested.Presenter.ApplyTemplate();
+            nested.Presenter!.ApplyTemplate();
 
             Assert.Equal(0, root.SelectedIndex);
             Assert.Equal(1, nested.SelectedIndex);
@@ -1342,7 +1342,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 var container = target.ContainerFromIndex(1)!;
                 _helper.Down(container);
 
-                var panel = target.Presenter.Panel;
+                var panel = target.Presenter!.Panel;
 
                 Assert.Same(container, KeyboardNavigation.GetTabOnceActiveElement(target));
             }
@@ -1363,12 +1363,11 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
 
-                _helper.Down(target.Presenter.Panel.Children[1]);
+                var panel = target.Presenter!.Panel!;
+                _helper.Down(panel.Children[1]);
 
                 items.RemoveAt(1);
 
-                var panel = target.Presenter.Panel;
-
                 Assert.Null(KeyboardNavigation.GetTabOnceActiveElement((InputElement)panel));
             }
         }
@@ -1459,7 +1458,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 };
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
-                _helper.Down((Interactive)target.Presenter.Panel.Children[3]);
+                _helper.Down((Interactive)target.Presenter!.Panel!.Children[3]);
 
                 Assert.Equal(3, target.SelectedIndex);
             }
@@ -1477,7 +1476,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 };
                 AvaloniaLocator.CurrentMutable.Bind<PlatformHotkeyConfiguration>().ToConstant(new PlatformHotkeyConfiguration());
                 Prepare(target);
-                _helper.Down((Interactive)target.Presenter.Panel.Children[3]);
+                _helper.Down((Interactive)target.Presenter!.Panel!.Children[3]);
 
                 Assert.Equal(new[] { ":pressed", ":selected" }, target.Presenter.Panel.Children[3].Classes);
             }
@@ -1592,7 +1591,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             Prepare(target);
             other.ApplyTemplate();
-            other.Presenter.ApplyTemplate();
+            other.Presenter!.ApplyTemplate();
 
             items.Add("Foo");
 
@@ -1706,8 +1705,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 root.Measure(new Size(100, 100));
                 root.Arrange(new Rect(0, 0, 100, 100));
 
-                Assert.True(target.Presenter.Panel.Children.Count > 0);
+                var panel = target.Presenter!.Panel!;
-                Assert.True(target.Presenter.Panel.Children.Count < 100);
+                Assert.True(panel.Children.Count > 0);
+                Assert.True(panel.Children.Count < 100);
 
                 target.SelectedItem = "Item99";
 
@@ -1716,7 +1716,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 Layout(target);
 
                 Assert.Equal(0, target.SelectedIndex);
-                Assert.Equal(1, target.Presenter.Panel.Children.Where(x => x.IsVisible).Count());
+                Assert.Equal(1, panel.Children.Count(x => x.IsVisible));
             }
         }
 
@@ -1743,7 +1743,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             Prepare(target);
 
-            var root = (TestRoot)target.Parent;
+            var root = (TestRoot)target.Parent!;
 
             target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
 
@@ -1777,7 +1777,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             Prepare(target);
 
-            var root = (TestRoot)target.Parent;
+            var root = (TestRoot)target.Parent!;
 
             target.AddHandler(Control.RequestBringIntoViewEvent, (s, e) => raised = true);
 
@@ -2555,7 +2555,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private static void Layout(Control c)
         {
-            ((ILayoutRoot)c.GetVisualRoot()).LayoutManager.ExecuteLayoutPass();
+            ((ILayoutRoot)c.GetVisualRoot()!).LayoutManager.ExecuteLayoutPass();
         }
 
         private static FuncControlTemplate Template()
@@ -2573,7 +2573,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class Item : Control, ISelectable
         {
-            public string Value { get; set; }
+            public string? Value { get; set; }
 
             public bool IsSelected 
             {
@@ -2584,9 +2584,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class MasterViewModel : NotifyingBase
         {
-            private ChildViewModel _child;
+            private ChildViewModel? _child;
 
-            public ChildViewModel Child
+            public ChildViewModel? Child
             {
                 get { return _child; }
                 set
@@ -2599,15 +2599,15 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class ChildViewModel : NotifyingBase
         {
-            public IList<Item> Items { get; set; }
+            public IList<Item> Items { get; set; } = [];
-            public Item SelectedItem { get; set; }
+            public Item? SelectedItem { get; set; }
             public int SelectedIndex { get; set; }
         }
 
         private class SelectionViewModel : NotifyingBase
         {
             private int _selectedIndex = -1;
-            private object _selectedItem;
+            private object? _selectedItem;
 
             public SelectionViewModel()
             {
@@ -2625,7 +2625,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 }
             }
 
-            public object SelectedItem
+            public object? SelectedItem
             {
                 get => _selectedItem;
                 set
@@ -2647,7 +2647,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class TestSelector : SelectingItemsControl
         {
-            public new static readonly DirectProperty<SelectingItemsControl, IList> SelectedItemsProperty =
+            public new static readonly DirectProperty<SelectingItemsControl, IList?> SelectedItemsProperty =
                 SelectingItemsControl.SelectedItemsProperty;
 
             public TestSelector()
@@ -2666,7 +2666,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
                 set => base.Selection = value;
             }
 
-            public new IList SelectedItems
+            public new IList? SelectedItems
             {
                 get => base.SelectedItems;
                 set => base.SelectedItems = value;
@@ -2700,11 +2700,9 @@ namespace Avalonia.Controls.UnitTests.Primitives
                     new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
             }
 
-            public event NotifyCollectionChangedEventHandler CollectionChanged;
+            public event NotifyCollectionChangedEventHandler? CollectionChanged;
         }
 
-#nullable enable
-
         private sealed class FullSelectionViewModel : NotifyingBase
         {
             private ItemModel? _selectedItem;

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_AutoSelect.cs

@@ -111,7 +111,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             };
 
             target.ApplyTemplate();
-            target.Presenter.ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
             items.RemoveAt(0);
 
             Assert.Equal(0, target.SelectedIndex);
@@ -138,7 +138,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
         private class ResetOnAdd : List<string>, INotifyCollectionChanged
         {
-            public event NotifyCollectionChangedEventHandler CollectionChanged;
+            public event NotifyCollectionChangedEventHandler? CollectionChanged;
 
             public new void Add(string item)
             {

tests/Avalonia.Controls.UnitTests/Primitives/SelectingItemsControlTests_SelectedValue.cs

@@ -319,7 +319,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
         public void Handles_Null_SelectedItem_When_SelectedValueBinding_Assigned()
         {
             // Issue #11220
-            var items = new object[] { null };
+            var items = new object?[] { null };
             var sic = new SelectingItemsControl
             {
                 ItemsSource = items,
@@ -366,15 +366,15 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
     internal class TestClass
     {
-        public TestClass(string name, string alt)
+        public TestClass(string? name, string? alt)
         {
             Name = name;
             AltProperty = alt;
         }
 
-        public string Name { get; set; }
+        public string? Name { get; set; }
 
-        public string AltProperty { get; set; }
+        public string? AltProperty { get; set; }
 
         public static List<TestClass> GetItems()
         {

tests/Avalonia.Controls.UnitTests/Primitives/TabStripTests.cs

@@ -93,7 +93,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             Assert.Equal(0, target.SelectedIndex);
             Assert.Same(target.Items[0], target.SelectedItem);
-            Assert.Same("first", ((TabItem)target.SelectedItem).Name);
+            Assert.Same("first", ((TabItem)target.SelectedItem!).Name);
         }
 
         private Control CreateTabStripTemplate(TabStrip parent, INameScope scope)

tests/Avalonia.Controls.UnitTests/Primitives/TemplatedControlTests.cs

@@ -242,8 +242,8 @@ namespace Avalonia.Controls.UnitTests.Primitives
 
             var border = contentControl.GetTemplateChildren().OfType<Border>().Single();
             var presenter = contentControl.GetTemplateChildren().OfType<ContentPresenter>().Single();
-            var decorator = (Decorator)presenter.Content;
+            var decorator = (Decorator)presenter.Content!;
-            var textBlock = (TextBlock)decorator.Child;
+            var textBlock = (TextBlock)decorator.Child!;
 
