Make OnPropertyChanged non-generic.

samples/SampleControls/HamburgerMenu/HamburgerMenu.cs

@@ -43,14 +43,13 @@ namespace ControlSamples
             _splitView = e.NameScope.Find<SplitView>("PART_NavigationPane");
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == BoundsProperty && _splitView is not null)
             {
-                var oldBounds = change.OldValue.GetValueOrDefault<Rect>();
-                var newBounds = change.NewValue.GetValueOrDefault<Rect>();
+                var (oldBounds, newBounds) = change.GetOldAndNewValue<Rect>();
                 EnsureSplitViewMode(oldBounds, newBounds);
             }
         }

src/Avalonia.Animation/Animatable.cs

@@ -87,12 +87,13 @@ namespace Avalonia.Animation
             }
         }
 
-        protected sealed override void OnPropertyChangedCore<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected sealed override void OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == TransitionsProperty && change.IsEffectiveValueChange)
             {
-                var oldTransitions = change.OldValue.GetValueOrDefault<Transitions>();
-                var newTransitions = change.NewValue.GetValueOrDefault<Transitions>();
+                var e = (AvaloniaPropertyChangedEventArgs<Transitions?>)change;
+                var oldTransitions = e.OldValue.GetValueOrDefault();
+                var newTransitions = e.NewValue.GetValueOrDefault();
 
                 // When transitions are replaced, we add the new transitions before removing the old
                 // transitions, so that when the old transition being disposed causes the value to

src/Avalonia.Base/AvaloniaObject.cs

@@ -657,7 +657,7 @@ namespace Avalonia
         /// Called when a avalonia property changes on the object.
         /// </summary>
         /// <param name="change">The property change details.</param>
-        protected virtual void OnPropertyChangedCore<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected virtual void OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.IsEffectiveValueChange)
             {
@@ -669,7 +669,7 @@ namespace Avalonia
         /// Called when a avalonia property changes on the object.
         /// </summary>
         /// <param name="change">The property change details.</param>
-        protected virtual void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected virtual void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
         }
 

src/Avalonia.Base/AvaloniaPropertyChangedExtensions.cs

@@ -0,0 +1,43 @@
+namespace Avalonia
+{
+    /// <summary>
+    /// Provides extensions for <see cref="AvaloniaPropertyChangedEventArgs"/>.
+    /// </summary>
+    public static class AvaloniaPropertyChangedExtensions
+    {
+        /// <summary>
+        /// Gets a typed value from <see cref="AvaloniaPropertyChangedEventArgs.OldValue"/>.
+        /// </summary>
+        /// <typeparam name="T">The value type.</typeparam>
+        /// <param name="e">The event args.</param>
+        /// <returns>The value.</returns>
+        public static T GetOldValue<T>(this AvaloniaPropertyChangedEventArgs e)
+        {
+            return ((AvaloniaPropertyChangedEventArgs<T>)e).OldValue.GetValueOrDefault()!;
+        }
+
+        /// <summary>
+        /// Gets a typed value from <see cref="AvaloniaPropertyChangedEventArgs.NewValue"/>.
+        /// </summary>
+        /// <typeparam name="T">The value type.</typeparam>
+        /// <param name="e">The event args.</param>
+        /// <returns>The value.</returns>
+        public static T GetNewValue<T>(this AvaloniaPropertyChangedEventArgs e)
+        {
+            return ((AvaloniaPropertyChangedEventArgs<T>)e).NewValue.GetValueOrDefault()!;
+        }
+
+        /// <summary>
+        /// Gets a typed value from <see cref="AvaloniaPropertyChangedEventArgs.OldValue"/> and
+        /// <see cref="AvaloniaPropertyChangedEventArgs.NewValue"/>.
+        /// </summary>
+        /// <typeparam name="T">The value type.</typeparam>
+        /// <param name="e">The event args.</param>
+        /// <returns>The value.</returns>
+        public static (T oldValue, T newValue) GetOldAndNewValue<T>(this AvaloniaPropertyChangedEventArgs e)
+        {
+            var ev = (AvaloniaPropertyChangedEventArgs<T>)e;
+            return (ev.OldValue.GetValueOrDefault()!, ev.NewValue.GetValueOrDefault()!);
+        }
+    }
+}

src/Avalonia.Controls.DataGrid/DataGridCheckBoxColumn.cs

@@ -46,7 +46,7 @@ namespace Avalonia.Controls
             set => SetValue(IsThreeStateProperty, value);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls.DataGrid/DataGridColumn.cs

