fixes: some null annotation

src/Avalonia.Base/Data/BindingValue.cs

@@ -247,7 +247,9 @@ public static BindingValue<T> FromUntyped(object? value)
                 UnsetValueType _ => Unset,
                 DoNothingType _ => DoNothing,
                 BindingNotification n => n.ToBindingValue().Cast<T>(),
+#pragma warning disable CS8600 // Converting null literal or possible null value to non-nullable type.
                 _ => new BindingValue<T>((T)value)
+#pragma warning restore CS8600 // Converting null literal or possible null value to non-nullable type.
             };
         }
 

src/Avalonia.Base/Data/Converters/MethodToCommandConverter.cs

@@ -170,7 +170,7 @@ static Func<object, bool> CreateCanExecute(object target
                 .Compile();
         }
 
-        private static Expression? ConvertTarget(object? target, MethodInfo method) =>
+        private static Expression ConvertTarget(object target, MethodInfo method) =>
             target is null ? null : Expression.Convert(Expression.Constant(target), method.DeclaringType);
 
         internal class WeakPropertyChangedProxy

src/Avalonia.Controls/Application.cs

@@ -175,7 +175,7 @@ public IResourceDictionary Resources
         /// - <see cref="ISingleViewApplicationLifetime"/>
         /// - <see cref="IControlledApplicationLifetime"/> 
         /// </summary>
-        public IApplicationLifetime ApplicationLifetime { get; set; }
+        public IApplicationLifetime? ApplicationLifetime { get; set; }
 
         event Action<IReadOnlyList<IStyle>> IGlobalStyles.GlobalStylesAdded
         {

src/Avalonia.Controls/Primitives/Popup.cs

@@ -91,8 +91,8 @@ public class Popup : Control, IVisualTreeHost, IPopupHostProvider
         public static readonly StyledProperty<bool> OverlayDismissEventPassThroughProperty =
             AvaloniaProperty.Register<Popup, bool>(nameof(OverlayDismissEventPassThrough));
 
-        public static readonly DirectProperty<Popup, IInputElement> OverlayInputPassThroughElementProperty =
-            AvaloniaProperty.RegisterDirect<Popup, IInputElement>(
+        public static readonly DirectProperty<Popup, IInputElement?> OverlayInputPassThroughElementProperty =
+            AvaloniaProperty.RegisterDirect<Popup, IInputElement?>(
                 nameof(OverlayInputPassThroughElement),
                 o => o.OverlayInputPassThroughElement,
                 (o, v) => o.OverlayInputPassThroughElement = v);
@@ -136,7 +136,7 @@ public class Popup : Control, IVisualTreeHost, IPopupHostProvider
         private bool _isOpen;
         private bool _ignoreIsOpenChanged;
         private PopupOpenState? _openState;
-        private IInputElement _overlayInputPassThroughElement;
+        private IInputElement? _overlayInputPassThroughElement;
         private Action<IPopupHost?>? _popupHostChangedHandler;
 
         /// <summary>
@@ -308,7 +308,7 @@ public bool OverlayDismissEventPassThrough
         /// Gets or sets an element that should receive pointer input events even when underneath
         /// the popup's overlay.
         /// </summary>
-        public IInputElement OverlayInputPassThroughElement
+        public IInputElement? OverlayInputPassThroughElement
         {
             get => _overlayInputPassThroughElement;
             set => SetAndRaise(OverlayInputPassThroughElementProperty, ref _overlayInputPassThroughElement, value);

src/Avalonia.Controls/SplitView.cs

@@ -129,14 +129,14 @@ Pseudo classes & combos
         /// <summary>
         /// Defines the <see cref="Pane"/> property
         /// </summary>
-        public static readonly StyledProperty<object?> PaneProperty =
-            AvaloniaProperty.Register<SplitView, object?>(nameof(Pane));
+        public static readonly StyledProperty<object> PaneProperty =
+            AvaloniaProperty.Register<SplitView, object>(nameof(Pane));
 
         /// <summary>
         /// Defines the <see cref="PaneTemplate"/> property.
         /// </summary>
-        public static readonly StyledProperty<IDataTemplate?> PaneTemplateProperty =
-            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate?>(nameof(PaneTemplate));
+        public static readonly StyledProperty<IDataTemplate> PaneTemplateProperty =
+            AvaloniaProperty.Register<HeaderedContentControl, IDataTemplate>(nameof(PaneTemplate));
 
         /// <summary>
         /// Defines the <see cref="UseLightDismissOverlayMode"/> property
@@ -267,7 +267,7 @@ public object Pane
         /// <summary>
         /// Gets or sets the data template used to display the header content of the control.
         /// </summary>
-        public IDataTemplate? PaneTemplate
+        public IDataTemplate PaneTemplate
         {
             get => GetValue(PaneTemplateProperty);
             set => SetValue(PaneTemplateProperty, value);

src/Avalonia.Input/ICommandSource.cs

@@ -1,5 +1,5 @@
 using System.Windows.Input;
-
+#nullable enable
 namespace Avalonia.Input
 {
     ///<summary>
@@ -12,13 +12,13 @@ public interface ICommandSource
         /// Classes that implement this interface should enable or disable based on the command's CanExecute return value.
         /// The property may be implemented as read-write if desired.
         /// </summary>
-        ICommand Command { get; }
+        ICommand? Command { get; }
 
         /// <summary>
         /// The parameter that will be passed to the command when executing the command.
         /// The property may be implemented as read-write if desired.
         /// </summary>
-        object CommandParameter { get; }
+        object? CommandParameter { get; }
 
 
         /// <summary>

src/Avalonia.Styling/Styling/Setter.cs

@@ -101,7 +101,11 @@ void IAvaloniaPropertyVisitor<SetterVisitorData>.Visit<T>(
                 data.result = new PropertySetterInstance<T>(
                     data.target,
                     property,
+#pragma warning disable CS8600 // Converting null literal or possible null value to non-nullable type.
+#pragma warning disable CS8604 // Possible null reference argument.
                     (T)data.value);
+#pragma warning restore CS8604 // Possible null reference argument.
+#pragma warning restore CS8600 // Converting null literal or possible null value to non-nullable type.
             }
         }
 
@@ -128,7 +132,11 @@ void IAvaloniaPropertyVisitor<SetterVisitorData>.Visit<T>(
                 data.result = new PropertySetterInstance<T>(
                     data.target,
                     property,
+#pragma warning disable CS8600 // Converting null literal or possible null value to non-nullable type.
+#pragma warning disable CS8604 // Possible null reference argument.
                     (T)data.value);
+#pragma warning restore CS8604 // Possible null reference argument.
+#pragma warning restore CS8600 // Converting null literal or possible null value to non-nullable type.
             }
         }
 

src/tools/MicroComGenerator/CSharpGen.Utils.cs

@@ -40,7 +40,7 @@ string Format(CompilationUnitSyntax unit)
         SyntaxToken Semicolon() => Token(SyntaxKind.SemicolonToken);
 
         static VariableDeclarationSyntax DeclareVar(string type, string name,
-            ExpressionSyntax? initializer = null)
+            ExpressionSyntax initializer = null)
             => VariableDeclaration(ParseTypeName(type),
                 SingletonSeparatedList(VariableDeclarator(name)
                     .WithInitializer(initializer == null ? null : EqualsValueClause(initializer))));