             Assert.Equal(target, contentControl.TemplatedParent);
             Assert.Equal(contentControl, border.TemplatedParent);
@@ -289,7 +289,7 @@ namespace Avalonia.Controls.UnitTests.Primitives
             target.ApplyTemplate();
 
             var decorator = (Decorator)target.GetVisualChildren().Single();
-            var border = (Border)decorator.Child;
+            var border = (Border)decorator.Child!;
 
             Assert.Equal(target, decorator.TemplatedParent);
             Assert.Equal(target, border.TemplatedParent);

tests/Avalonia.Controls.UnitTests/ScrollViewerTests.cs

@@ -28,7 +28,7 @@ namespace Avalonia.Controls.UnitTests
 
             InitializeScrollViewer(target);
 
-            Assert.IsType<TextBlock>(target.Presenter.Child);
+            Assert.IsType<TextBlock>(target.Presenter!.Child);
         }
 
         [Fact]
@@ -473,7 +473,7 @@ namespace Avalonia.Controls.UnitTests
         public void MenuScrollBar_Should_Be_Visible_When_Specified_Visible()
         {
             Converters.MenuScrollingVisibilityConverter converter = Converters.MenuScrollingVisibilityConverter.Instance;
-            IList<object> args = new List<object> {ScrollBarVisibility.Visible,400d,1800d,500d};
+            var args = new List<object?> {ScrollBarVisibility.Visible,400d,1800d,500d};
             var result = converter.Convert(args, typeof(ScrollBarVisibility), "0", System.Globalization.CultureInfo.CurrentCulture);
             Assert.Equal(true, result);
         }
@@ -590,7 +590,7 @@ namespace Avalonia.Controls.UnitTests
                     [!!Track.ValueProperty] = scrollBar[!!RangeBase.ValueProperty],
                     [!Track.ViewportSizeProperty] = scrollBar[!ScrollBar.ViewportSizeProperty],
                     [!Track.OrientationProperty] = scrollBar[!ScrollBar.OrientationProperty],
-                    [!Track.DeferThumbDragProperty] = scrollBar.TemplatedParent[!ScrollViewer.IsDeferredScrollingEnabledProperty],
+                    [!Track.DeferThumbDragProperty] = scrollBar.TemplatedParent![!ScrollViewer.IsDeferredScrollingEnabledProperty],
                     Thumb = new Thumb
                     {
                         Template = new FuncControlTemplate<Thumb>(CreateThumbTemplate),
@@ -620,7 +620,7 @@ namespace Avalonia.Controls.UnitTests
         {
             target.ApplyTemplate();
 
-            var presenter = (ScrollContentPresenter)target.Presenter;
+            var presenter = (ScrollContentPresenter)target.Presenter!;
             presenter.AttachToScrollViewer();
             presenter.UpdateChild();
         }

tests/Avalonia.Controls.UnitTests/SelectableTextBlockTests.cs

@@ -26,12 +26,12 @@ namespace Avalonia.Controls.UnitTests
                     FontSize = 20
                 };
 
-                target.Inlines.Add(run);
+                target.Inlines!.Add(run);
 
                 target.Measure(Size.Infinity);
 
                 target.SelectionStart = 0;
-                target.SelectionEnd = run.Text.Length;
+                target.SelectionEnd = run.Text!.Length;
 
                 target.Measure(Size.Infinity);
 

tests/Avalonia.Controls.UnitTests/Selection/InternalSelectionModelTests.cs

@@ -147,7 +147,7 @@ namespace Avalonia.Controls.UnitTests.Selection
             var target = CreateTarget(nullSource: true);
 
             target.SelectRange(1, 2);
-            Assert.Equal(new object[] { null, null }, target.WritableSelectedItems);
+            Assert.Equal(new object?[] { null, null }, target.WritableSelectedItems);
         }
 
         [Fact]
@@ -156,7 +156,7 @@ namespace Avalonia.Controls.UnitTests.Selection
             var target = CreateTarget(nullSource: true);
 
             target.SelectRange(1, 2);
-            Assert.Equal(new object[] { null, null }, target.WritableSelectedItems);
+            Assert.Equal(new object?[] { null, null }, target.WritableSelectedItems);
 
             target.Source = new[] { "foo", "bar", "baz" };
             Assert.Equal(new[] { "bar", "baz" }, target.WritableSelectedItems);
@@ -171,7 +171,7 @@ namespace Avalonia.Controls.UnitTests.Selection
             Assert.Equal(new[] { "bar", "baz" }, target.WritableSelectedItems);
 
             target.Source = null;
-            Assert.Equal(new object[] { null, null }, target.WritableSelectedItems);
+            Assert.Equal(new object?[] { null, null }, target.WritableSelectedItems);
         }
 
         [Fact]
@@ -218,7 +218,7 @@ namespace Avalonia.Controls.UnitTests.Selection
 
             target.SelectedIndex = 1;
 
-            var changed = new List<string>();
+            var changed = new List<string?>();
 
             target.PropertyChanged += (s, e) => changed.Add(e.PropertyName);
 
@@ -248,7 +248,7 @@ namespace Avalonia.Controls.UnitTests.Selection
 
             Assert.Equal(0, target.SelectedIndex);
 
-            items.Reset(new string[] { "baz", "foo", "bar" });
+            items.Reset(["baz", "foo", "bar"]);
 
             Assert.Equal("foo", target.SelectedItem);
             Assert.Equal(1, target.SelectedIndex);
@@ -269,7 +269,7 @@ namespace Avalonia.Controls.UnitTests.Selection
 
         private static InternalSelectionModel CreateTarget(
             bool singleSelect = false,
-            IList source = null,
+            IList? source = null,
             bool nullSource = false)
         {
             source ??= !nullSource ? new[] { "foo", "bar", "baz" } : null;
@@ -299,7 +299,7 @@ namespace Avalonia.Controls.UnitTests.Selection
                     new NotifyCollectionChangedEventArgs(NotifyCollectionChangedAction.Reset));
             }
 
-            public event NotifyCollectionChangedEventHandler CollectionChanged;
+            public event NotifyCollectionChangedEventHandler? CollectionChanged;
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Shapes/PathTests.cs

@@ -88,6 +88,7 @@ namespace Avalonia.Controls.UnitTests.Shapes
 
             target.Measure(new Size(500, 500));
 
+            Assert.NotNull(target.RenderedGeometry);
             Assert.Null(target.RenderedGeometry.Transform);
         }
 
@@ -109,12 +110,15 @@ namespace Avalonia.Controls.UnitTests.Shapes
             target.Measure(new Size(500, 500));
             target.Arrange(new Rect(0, 0, 500, 500));
 
+            Assert.NotNull(target.RenderedGeometry);
+
             if (expectedScaleX == 1 && expectedScaleY == 1)
             {
                 Assert.Null(target.RenderedGeometry.Transform);
             }
             else
             {
+                Assert.NotNull(target.RenderedGeometry.Transform);
                 Assert.Equal(Matrix.CreateScale(expectedScaleX, expectedScaleY), target.RenderedGeometry.Transform.Value);
             }
         }
@@ -135,6 +139,8 @@ namespace Avalonia.Controls.UnitTests.Shapes
             target.Arrange(new Rect(0, 0, 400, 400));
 