@@ -192,14 +192,14 @@ namespace Avalonia.Controls
             set => SetValue(IsVisibleProperty, value);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == IsVisibleProperty)
             {
                 OwningGrid?.OnColumnVisibleStateChanging(this);
-                var isVisible = (change as AvaloniaPropertyChangedEventArgs<bool>).NewValue.Value;
+                var isVisible = change.GetNewValue<bool>();
 
                 if (_headerCell != null)
                 {

src/Avalonia.Controls.DataGrid/DataGridRow.cs

@@ -1092,7 +1092,7 @@ namespace Avalonia.Controls
         }
         
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == DataContextProperty)
             {

src/Avalonia.Controls.DataGrid/DataGridTextColumn.cs

@@ -121,7 +121,7 @@ namespace Avalonia.Controls
             set => SetValue(ForegroundProperty, value);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/Button.cs

@@ -413,7 +413,7 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
@@ -421,12 +421,13 @@ namespace Avalonia.Controls
             {
                 if (((ILogical)this).IsAttachedToLogicalTree)
                 {
-                    if (change.OldValue.GetValueOrDefault() is ICommand oldCommand)
+                    var (oldValue, newValue) = change.GetOldAndNewValue<ICommand?>();
+                    if (oldValue is ICommand oldCommand)
                     {
                         oldCommand.CanExecuteChanged -= CanExecuteChanged;
                     }
 
-                    if (change.NewValue.GetValueOrDefault() is ICommand newCommand)
+                    if (newValue is ICommand newCommand)
                     {
                         newCommand.CanExecuteChanged += CanExecuteChanged;
                     }
@@ -440,7 +441,7 @@ namespace Avalonia.Controls
             }
             else if (change.Property == IsCancelProperty)
             {
-                var isCancel = change.NewValue.GetValueOrDefault<bool>();
+                var isCancel = change.GetNewValue<bool>();
 
                 if (VisualRoot is IInputElement inputRoot)
                 {
@@ -456,7 +457,7 @@ namespace Avalonia.Controls
             }
             else if (change.Property == IsDefaultProperty)
             {
-                var isDefault = change.NewValue.GetValueOrDefault<bool>();
+                var isDefault = change.GetNewValue<bool>();
 
                 if (VisualRoot is IInputElement inputRoot)
                 {
@@ -476,8 +477,7 @@ namespace Avalonia.Controls
             }
             else if (change.Property == FlyoutProperty)
             {
-                var oldFlyout = change.OldValue.GetValueOrDefault() as FlyoutBase;
-                var newFlyout = change.NewValue.GetValueOrDefault() as FlyoutBase;
+                var (oldFlyout, newFlyout) = change.GetOldAndNewValue<FlyoutBase?>();
 
                 // If flyout is changed while one is already open, make sure we 
                 // close the old one first

src/Avalonia.Controls/ButtonSpinner.cs

@@ -210,13 +210,13 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == ButtonSpinnerLocationProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<Location>());
+                UpdatePseudoClasses(change.GetNewValue<Location>());
             }
         }
 

src/Avalonia.Controls/ContextMenu.cs

@@ -241,13 +241,13 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == WindowManagerAddShadowHintProperty && _popup != null)
             {
-                _popup.WindowManagerAddShadowHint = change.NewValue.GetValueOrDefault<bool>();
+                _popup.WindowManagerAddShadowHint = change.GetNewValue<bool>();
             }
         }
 

src/Avalonia.Controls/Control.cs

@@ -348,7 +348,7 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/Documents/Inline.cs

@@ -55,7 +55,7 @@ namespace Avalonia.Controls.Documents
                 TextDecorations, Foreground, Background, BaselineAlignment);
         }
         
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/Documents/Run.cs

@@ -71,7 +71,7 @@ namespace Avalonia.Controls.Documents
             return text.Length;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/Documents/TextElement.cs

@@ -256,7 +256,7 @@ namespace Avalonia.Controls.Documents
         /// </summary>
         public event EventHandler? Invalidated;
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/Expander.cs

@@ -106,13 +106,13 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == ExpandDirectionProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<ExpandDirection>());
+                UpdatePseudoClasses(change.GetNewValue<ExpandDirection>());
             }
         }
 

src/Avalonia.Controls/ItemsControl.cs

@@ -341,13 +341,13 @@ namespace Avalonia.Controls
             return new ItemsControlAutomationPeer(this);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == ItemCountProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<int>());
+                UpdatePseudoClasses(change.GetNewValue<int>());
             }
         }
 

src/Avalonia.Controls/MaskedTextBox.cs

@@ -280,7 +280,7 @@ namespace Avalonia.Controls
             base.OnLostFocus(e);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             void UpdateMaskProvider()
             {

src/Avalonia.Controls/Notifications/WindowNotificationManager.cs

@@ -139,13 +139,13 @@ namespace Avalonia.Controls.Notifications
             notificationControl.Close();
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == PositionProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<NotificationPosition>());
+                UpdatePseudoClasses(change.GetNewValue<NotificationPosition>());
             }
         }
 

src/Avalonia.Controls/Presenters/ContentPresenter.cs

@@ -403,7 +403,7 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
             switch (change.Property.Name)

src/Avalonia.Controls/Presenters/ScrollContentPresenter.cs

@@ -469,7 +469,7 @@ namespace Avalonia.Controls.Presenters
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == OffsetProperty && !_arranging)
             {

src/Avalonia.Controls/Presenters/TextPresenter.cs

@@ -776,7 +776,7 @@ namespace Avalonia.Controls.Presenters
             _caretTimer.Tick -= CaretTimerTick;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
             

src/Avalonia.Controls/Primitives/ScrollBar.cs

@@ -194,13 +194,13 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == OrientationProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<Orientation>());
+                UpdatePseudoClasses(change.GetNewValue<Orientation>());
             }
             else if (change.Property == AllowAutoHideProperty)
             {

src/Avalonia.Controls/Primitives/SelectingItemsControl.cs

@@ -585,7 +585,7 @@ namespace Avalonia.Controls.Primitives
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
@@ -595,7 +595,7 @@ namespace Avalonia.Controls.Primitives
             }
             if (change.Property == ItemsProperty && _updateState is null && _selection is object)
             {
-                var newValue = change.NewValue.GetValueOrDefault<IEnumerable>();
+                var newValue = change.GetNewValue<IEnumerable>();
                 _selection.Source = newValue;
 
                 if (newValue is null)
@@ -605,7 +605,7 @@ namespace Avalonia.Controls.Primitives
             }
             else if (change.Property == SelectionModeProperty && _selection is object)
             {
-                var newValue = change.NewValue.GetValueOrDefault<SelectionMode>();
+                var newValue = change.GetNewValue<SelectionMode>();
                 _selection.SingleSelect = !newValue.HasAllFlags(SelectionMode.Multiple);
             }
             else if (change.Property == WrapSelectionProperty)

src/Avalonia.Controls/Primitives/Track.cs

@@ -291,13 +291,13 @@ namespace Avalonia.Controls.Primitives
             return arrangeSize;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == OrientationProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<Orientation>());
+                UpdatePseudoClasses(change.GetNewValue<Orientation>());
             }
         }
 

src/Avalonia.Controls/ProgressBar.cs

@@ -178,17 +178,17 @@ namespace Avalonia.Controls
             return base.ArrangeOverride(finalSize);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == IsIndeterminateProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<bool>(), null);
+                UpdatePseudoClasses(change.GetNewValue<bool>(), null);
             }
             else if (change.Property == OrientationProperty)
             {
-                UpdatePseudoClasses(null, change.NewValue.GetValueOrDefault<Orientation>());
+                UpdatePseudoClasses(null, change.GetNewValue<Orientation>());
             }
         }
 