             Assert.Equal(new Rect(0, 0, 100, 200), geometry.Rect);
+            Assert.NotNull(target.RenderedGeometry);
+            Assert.NotNull(target.RenderedGeometry.Transform);
             Assert.Equal(Matrix.CreateScale(2, 2), target.RenderedGeometry.Transform.Value);
             Assert.Equal(new Rect(0, 0, 400, 400), target.Bounds);
         }
@@ -153,6 +159,8 @@ namespace Avalonia.Controls.UnitTests.Shapes
             target.Measure(new Size(200, 200));
             target.Arrange(new Rect(0, 0, 200, 200));
 
+            Assert.NotNull(target.RenderedGeometry);
+            Assert.NotNull(target.RenderedGeometry.Transform);
             Assert.Equal(Matrix.CreateScale(2, 2), target.RenderedGeometry.Transform.Value);
 
             target.Measure(new Size(300, 300));
@@ -173,6 +181,8 @@ namespace Avalonia.Controls.UnitTests.Shapes
 
             target.Measure(new Size(200, 200));
             target.Arrange(new Rect(0, 0, 200, 200));
+            Assert.NotNull(target.RenderedGeometry);
+            Assert.NotNull(target.RenderedGeometry.Transform);
             Assert.Equal(Matrix.CreateScale(2, 2), target.RenderedGeometry.Transform.Value);
 
             target.Arrange(new Rect(0, 0, 300, 300));

tests/Avalonia.Controls.UnitTests/SplitButtonTests.cs

@@ -13,7 +13,7 @@ public class SplitButtonTests : ScopedTestBase
     public void SplitButton_CommandParameter_Does_Not_Change_While_Execution()
     {
         var target = new SplitButton();
-        object lastParamenter = "A";
+        object? lastParamenter = "A";
         var generator = new Random();
         var command = new TestCommand(parameter =>
         {
@@ -56,12 +56,12 @@ public class SplitButtonTests : ScopedTestBase
         }
     }
 
-    private static KeyEventArgs CreateKeyDownEvent(Key key, Interactive source = null)
+    private static KeyEventArgs CreateKeyDownEvent(Key key, Interactive? source = null)
     {
         return new KeyEventArgs { RoutedEvent = InputElement.KeyDownEvent, Key = key, Source = source };
     }
 
-    private static KeyEventArgs CreateKeyUpEvent(Key key, Interactive source = null)
+    private static KeyEventArgs CreateKeyUpEvent(Key key, Interactive? source = null)
     {
         return new KeyEventArgs { RoutedEvent = InputElement.KeyUpEvent, Key = key, Source = source };
     }

tests/Avalonia.Controls.UnitTests/SplitViewTests.cs

@@ -160,7 +160,7 @@ namespace Avalonia.Controls.UnitTests
             splitView.IsPaneOpen = true;
 
             splitView.RaiseEvent(new PointerReleasedEventArgs(splitView,
-                null, wnd, new Point(1270, 30), 0,
+                null!, wnd, new Point(1270, 30), 0,
                 new PointerPointProperties(),
                 KeyModifiers.None,
                 MouseButton.Left));
@@ -172,7 +172,7 @@ namespace Avalonia.Controls.UnitTests
             splitView.IsPaneOpen = true;
 
             splitView.RaiseEvent(new PointerReleasedEventArgs(splitView,
-                null, wnd, new Point(1270, 30), 0,
+                null!, wnd, new Point(1270, 30), 0,
                 new PointerPointProperties(),
                 KeyModifiers.None,
                 MouseButton.Left));
@@ -207,7 +207,7 @@ namespace Avalonia.Controls.UnitTests
             splitView.IsPaneOpen = true;
 
             clickBorder.RaiseEvent(new PointerReleasedEventArgs(splitView,
-                null, wnd, new Point(5, 5), 0,
+                null!, wnd, new Point(5, 5), 0,
                 new PointerPointProperties(),
                 KeyModifiers.None,
                 MouseButton.Left));
@@ -322,7 +322,7 @@ namespace Avalonia.Controls.UnitTests
             splitView.IsPaneOpen = true;
 
             splitView.RaiseEvent(new PointerReleasedEventArgs(splitView,
-                null, wnd, new Point(1270, 30), 0,
+                null!, wnd, new Point(1270, 30), 0,
                 new PointerPointProperties(),
                 KeyModifiers.None,
                 MouseButton.Left));
@@ -336,7 +336,7 @@ namespace Avalonia.Controls.UnitTests
             splitView.DisplayMode = SplitViewDisplayMode.Inline;
 
             splitView.RaiseEvent(new PointerReleasedEventArgs(splitView,
-                null, wnd, new Point(1270, 30), 0,
+                null!, wnd, new Point(1270, 30), 0,
                 new PointerPointProperties(),
                 KeyModifiers.None,
                 MouseButton.Left));

tests/Avalonia.Controls.UnitTests/TabControlTests.cs

@@ -281,8 +281,8 @@ namespace Avalonia.Controls.UnitTests
 
             ApplyTemplate(target);
 
-            target.ContentPart.UpdateChild();
+            target.ContentPart!.UpdateChild();
-            var dataContext = ((TextBlock)target.ContentPart.Child).DataContext;
+            var dataContext = ((TextBlock)target.ContentPart.Child!).DataContext;
             Assert.Equal(items[0], dataContext);
 
             target.SelectedIndex = 1;
@@ -337,7 +337,7 @@ namespace Avalonia.Controls.UnitTests
                 .Select(x => x.Header)
                 .ToList();
 
-            Assert.Equal(new object[] { null, null }, result);
+            Assert.Equal(new object?[] { null, null }, result);
         }
 
         [Fact]
@@ -358,7 +358,7 @@ namespace Avalonia.Controls.UnitTests
 
             target.SelectedIndex = 2;
 
-            var page = (TabItem)target.SelectedItem;
+            var page = Assert.IsType<TabItem>(target.SelectedItem);
 
             Assert.Null(page.Content);
         }
@@ -376,7 +376,7 @@ namespace Avalonia.Controls.UnitTests
             var root = new TestRoot(target);
 
             ApplyTemplate(target);
-            target.ContentPart.UpdateChild();
+            target.ContentPart!.UpdateChild();
 
             var content = Assert.IsType<TextBlock>(target.ContentPart.Child);
             Assert.Equal("bar", content.Tag);
@@ -395,7 +395,7 @@ namespace Avalonia.Controls.UnitTests
             var root = new TestRoot(target);
 
             ApplyTemplate(target);
-            ((ContentPresenter)target.ContentPart).UpdateChild();
+            target.ContentPart!.UpdateChild();
 
             Assert.Equal(null, Assert.IsType<TextBlock>(target.ContentPart.Child).Tag);
 
@@ -473,7 +473,7 @@ namespace Avalonia.Controls.UnitTests
     <TabControl Name='tabs' ItemsSource='{Binding Tabs}'/>
 </Window>";
                 var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
-                var tabControl = window.FindControl<TabControl>("tabs");
+                var tabControl = window.GetControl<TabControl>("tabs");
 
                 tabControl.DataContext = new { Tabs = new List<string>() };
                 window.ApplyTemplate();
@@ -601,6 +601,7 @@ namespace Avalonia.Controls.UnitTests
             RaiseKeyEvent(button, Key.Tab);
 
             var item = target.ContainerFromIndex(1);
+            Assert.NotNull(item);
             Assert.Same(item, root.FocusManager.GetFocusedElement());
 
             RaiseKeyEvent(item, Key.Tab);
@@ -747,7 +748,7 @@ namespace Avalonia.Controls.UnitTests
                         new TabItem { Header = "_Disabled", IsEnabled = false },
                     }
                 };
-                kd.SetFocusedElement((TabItem)tabControl.Items[selectedTabIndex], NavigationMethod.Unspecified, KeyModifiers.None);
+                kd.SetFocusedElement((TabItem?)tabControl.Items[selectedTabIndex], NavigationMethod.Unspecified, KeyModifiers.None);
                 
                 var root = new TestTopLevel(impl.Object)
                 {
@@ -756,7 +757,7 @@ namespace Avalonia.Controls.UnitTests
                 };
 
                 root.ApplyTemplate();
-                root.Presenter.UpdateChild();
+                root.Presenter!.UpdateChild();
                 ApplyTemplate(tabControl);
 
                 KeyDown(root, Key.LeftAlt);
@@ -847,7 +848,7 @@ namespace Avalonia.Controls.UnitTests
             private readonly ILayoutManager _layoutManager;
             public bool IsClosed { get; private set; }
 
-            public TestTopLevel(ITopLevelImpl impl, ILayoutManager layoutManager = null)
+            public TestTopLevel(ITopLevelImpl impl, ILayoutManager? layoutManager = null)
                 : base(impl)
             {
                 _layoutManager = layoutManager ?? new LayoutManager(this);
@@ -877,7 +878,7 @@ namespace Avalonia.Controls.UnitTests
         {
             target.ApplyTemplate();
 
-            target.Presenter.ApplyTemplate();
+            target.Presenter!.ApplyTemplate();
 
             foreach (var tabItem in target.GetLogicalChildren().OfType<TabItem>())
             {
@@ -885,10 +886,10 @@ namespace Avalonia.Controls.UnitTests
 
                 tabItem.ApplyTemplate();
 
-                tabItem.Presenter.UpdateChild();
+                tabItem.Presenter!.UpdateChild();
             }
 
-            target.ContentPart.ApplyTemplate();
+            target.ContentPart!.ApplyTemplate();
         }
 
         private class Item

tests/Avalonia.Controls.UnitTests/TextBlockTests.cs

@@ -141,7 +141,7 @@ namespace Avalonia.Controls.UnitTests
 
                 Assert.True(target.IsMeasureValid);
 
-                target.Inlines.Add(new Run("Hello"));
+                target.Inlines!.Add(new Run("Hello"));
 
                 Assert.False(target.IsMeasureValid);
 
@@ -177,7 +177,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 var target = new TextBlock();
 
-                target.Inlines.Add(new TextBox { Text = "Hello"});
+                target.Inlines!.Add(new TextBox { Text = "Hello"});
 
                 target.Measure(Size.Infinity);
 
@@ -201,7 +201,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var textBox = new TextBox { Text = "Hello", Template = TextBoxTests.CreateTemplate() };
 
-                target.Inlines.Add(textBox);
+                target.Inlines!.Add(textBox);
 
                 target.Measure(Size.Infinity);
 
@@ -228,7 +228,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var inline = new Run("Hello");
 
-                target.Inlines.Add(inline);
+                target.Inlines!.Add(inline);
 
                 target.Measure(Size.Infinity);
 
@@ -268,7 +268,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var run = new Run("Hello");
 
-                target.Inlines.Add(run);
+                target.Inlines!.Add(run);
 
                 target.Measure(Size.Infinity);
 
@@ -308,7 +308,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 var target = new TextBlock();
 
-                target.Inlines.Add(new Border());
+                target.Inlines!.Add(new Border());
 
                 target.Measure(Size.Infinity);
 
@@ -331,7 +331,7 @@ namespace Avalonia.Controls.UnitTests
 
                 var run = new InlineUIContainer(control);
 
-                target.Inlines.Add(run);
+                target.Inlines!.Add(run);
 
                 target.Measure(Size.Infinity);
 
@@ -402,7 +402,7 @@ namespace Avalonia.Controls.UnitTests
                 Image imageControl = new Image { Source = image };
                 InlineUIContainer container = new InlineUIContainer(imageControl);
 
-                target.Inlines.Add(new Run("The child should not be limited by position on line."));
+                target.Inlines!.Add(new Run("The child should not be limited by position on line."));
                 target.Inlines.Add(container);
 
                 target.Measure(new Size(100, 100));
@@ -420,7 +420,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 var target = new TextBlock();
 
-                target.Inlines.Add(new Run("Hello World"));
+                target.Inlines!.Add(new Run("Hello World"));
 
                 Assert.Equal(null, target.Text);
 
@@ -441,7 +441,7 @@ namespace Avalonia.Controls.UnitTests
             {
                 var target = new TextBlock();
 
-                target.Inlines.Add(new Run("Hello World"));
+                target.Inlines!.Add(new Run("Hello World"));
 
                 Assert.Equal(1, target.Inlines.Count);
 

tests/Avalonia.Controls.UnitTests/TextBoxTests_DataValidation.cs

@@ -59,9 +59,10 @@ namespace Avalonia.Controls.UnitTests
                 Assert.Null(DataValidationErrors.GetErrors(target));
                 target.Text = "20";
 
-                IEnumerable<object> errors = DataValidationErrors.GetErrors(target);
+                var errors = DataValidationErrors.GetErrors(target);
-                Assert.Single(errors);
+                Assert.NotNull(errors);
-                Assert.IsType<InvalidOperationException>(errors.Single());
+                var error = Assert.Single(errors);
+                Assert.IsType<InvalidOperationException>(error);
                 target.Text = "1";
                 Assert.Null(DataValidationErrors.GetErrors(target));
             }
@@ -84,9 +85,9 @@ namespace Avalonia.Controls.UnitTests
 
                 target.Text = "20";
 
-                IEnumerable<object> errors = DataValidationErrors.GetErrors(target);
+                var errors = DataValidationErrors.GetErrors(target);
-                Assert.Single(errors);
+                Assert.NotNull(errors);
-                var error = Assert.IsType<string>(errors.Single());
+                var error = Assert.IsType<string>(Assert.Single(errors));
                 Assert.StartsWith("Error: ", error);
             }
         }
@@ -121,7 +122,7 @@ namespace Avalonia.Controls.UnitTests
                 new ClrPropertyInfo(
                     nameof(ExceptionTest.LessThan10),
                     target => ((ExceptionTest)target).LessThan10,
-                    (target, value) => ((ExceptionTest)target).LessThan10 = (int)value,
+                    (target, value) => ((ExceptionTest)target).LessThan10 = (int)value!,
                     typeof(int)),
                 PropertyInfoAccessorFactory.CreateInpcPropertyAccessor)
             .Build();
@@ -215,13 +216,13 @@ namespace Avalonia.Controls.UnitTests
 
             public bool HasErrors => _lessThan10 >= 10;
 
-            public event EventHandler<DataErrorsChangedEventArgs> ErrorsChanged;
+            public event EventHandler<DataErrorsChangedEventArgs>? ErrorsChanged;
 
-            public IEnumerable GetErrors(string propertyName)
+            public IEnumerable GetErrors(string? propertyName)
             {
-                IList<string> result;
+                if (propertyName is not null && _errors.TryGetValue(propertyName, out var result))
-                _errors.TryGetValue(propertyName, out result);
+                    return result;
-                return result;
+                return Array.Empty<string?>();
             }
         }
     }

tests/Avalonia.Controls.UnitTests/TimePickerTests.cs

@@ -49,15 +49,11 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = timePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                Grid container = null;
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
-                container = (desc.ElementAt(1) as Button).Content as Grid;
-                Assert.True(container != null);
 