src/Avalonia.Controls/RepeatButton.cs

@@ -70,11 +70,11 @@ namespace Avalonia.Controls
             _repeatTimer?.Stop();
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
-            if (change.Property == IsPressedProperty && change.NewValue.GetValueOrDefault<bool>() == false)
+            if (change.Property == IsPressedProperty && change.GetNewValue<bool>() == false)
             {
                 StopTimer();
             }

src/Avalonia.Controls/Repeater/ItemsRepeater.cs

@@ -424,12 +424,11 @@ namespace Avalonia.Controls
             _viewportManager.ResetScrollers();
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == ItemsProperty)
             {
-                var oldEnumerable = change.OldValue.GetValueOrDefault<IEnumerable>();
-                var newEnumerable = change.NewValue.GetValueOrDefault<IEnumerable>();
+                var (oldEnumerable, newEnumerable) = change.GetOldAndNewValue<IEnumerable?>();
 
                 if (oldEnumerable != newEnumerable)
                 {
@@ -444,23 +443,21 @@ namespace Avalonia.Controls
             }
             else if (change.Property == ItemTemplateProperty)
             {
-                OnItemTemplateChanged(
-                    change.OldValue.GetValueOrDefault<IDataTemplate>(),
-                    change.NewValue.GetValueOrDefault<IDataTemplate>());
+                var (oldvalue, newValue) = change.GetOldAndNewValue<IDataTemplate?>();
+                OnItemTemplateChanged(oldvalue, newValue);
             }
             else if (change.Property == LayoutProperty)
             {
-                OnLayoutChanged(
-                    change.OldValue.GetValueOrDefault<AttachedLayout>(),
-                    change.NewValue.GetValueOrDefault<AttachedLayout>());
+                var (oldvalue, newValue) = change.GetOldAndNewValue<AttachedLayout>();
+                OnLayoutChanged(oldvalue, newValue);
             }
             else if (change.Property == HorizontalCacheLengthProperty)
             {
-                _viewportManager.HorizontalCacheLength = change.NewValue.GetValueOrDefault<double>();
+                _viewportManager.HorizontalCacheLength = change.GetNewValue<double>();
             }
             else if (change.Property == VerticalCacheLengthProperty)
             {
-                _viewportManager.VerticalCacheLength = change.NewValue.GetValueOrDefault<double>();
+                _viewportManager.VerticalCacheLength = change.GetNewValue<double>();
             }
 
             base.OnPropertyChanged(change);

src/Avalonia.Controls/Slider.cs

@@ -369,13 +369,13 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == OrientationProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<Orientation>());
+                UpdatePseudoClasses(change.GetNewValue<Orientation>());
             }
         }
 

src/Avalonia.Controls/SplitButton/SplitButton.cs

@@ -276,19 +276,21 @@ namespace Avalonia.Controls
         }
 
         /// <inheritdoc/>
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> e)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
             if (e.Property == CommandProperty)
             {
                 if (_isAttachedToLogicalTree)
                 {
                     // Must unregister events here while a reference to the old command still exists
-                    if (e.OldValue.GetValueOrDefault() is ICommand oldCommand)
+                    var (oldValue, newValue) = e.GetOldAndNewValue<ICommand?>();
+
+                    if (oldValue is ICommand oldCommand)
                     {
                         oldCommand.CanExecuteChanged -= CanExecuteChanged;
                     }
 
-                    if (e.NewValue.GetValueOrDefault() is ICommand newCommand)
+                    if (newValue is ICommand newCommand)
                     {
                         newCommand.CanExecuteChanged += CanExecuteChanged;
                     }
@@ -302,8 +304,7 @@ namespace Avalonia.Controls
             }
             else if (e.Property == FlyoutProperty)
             {
-                var oldFlyout = e.OldValue.GetValueOrDefault() as FlyoutBase;
-                var newFlyout = e.NewValue.GetValueOrDefault() as FlyoutBase;
+                var (oldFlyout, newFlyout) = e.GetOldAndNewValue<FlyoutBase?>();
 