-                var periodTextHost = container.Children[6] as Border;
+                var periodTextHost = Assert.IsAssignableFrom<Border>(container.Children[6]);
-                Assert.True(periodTextHost != null);
                 Assert.True(periodTextHost.IsVisible);
 
                 timePicker.ClockIdentifier = "24HourClock";
@@ -79,15 +75,11 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = timePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                Grid container = null;
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
 
-                container = (desc.ElementAt(1) as Button).Content as Grid;
+                var periodTextHost = Assert.IsAssignableFrom<Border>(container.Children[4]);
-                Assert.True(container != null);
-
-                var periodTextHost = container.Children[4] as Border;
-                Assert.True(periodTextHost != null);
                 Assert.True(periodTextHost.IsVisible);
 
                 timePicker.UseSeconds = false;
@@ -117,9 +109,7 @@ namespace Avalonia.Controls.UnitTests
                 Assert.True(desc.ElementAt(2) is Popup);
                 var popup = (Popup)desc.ElementAt(2);
 
-                Assert.True(popup.Child is TimePickerPresenter);
+                var timePickerPresenter = Assert.IsAssignableFrom<TimePickerPresenter>(popup.Child);
-                var timePickerPresenter = (TimePickerPresenter)popup.Child;
-
                 var panel = (Panel)timePickerPresenter.VisualChildren[0];
                 var acceptBtn = (Button)panel.VisualChildren[0];
 
@@ -169,22 +159,16 @@ namespace Avalonia.Controls.UnitTests
 
                 var desc = timePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1);//Should be layoutroot grid & button
-                Grid container = null;
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
-                container = (desc.ElementAt(1) as Button).Content as Grid;
-                Assert.True(container != null);
 
-                var hourTextHost = container.Children[0] as Border;
+                var hourTextHost = Assert.IsAssignableFrom<Border>(container.Children[0]);
-                Assert.True(hourTextHost != null);
+                var hourText = Assert.IsAssignableFrom<TextBlock>(hourTextHost.Child);
-                var hourText = hourTextHost.Child as TextBlock;
+                var minuteTextHost = Assert.IsAssignableFrom<Border>(container.Children[2]);
-                var minuteTextHost = container.Children[2] as Border;
+                var minuteText = Assert.IsAssignableFrom<TextBlock>(minuteTextHost.Child);
-                Assert.True(minuteTextHost != null);
+                var secondTextHost = Assert.IsAssignableFrom<Border>(container.Children[4]);
-                var minuteText = minuteTextHost.Child as TextBlock;
+                var secondText = Assert.IsAssignableFrom<TextBlock>(secondTextHost.Child);
-                var secondTextHost = container.Children[4] as Border;
-                Assert.True(secondTextHost != null);
-                var secondText = secondTextHost.Child as TextBlock;
 
                 TimeSpan ts = TimeSpan.FromHours(10);
                 timePicker.SelectedTime = ts;
@@ -214,15 +198,11 @@ namespace Avalonia.Controls.UnitTests
                 var desc = timePicker.GetVisualDescendants();
                 Assert.True(desc.Count() > 1); //Should be layoutroot grid & button
 
-                Assert.True(desc.ElementAt(1) is Button);
+                var button = Assert.IsAssignableFrom<Button>(desc.ElementAt(1));
-
+                var container = Assert.IsAssignableFrom<Grid>(button.Content);
-                var container = (desc.ElementAt(1) as Button).Content as Grid;
-                Assert.True(container != null);
 
-                var periodTextHost = container.Children[6] as Border;
+                var periodTextHost = Assert.IsAssignableFrom<Border>(container.Children[6]);
-                Assert.NotNull(periodTextHost);
+                var periodText = Assert.IsAssignableFrom<TextBlock>(periodTextHost.Child);
-                var periodText = periodTextHost.Child as TextBlock;
-                Assert.NotNull(periodTextHost);
 
                 TimeSpan ts = TimeSpan.FromHours(10);
                 timePicker.SelectedTime = ts;

tests/Avalonia.Controls.UnitTests/ToolTipTests.cs

@@ -24,6 +24,7 @@ namespace Avalonia.Controls.UnitTests
         protected override void VerifyToolTipType(Control control)
         {
             var toolTip = control.GetValue(ToolTip.ToolTipProperty);
+            Assert.NotNull(toolTip);
             Assert.IsType<PopupRoot>(toolTip.PopupHost);
             Assert.Same(toolTip.VisualRoot, toolTip.PopupHost);
         }
@@ -38,7 +39,7 @@ namespace Avalonia.Controls.UnitTests
             _toolTipOpenSubscription = ToolTip.IsOpenProperty.Changed.Subscribe(new AnonymousObserver<AvaloniaPropertyChangedEventArgs<bool>>(e =>
             {
                 if (e.Sender is Visual { VisualRoot: {} root } visual)
-                    OverlayLayer.GetOverlayLayer(visual).Measure(root.ClientSize);
+                    OverlayLayer.GetOverlayLayer(visual)!.Measure(root.ClientSize);
             }));
         }
 
@@ -59,6 +60,7 @@ namespace Avalonia.Controls.UnitTests
         protected override void VerifyToolTipType(Control control)
         {
             var toolTip = control.GetValue(ToolTip.ToolTipProperty);
+            Assert.NotNull(toolTip);
             Assert.IsType<OverlayPopupHost>(toolTip.PopupHost);
             Assert.Same(toolTip.VisualRoot, control.VisualRoot);
         }
@@ -164,10 +166,10 @@ namespace Avalonia.Controls.UnitTests
                 SetupWindowAndActivateToolTip(target);
 
                 AssertToolTipOpen(target);
-                Assert.Equal("Tip", target.GetValue(ToolTip.ToolTipProperty).Content);
+                Assert.Equal("Tip", target.GetValue(ToolTip.ToolTipProperty)?.Content);
 
                 ToolTip.SetTip(target, "Tip1");
-                Assert.Equal("Tip1", target.GetValue(ToolTip.ToolTipProperty).Content);
+                Assert.Equal("Tip1", target.GetValue(ToolTip.ToolTipProperty)?.Content);
             }
         }
 
@@ -211,7 +213,7 @@ namespace Avalonia.Controls.UnitTests
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 Assert.Empty(toolTip.Classes);
             }
@@ -234,7 +236,7 @@ namespace Avalonia.Controls.UnitTests
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 ToolTip.SetIsOpen(decorator, true);
 
@@ -263,7 +265,7 @@ namespace Avalonia.Controls.UnitTests
 
                 window.ApplyStyling();
                 window.ApplyTemplate();
-                window.Presenter.ApplyTemplate();
+                window.Presenter!.ApplyTemplate();
 
                 ToolTip.SetIsOpen(decorator, true);
                 AssertToolTipOpen(decorator);
@@ -436,7 +438,7 @@ namespace Avalonia.Controls.UnitTests
                 [ToolTip.ShowDelayProperty] = 0
             };
 
-            var eventsOrder = new List<(string eventName, object sender, object source)>();
+            var eventsOrder = new List<(string eventName, object? sender, object? source)>();
 
             ToolTip.AddToolTipOpeningHandler(target,
                 (sender, args) => eventsOrder.Add(("Opening", sender, args.Source)));
@@ -454,7 +456,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.Equal(
                 new[]
                 {
-                    ("Opening", (object)target, (object)target),
+                    ("Opening", (object?)target, (object?)target),
                     ("Closing", target, target)
                 },
                 eventsOrder);
@@ -522,14 +524,14 @@ namespace Avalonia.Controls.UnitTests
                 AssertToolTipOpen(target);
 
                 var topLevel = TopLevel.GetTopLevel(target);
-                topLevel.PlatformImpl.Input(new RawPointerEventArgs(s_mouseDevice, (ulong)DateTime.Now.Ticks, topLevel, 
+                topLevel!.PlatformImpl!.Input!(new RawPointerEventArgs(s_mouseDevice, (ulong)DateTime.Now.Ticks, topLevel,
                     RawPointerEventType.LeaveWindow, default(RawPointerPoint), RawInputModifiers.None));
 
                 Assert.False(ToolTip.GetIsOpen(target));
             }
         }
 
-        private Action<Control> SetupWindowAndGetMouseEnterAction(Control windowContent, [CallerMemberName] string testName = null)
+        private Action<Control?> SetupWindowAndGetMouseEnterAction(Control windowContent, [CallerMemberName] string? testName = null)
         {
             var windowImpl = MockWindowingPlatform.CreateWindowMock();
             SetupWindowMock(windowImpl);
@@ -545,7 +547,7 @@ namespace Avalonia.Controls.UnitTests
 
             window.ApplyStyling();
             window.ApplyTemplate();
-            window.Presenter.ApplyTemplate();
+            window.Presenter!.ApplyTemplate();
             window.Show();
 
             Assert.True(windowContent.IsAttachedToVisualTree);
@@ -553,7 +555,7 @@ namespace Avalonia.Controls.UnitTests
             Assert.True(windowContent.IsVisible);
 
             var controlIds = new Dictionary<Control, int>();
-            IInputRoot lastRoot = null;
+            IInputRoot? lastRoot = null;
 
             return control =>
             {
@@ -575,15 +577,15 @@ namespace Avalonia.Controls.UnitTests
                 hitTesterMock.Setup(m => m.HitTestFirst(point, window, It.IsAny<Func<Visual, bool>>()))
                     .Returns(control);
 
-                var root = (IInputRoot)control?.VisualRoot ?? window;
+                var root = (IInputRoot?)control?.VisualRoot ?? window;
                 var timestamp = (ulong)DateTime.Now.Ticks;
 
-                windowImpl.Object.Input(new RawPointerEventArgs(s_mouseDevice, timestamp, root,
+                windowImpl.Object.Input!(new RawPointerEventArgs(s_mouseDevice, timestamp, root,
                         RawPointerEventType.Move, point, RawInputModifiers.None));
 
                 if (lastRoot != null && lastRoot != root)
                 {
-                    ((TopLevel)lastRoot).PlatformImpl?.Input(new RawPointerEventArgs(s_mouseDevice, timestamp, 
+                    ((TopLevel)lastRoot).PlatformImpl?.Input!(new RawPointerEventArgs(s_mouseDevice, timestamp,
                         lastRoot, RawPointerEventType.LeaveWindow, new Point(-1,-1), RawInputModifiers.None));
                 }
 
@@ -593,7 +595,7 @@ namespace Avalonia.Controls.UnitTests
             };
         }
 
-        private void SetupWindowAndActivateToolTip(Control windowContent, Control targetOverride = null, [CallerMemberName] string testName = null) =>
+        private void SetupWindowAndActivateToolTip(Control windowContent, Control? targetOverride = null, [CallerMemberName] string? testName = null) =>
             SetupWindowAndGetMouseEnterAction(windowContent, testName)(targetOverride ?? windowContent);
     }
 

tests/Avalonia.Controls.UnitTests/TopLevelTests.cs

@@ -142,7 +142,7 @@ namespace Avalonia.Controls.UnitTests
 
                 // The user has resized the window, so we can no longer auto-size.
                 var target = new TestTopLevel(impl.Object);
-                impl.Object.Resized(new Size(100, 200), WindowResizeReason.Unspecified);
+                impl.Object.Resized!(new Size(100, 200), WindowResizeReason.Unspecified);
 
                 Assert.Equal(100, target.Width);
                 Assert.Equal(200, target.Height);
@@ -160,7 +160,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new TestTopLevel(impl.Object);
                 target.Closed += (s, e) => raised = true;
 
-                impl.Object.Closed();
+                impl.Object.Closed!();
 
                 Assert.True(raised);
             }
@@ -184,7 +184,7 @@ namespace Avalonia.Controls.UnitTests
                     ++raised;
                 };
 
-                impl.Object.Closed();
+                impl.Object.Closed!();
 
                 Assert.Equal(1, raised);
             }
@@ -215,7 +215,7 @@ namespace Avalonia.Controls.UnitTests
                     PhysicalKey.A,
                     "a");
 
-                impl.Object.Input(input);
+                impl.Object.Input!(input);
 
                 inputManagerMock.Verify(x => x.ProcessInput(input));
             }
@@ -234,7 +234,7 @@ namespace Avalonia.Controls.UnitTests
                 target.Content = child;
                 target.ApplyTemplate();
 
-                Assert.Throws<InvalidOperationException>(() => target.Presenter.ApplyTemplate());
+                Assert.Throws<InvalidOperationException>(() => target.Presenter!.ApplyTemplate());
             }
         }
 
@@ -248,7 +248,7 @@ namespace Avalonia.Controls.UnitTests
                 var raised = false;
 
                 target.ResourcesChanged += (_, __) => raised = true;
-                Application.Current.Resources.Add("foo", "bar");
+                Application.Current!.Resources.Add("foo", "bar");
 
                 Assert.True(raised);
             }
@@ -264,7 +264,7 @@ namespace Avalonia.Controls.UnitTests
                 var layoutManager = new Mock<ILayoutManager>();
                 var target = new TestTopLevel(impl.Object, layoutManager.Object);
 
-                impl.Object.Closed();
+                impl.Object.Closed!();
 
                 layoutManager.Verify(x => x.Dispose());
             }
@@ -297,12 +297,13 @@ namespace Avalonia.Controls.UnitTests
                     }
                 };
 
-                Application.Current.Styles.Add(style);
+                var application = Application.Current!;
+                application.Styles.Add(style);
                 target.LayoutManager.ExecuteInitialLayoutPass();
 
                 Assert.Equal(new Thickness(2), child.BorderThickness);
 
-                Application.Current.Styles.Remove(style);
+                application.Styles.Remove(style);
 
                 Assert.Equal(new Thickness(0), child.BorderThickness);
             }
@@ -333,7 +334,7 @@ namespace Avalonia.Controls.UnitTests
             private readonly ILayoutManager _layoutManager;
             public bool IsClosed { get; private set; }
 
-            public TestTopLevel(ITopLevelImpl impl, ILayoutManager layoutManager = null)
+            public TestTopLevel(ITopLevelImpl impl, ILayoutManager? layoutManager = null)
                 : base(impl)
             {
                 _layoutManager = layoutManager ?? new LayoutManager(this);

tests/Avalonia.Controls.UnitTests/Utils/AncestorFinderTests.cs

@@ -1,11 +1,6 @@
 using System;
-using System.Collections.Generic;
-using System.Linq;
-using System.Text;
-using System.Threading.Tasks;
 using Avalonia.Controls.Utils;
 using Avalonia.UnitTests;
-using Avalonia.VisualTree;
 using Xunit;
 
 namespace Avalonia.Controls.UnitTests.Utils
@@ -20,7 +15,7 @@ namespace Avalonia.Controls.UnitTests.Utils
             var grandParent = new Border();
             var grandParent2 = new Border();
 
-            StyledElement currentParent = null;
+            StyledElement? currentParent = null;
             var subscription = AncestorFinder.Create(child, typeof (Border)).Subscribe(s => currentParent = s);
 