                 // If flyout is changed while one is already open, make sure we 
                 // close the old one first

src/Avalonia.Controls/SplitButton/ToggleSplitButton.cs

@@ -90,7 +90,7 @@ namespace Avalonia.Controls
         ////////////////////////////////////////////////////////////////////////
 
         /// <inheritdoc/>
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> e)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs e)
         {
             if (e.Property == IsCheckedProperty)
             {

src/Avalonia.Controls/TextBlock.cs

@@ -639,7 +639,7 @@ namespace Avalonia.Controls
 
         private static bool IsValidLineHeight(double lineHeight) => double.IsNaN(lineHeight) || lineHeight > 0;
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Controls/TextBox.cs

@@ -585,7 +585,7 @@ namespace Avalonia.Controls
             _imClient.SetPresenter(null, null);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
@@ -594,7 +594,7 @@ namespace Avalonia.Controls
                 UpdatePseudoclasses();
                 UpdateCommandStates();
             }
-            else if (change.Property == IsUndoEnabledProperty && change.NewValue.GetValueOrDefault<bool>() == false)
+            else if (change.Property == IsUndoEnabledProperty && change.GetNewValue<bool>() == false)
             {
                 // from docs at
                 // https://docs.microsoft.com/en-us/dotnet/api/system.windows.controls.primitives.textboxbase.isundoenabled:

src/Avalonia.Controls/TransitioningContentControl.cs

@@ -61,7 +61,7 @@ public class TransitioningContentControl : ContentControl
         _lastTransitionCts?.Cancel();
     }
 
-    protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+    protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
     {
         base.OnPropertyChanged(change);
 

src/Avalonia.Controls/TrayIcon.cs

@@ -206,7 +206,7 @@ namespace Avalonia.Controls
             }
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
@@ -216,15 +216,15 @@ namespace Avalonia.Controls
             }
             else if (change.Property == IsVisibleProperty)
             {
-                _impl?.SetIsVisible(change.NewValue.GetValueOrDefault<bool>());
+                _impl?.SetIsVisible(change.GetNewValue<bool>());
             }
             else if (change.Property == ToolTipTextProperty)
             {
-                _impl?.SetToolTipText(change.NewValue.GetValueOrDefault<string?>());
+                _impl?.SetToolTipText(change.GetNewValue<string?>());
             }
             else if (change.Property == MenuProperty)
             {
-                _impl?.MenuExporter?.SetNativeMenu(change.NewValue.GetValueOrDefault<NativeMenu>());
+                _impl?.MenuExporter?.SetNativeMenu(change.GetNewValue<NativeMenu?>());
             }
         }
 

src/Avalonia.Controls/Viewbox.cs

@@ -82,13 +82,13 @@ namespace Avalonia.Controls
             set => _containerVisual.RenderTransform = value;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == ChildProperty)
             {
-                _containerVisual.Child = change.NewValue.GetValueOrDefault<IControl>();
+                _containerVisual.Child = change.GetNewValue<IControl>();
                 InvalidateMeasure();
             }
         }

src/Avalonia.Controls/Window.cs

@@ -1019,16 +1019,16 @@ namespace Avalonia.Controls
         /// </remarks>
         protected virtual void OnClosing(CancelEventArgs e) => Closing?.Invoke(this, e);
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
             if (change.Property == SystemDecorationsProperty)
             {
-                var typedNewValue = change.NewValue.GetValueOrDefault<SystemDecorations>();
+                var (typedOldValue, typedNewValue) = change.GetOldAndNewValue<SystemDecorations>();
 
                 PlatformImpl?.SetSystemDecorations(typedNewValue);
 
-                var o = change.OldValue.GetValueOrDefault<SystemDecorations>() == SystemDecorations.Full;
+                var o = typedOldValue == SystemDecorations.Full;
                 var n = typedNewValue == SystemDecorations.Full;
 
                 if (o != n)

src/Avalonia.Diagnostics/Diagnostics/Controls/ThicknessEditor.cs

@@ -92,7 +92,7 @@ namespace Avalonia.Diagnostics.Controls
             set => SetValue(HighlightProperty, value);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
@@ -102,7 +102,7 @@ namespace Avalonia.Diagnostics.Controls
                 {
                     _isUpdatingThickness = true;
 