             Assert.Null(currentParent);

tests/Avalonia.Controls.UnitTests/Utils/CollectionChangedEventManagerTests.cs

@@ -80,7 +80,7 @@ namespace Avalonia.Controls.UnitTests.Utils
 
             public void Add(string s) => _inner.Add(s);
 
-            public event NotifyCollectionChangedEventHandler CollectionChanged
+            public event NotifyCollectionChangedEventHandler? CollectionChanged
             {
                 add => _inner.CollectionChanged += value;
                 remove => _inner.CollectionChanged -= value;

tests/Avalonia.Controls.UnitTests/Utils/HotKeyManagerTests.cs

@@ -6,7 +6,7 @@ using Avalonia.Input.Raw;
 using Avalonia.Platform;
 using Avalonia.UnitTests;
 using Xunit;
-using Factory = System.Func<int, System.Action<object>, Avalonia.Controls.Window, Avalonia.AvaloniaObject>;
+using Factory = System.Func<int, System.Action<object?>?, Avalonia.Controls.Window, Avalonia.AvaloniaObject>;
 
 namespace Avalonia.Controls.UnitTests.Utils
 {
@@ -28,7 +28,7 @@ namespace Avalonia.Controls.UnitTests.Utils
                 tl.Content = button;
                 tl.Template = CreateWindowTemplate();
                 tl.ApplyTemplate();
-                tl.Presenter.ApplyTemplate();
+                tl.Presenter!.ApplyTemplate();
 
                 HotKeyManager.SetHotKey(button, gesture1);
 
@@ -66,7 +66,7 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 var gesture = new KeyGesture(Key.A, KeyModifiers.Control);
 
-                var action = new Action<object>(parameter =>
+                var action = new Action<object?>(parameter =>
                 {
                     if (parameter is int value)
                     {
@@ -79,7 +79,7 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 root.Template = CreateWindowTemplate();
                 root.ApplyTemplate();
-                root.Presenter.ApplyTemplate();
+                root.Presenter!.ApplyTemplate();
 
                 HotKeyManager.SetHotKey(element, gesture);
 
@@ -110,19 +110,20 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 var gesture = new KeyGesture(Key.A, KeyModifiers.Control);
 
-                var action = new Action<object>(parameter =>
+                var action = new Action<object?>(parameter =>
                 {
                     isExecuted = true;
                 });
 
                 var root = new Window();
-                var element = factory(0, action, root) as InputElement;
+                var element = (InputElement)factory(0, action, root);
+                Assert.NotNull(element);
 
                 element.IsEnabled = false;
 
                 root.Template = CreateWindowTemplate();
                 root.ApplyTemplate();
-                root.Presenter.ApplyTemplate();
+                root.Presenter!.ApplyTemplate();
 
                 HotKeyManager.SetHotKey(element, gesture);
 
@@ -152,13 +153,13 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 var gesture = new KeyGesture(Key.A, KeyModifiers.Control);
 
-                void Clickable_Click(object sender, Interactivity.RoutedEventArgs e)
+                void Clickable_Click(object? sender, Interactivity.RoutedEventArgs e)
                 {
                     clickExecutedCount++;
                 }
 
                 var root = new Window();
-                var element = factory(0, default, root) as InputElement;
+                var element = (InputElement)factory(0, null, root);
                 if (element is IClickableControl clickable)
                 {
                     clickable.Click += Clickable_Click;
@@ -166,7 +167,7 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 root.Template = CreateWindowTemplate();
                 root.ApplyTemplate();
-                root.Presenter.ApplyTemplate();
+                root.Presenter!.ApplyTemplate();
 
                 HotKeyManager.SetHotKey(element, gesture);
 
@@ -209,18 +210,18 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 var gesture = new KeyGesture(Key.A, KeyModifiers.Control);
 
-                void DoExecute(object parameter)
+                void DoExecute(object? parameter)
                 {
                     commandExecutedCount++;
                 }
 
-                void Clickable_Click(object sender, Interactivity.RoutedEventArgs e)
+                void Clickable_Click(object? sender, Interactivity.RoutedEventArgs e)
                 {
                     clickExecutedCount++;
                 }
 
                 var root = new Window();
-                var element = factory(0, DoExecute, root) as InputElement;
+                var element = (InputElement)factory(0, DoExecute, root);
                 if (element is IClickableControl clickable)
                 {
                     clickable.Click += Clickable_Click;
@@ -228,7 +229,7 @@ namespace Avalonia.Controls.UnitTests.Utils
 
                 root.Template = CreateWindowTemplate();
                 root.ApplyTemplate();
-                root.Presenter.ApplyTemplate();
+                root.Presenter!.ApplyTemplate();
 
                 HotKeyManager.SetHotKey(element, gesture);
 
@@ -266,7 +267,7 @@ namespace Avalonia.Controls.UnitTests.Utils
                 {nameof(MenuItem),withCommand ? MakeMenu : MakeMenuWithoutCommand},
             };
 
-        private static AvaloniaObject MakeMenu(int expectedParameter, Action<object> action, Window root)
+        private static AvaloniaObject MakeMenu(int expectedParameter, Action<object?>? action, Window root)
         {
             var menuitem = new MenuItem()
             {
@@ -281,7 +282,7 @@ namespace Avalonia.Controls.UnitTests.Utils
             return menuitem;
         }
 
-        private static AvaloniaObject MakeButton(int expectedParameter, Action<object> action, Window root)
+        private static AvaloniaObject MakeButton(int expectedParameter, Action<object?>? action, Window root)
         {
             var button = new Button()
             {
@@ -293,7 +294,7 @@ namespace Avalonia.Controls.UnitTests.Utils
             return button;
         }
 
-        private static AvaloniaObject MakeMenuWithoutCommand(int expectedParameter, Action<object> action, Window root)
+        private static AvaloniaObject MakeMenuWithoutCommand(int expectedParameter, Action<object?>? action, Window root)
         {
             var menuitem = new MenuItem()
             {
@@ -306,7 +307,7 @@ namespace Avalonia.Controls.UnitTests.Utils
             return menuitem;
         }
 
-        private static AvaloniaObject MakeButtonWithoutCommand(int expectedParameter, Action<object> action, Window root)
+        private static AvaloniaObject MakeButtonWithoutCommand(int expectedParameter, Action<object?>? action, Window root)
         {
             var button = new Button()
             {
@@ -330,19 +331,19 @@ namespace Avalonia.Controls.UnitTests.Utils
 
         class Command : System.Windows.Input.ICommand
         {
-            private readonly Action<object> _execeute;
+            private readonly Action<object?>? _execute;
 
 #pragma warning disable 67 // Event not used
-            public event EventHandler CanExecuteChanged;
+            public event EventHandler? CanExecuteChanged;
 #pragma warning restore 67 // Event not used
 
-            public Command(Action<object> execeute)
+            public Command(Action<object?>? execute)
             {
-                _execeute = execeute;
+                _execute = execute;
             }
-            public bool CanExecute(object parameter) => true;
+            public bool CanExecute(object? parameter) => true;
 
-            public void Execute(object parameter) => _execeute?.Invoke(parameter);
+            public void Execute(object? parameter) => _execute?.Invoke(parameter);
         }
     }
 }

tests/Avalonia.Controls.UnitTests/Utils/TestCommand.cs

@@ -5,9 +5,9 @@ namespace Avalonia.Controls.UnitTests.Utils;
 
 internal class TestCommand : ICommand
 {
-    private readonly Func<object, bool> _canExecute;
+    private readonly Func<object?, bool> _canExecute;
-    private readonly Action<object> _execute;
+    private readonly Action<object?> _execute;
-    private EventHandler _canExecuteChanged;
+    private EventHandler? _canExecuteChanged;
     private bool _enabled = true;
 
     public TestCommand(bool enabled = true)
@@ -17,7 +17,7 @@ internal class TestCommand : ICommand
         _execute = _ => { };
     }
 
-    public TestCommand(Func<object, bool> canExecute, Action<object> execute = null)
+    public TestCommand(Func<object?, bool> canExecute, Action<object?>? execute = null)
     {
         _canExecute = canExecute;
         _execute = execute ?? (_ => { });
@@ -38,15 +38,15 @@ internal class TestCommand : ICommand
 
     public int SubscriptionCount { get; private set; }
 
-    public event EventHandler CanExecuteChanged
+    public event EventHandler? CanExecuteChanged
     {
         add { _canExecuteChanged += value; ++SubscriptionCount; }
         remove { _canExecuteChanged -= value; --SubscriptionCount; }
     }
 
-    public bool CanExecute(object parameter) => _canExecute(parameter);
+    public bool CanExecute(object? parameter) => _canExecute(parameter);
 
-    public void Execute(object parameter) => _execute(parameter);
+    public void Execute(object? parameter) => _execute(parameter);
 
     public void RaiseCanExecuteChanged() => _canExecuteChanged?.Invoke(this, EventArgs.Empty);
 }

tests/Avalonia.Controls.UnitTests/WindowBaseTests.cs

@@ -1,17 +1,9 @@
-using System;
-using System.Reactive;
-using System.Reactive.Subjects;
 using Moq;
 using Avalonia.Controls.Presenters;
 using Avalonia.Controls.Templates;
-using Avalonia.Input;
-using Avalonia.Input.Raw;
-using Avalonia.Layout;
 using Avalonia.Media;
 using Avalonia.Platform;
-using Avalonia.Rendering;
 using Avalonia.Rendering.Composition;
-using Avalonia.Styling;
 using Avalonia.UnitTests;
 using Xunit;
 
@@ -44,7 +36,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new TestWindowBase(impl.Object);
                 target.Activated += (s, e) => raised = true;
 
-                impl.Object.Activated();
+                impl.Object.Activated!();
 
                 Assert.True(raised);
             }
@@ -62,7 +54,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new TestWindowBase(impl.Object);
                 target.Deactivated += (s, e) => raised = true;
 
-                impl.Object.Deactivated();
+                impl.Object.Deactivated!();
 
                 Assert.True(raised);
             }
@@ -120,7 +112,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new TestWindowBase(windowImpl.Object);
 
                 target.Show();
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
 
                 Assert.False(target.IsVisible);
             }
@@ -219,7 +211,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new TestWindowBase(windowImpl.Object);
 
                 target.Show();
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
                 Assert.True(((CompositingRenderer)target.Renderer).IsDisposed);
             }
         }

tests/Avalonia.Controls.UnitTests/WindowTests.cs

@@ -116,7 +116,7 @@ namespace Avalonia.Controls.UnitTests
                 window.Show();
                 Assert.True(window.IsVisible);
 
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
 
                 Assert.False(window.IsVisible);
             }
@@ -196,7 +196,7 @@ namespace Avalonia.Controls.UnitTests
                 }
                 else
                 {
-                    var cancel = window.PlatformImpl.Closing(WindowCloseReason.WindowClosing);
+                    var cancel = window.PlatformImpl!.Closing!(WindowCloseReason.WindowClosing);
 
                     Assert.Equal(false, cancel);
                 }
@@ -258,7 +258,7 @@ namespace Avalonia.Controls.UnitTests
                 }
                 else
                 {
-                    var cancel = window.PlatformImpl.Closing(WindowCloseReason.WindowClosing);
+                    var cancel = window.PlatformImpl!.Closing!(WindowCloseReason.WindowClosing);
 
                     Assert.Equal(true, cancel);
                 }
@@ -344,7 +344,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new Window(windowImpl.Object);
                 var task = target.ShowDialog<bool>(parent);
 
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
 
                 var result = await task;
                 Assert.False(result);
@@ -387,7 +387,7 @@ namespace Avalonia.Controls.UnitTests
                 var target = new Window(windowImpl.Object);
                 var task = target.ShowDialog<bool>(parent);
 
-                windowImpl.Object.Closed();
+                windowImpl.Object.Closed!();
                 await task;
 
                 var openedRaised = false;
@@ -927,8 +927,8 @@ namespace Avalonia.Controls.UnitTests
 
                     // Size before and after DPI change is a real-world example, with size after DPI
                     // change coming from Win32 WM_DPICHANGED.
-                    target.PlatformImpl.ScalingChanged(1.5);
+                    target.PlatformImpl!.ScalingChanged!(1.5);
-                    target.PlatformImpl.Resized(
+                    target.PlatformImpl!.Resized!(
                         new Size(210.66666666666666, 118.66666666666667),
                         WindowResizeReason.DpiChange);
 
@@ -987,7 +987,7 @@ namespace Avalonia.Controls.UnitTests
                     target.Width = 410;
                     target.LayoutManager.ExecuteLayoutPass();
 
-                    var windowImpl = Mock.Get(target.PlatformImpl);
+                    var windowImpl = Mock.Get(target.PlatformImpl!);
                     windowImpl.Verify(x => x.Resize(new Size(410, 800), WindowResizeReason.Application));
                     Assert.Equal(410, target.Width);
                 }
@@ -1013,7 +1013,7 @@ namespace Avalonia.Controls.UnitTests
                     Assert.Equal(400, target.Width);
                     Assert.Equal(800, target.Height);
 
-                    target.PlatformImpl.Resized(new Size(410, 800), WindowResizeReason.User);
+                    target.PlatformImpl!.Resized!(new Size(410, 800), WindowResizeReason.User);
 
                     Assert.Equal(410, target.Width);
                     Assert.Equal(800, target.Height);
@@ -1040,7 +1040,7 @@ namespace Avalonia.Controls.UnitTests
                     Assert.Equal(400, target.Width);
                     Assert.Equal(800, target.Height);
 
-                    target.PlatformImpl.Resized(new Size(400, 810), WindowResizeReason.User);
+                    target.PlatformImpl!.Resized!(new Size(400, 810), WindowResizeReason.User);
 
                     Assert.Equal(400, target.Width);
                     Assert.Equal(810, target.Height);
@@ -1068,7 +1068,7 @@ namespace Avalonia.Controls.UnitTests
                     Assert.Equal(400, target.Width);
                     Assert.Equal(800, target.Height);
 
-                    target.PlatformImpl.Resized(new Size(410, 810), WindowResizeReason.Unspecified);
+                    target.PlatformImpl!.Resized!(new Size(410, 810), WindowResizeReason.Unspecified);
 
                     Assert.Equal(400, target.Width);
                     Assert.Equal(800, target.Height);

tests/Avalonia.UnitTests/MockWindowingPlatform.cs

@@ -12,11 +12,11 @@ namespace Avalonia.UnitTests
     {
         private static readonly Size s_screenSize = new Size(1280, 1024);
         private readonly Func<IWindowImpl>? _windowImpl;
-        private readonly Func<IWindowBaseImpl, IPopupImpl>? _popupImpl;
+        private readonly Func<IWindowBaseImpl, IPopupImpl?>? _popupImpl;
 
         public MockWindowingPlatform(
             Func<IWindowImpl>? windowImpl = null,
-            Func<IWindowBaseImpl, IPopupImpl>? popupImpl = null )
+            Func<IWindowBaseImpl, IPopupImpl?>? popupImpl = null )
         {
             _windowImpl = windowImpl;
             _popupImpl = popupImpl;