-                    var value = change.NewValue.GetValueOrDefault<Thickness>();
+                    var value = change.GetNewValue<Thickness>();
 
                     Left = value.Left;
                     Top = value.Top;

src/Avalonia.Input/InputElement.cs

@@ -601,21 +601,21 @@ namespace Avalonia.Input
         {
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == IsFocusedProperty)
             {
-                UpdatePseudoClasses(change.NewValue.GetValueOrDefault<bool>(), null);
+                UpdatePseudoClasses(change.GetNewValue<bool>(), null);
             }
             else if (change.Property == IsPointerOverProperty)
             {
-                UpdatePseudoClasses(null, change.NewValue.GetValueOrDefault<bool>());
+                UpdatePseudoClasses(null, change.GetNewValue<bool>());
             }
             else if (change.Property == IsKeyboardFocusWithinProperty)
             {
-                PseudoClasses.Set(":focus-within", change.NewValue.GetValueOrDefault<bool>());
+                PseudoClasses.Set(":focus-within", change.GetNewValue<bool>());
             }
         }
 

src/Avalonia.Layout/StackLayout.cs

@@ -320,11 +320,11 @@ namespace Avalonia.Layout
             InvalidateLayout();
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == OrientationProperty)
             {
-                var orientation = change.NewValue.GetValueOrDefault<Orientation>();
+                var orientation = change.GetNewValue<Orientation>();
 
                 //Note: For StackLayout Vertical Orientation means we have a Vertical ScrollOrientation.
                 //Horizontal Orientation means we have a Horizontal ScrollOrientation.

src/Avalonia.Layout/UniformGridLayout.cs

@@ -471,11 +471,11 @@ namespace Avalonia.Layout
             gridState.ClearElementOnDataSourceChange(context, args);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             if (change.Property == OrientationProperty)
             {
-                var orientation = change.NewValue.GetValueOrDefault<Orientation>();
+                var orientation = change.GetNewValue<Orientation>();
 
                 //Note: For UniformGridLayout Vertical Orientation means we have a Horizontal ScrollOrientation. Horizontal Orientation means we have a Vertical ScrollOrientation.
                 //i.e. the properties are the inverse of each other.
@@ -484,31 +484,31 @@ namespace Avalonia.Layout
             }
             else if (change.Property == MinColumnSpacingProperty)
             {
-                _minColumnSpacing = change.NewValue.GetValueOrDefault<double>();
+                _minColumnSpacing = change.GetNewValue<double>();
             }
             else if (change.Property == MinRowSpacingProperty)
             {
-                _minRowSpacing = change.NewValue.GetValueOrDefault<double>();
+                _minRowSpacing = change.GetNewValue<double>();
             }
             else if (change.Property == ItemsJustificationProperty)
             {
-                _itemsJustification = change.NewValue.GetValueOrDefault<UniformGridLayoutItemsJustification>();
+                _itemsJustification = change.GetNewValue<UniformGridLayoutItemsJustification>();
             }
             else if (change.Property == ItemsStretchProperty)
             {
-                _itemsStretch = change.NewValue.GetValueOrDefault<UniformGridLayoutItemsStretch>();
+                _itemsStretch = change.GetNewValue<UniformGridLayoutItemsStretch>();
             }
             else if (change.Property == MinItemWidthProperty)
             {
-                _minItemWidth = change.NewValue.GetValueOrDefault<double>();
+                _minItemWidth = change.GetNewValue<double>();
             }
             else if (change.Property == MinItemHeightProperty)
             {
-                _minItemHeight = change.NewValue.GetValueOrDefault<double>();
+                _minItemHeight = change.GetNewValue<double>();
             }
             else if (change.Property == MaximumRowsOrColumnsProperty)
             {
-                _maximumRowsOrColumns = change.NewValue.GetValueOrDefault<int>();
+                _maximumRowsOrColumns = change.GetNewValue<int>();
             }
 
             InvalidateLayout();

src/Avalonia.Layout/WrapLayout/WrapLayout.cs

@@ -322,7 +322,7 @@ namespace Avalonia.Layout
             return finalSize;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Themes.Default/SimpleTheme.cs

@@ -116,7 +116,7 @@ namespace Avalonia.Themes.Default
             return false;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
             if (change.Property == ModeProperty)

src/Avalonia.Themes.Fluent/FluentTheme.cs

@@ -78,7 +78,7 @@ namespace Avalonia.Themes.Fluent
             set => SetValue(DensityStyleProperty, value);
         }
         
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
             if (change.Property == ModeProperty)

src/Avalonia.Visuals/Media/DashStyle.cs

@@ -112,14 +112,13 @@ namespace Avalonia.Media
         /// <returns></returns>
         public ImmutableDashStyle ToImmutable() => new ImmutableDashStyle(Dashes, Offset);
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 
             if (change.Property == DashesProperty)
             {
-                var oldValue = change.OldValue.GetValueOrDefault<AvaloniaList<double>>();
-                var newValue = change.NewValue.GetValueOrDefault<AvaloniaList<double>>();
+                var (oldValue, newValue) = change.GetOldAndNewValue<AvaloniaList<double>>();
 
                 if (oldValue is object)
                 {

src/Avalonia.Visuals/Media/DrawingImage.cs

@@ -71,7 +71,7 @@ namespace Avalonia.Media
         }
 
         /// <inheritdoc/>
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Visuals/Media/GeometryGroup.cs

@@ -68,7 +68,7 @@ namespace Avalonia.Media
             return null;
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             base.OnPropertyChanged(change);
 

src/Avalonia.Visuals/Media/Pen.cs

@@ -192,7 +192,7 @@ namespace Avalonia.Media
                 MiterLimit);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             _invalidated?.Invoke(this, EventArgs.Empty);
             if(change.Property == BrushProperty)

tests/Avalonia.Base.UnitTests/AvaloniaObjectTests_OnPropertyChanged.cs

@@ -109,25 +109,26 @@ namespace Avalonia.Base.UnitTests
             public List<AvaloniaPropertyChangedEventArgs> Changes { get; }
             public List<AvaloniaPropertyChangedEventArgs> CoreChanges { get; }
 
-            protected override void OnPropertyChangedCore<T>(AvaloniaPropertyChangedEventArgs<T> change)
+            protected override void OnPropertyChangedCore(AvaloniaPropertyChangedEventArgs change)
             {
                 CoreChanges.Add(Clone(change));
                 base.OnPropertyChangedCore(change);
             }
 
-            protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+            protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
             {
                 Changes.Add(Clone(change));
                 base.OnPropertyChanged(change);
             }
 
-            private static AvaloniaPropertyChangedEventArgs<T> Clone<T>(AvaloniaPropertyChangedEventArgs<T> change)
+            private static AvaloniaPropertyChangedEventArgs Clone(AvaloniaPropertyChangedEventArgs change)
             {
-                var result = new AvaloniaPropertyChangedEventArgs<T>(
+                var e = (AvaloniaPropertyChangedEventArgs<string>)change;
+                var result = new AvaloniaPropertyChangedEventArgs<string>(
                     change.Sender,
-                    change.Property,
-                    change.OldValue,
-                    change.NewValue,
+                    e.Property,
+                    e.OldValue,
+                    e.NewValue,
                     change.Priority);
 
                 if (!change.IsEffectiveValueChange)

tests/Avalonia.Markup.Xaml.UnitTests/Xaml/InitializationOrderTracker.cs

@@ -18,7 +18,7 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             base.OnAttachedToLogicalTree(e);
         }
 
-        protected override void OnPropertyChanged<T>(AvaloniaPropertyChangedEventArgs<T> change)
+        protected override void OnPropertyChanged(AvaloniaPropertyChangedEventArgs change)
         {
             Order.Add($"Property {change.Property.Name} Changed");
             base.OnPropertyChanged(change);