Fix negation of null values. (#16101)

* Add tests for binding negation operator.

Expected results come from 11.0.x branch.

* Unset and null need to be distinct.

Also remove `ExpressionNode.Reset` as it was doing unneeded stuff and should just be the same as `SetSource(AvaloniaProperty.UnsetValue, null);`.

* Make ExpressionNode.OnSourceChanged accept null.

The previous design assumed that a source of `null` was an invalid input to all types of expression nodes. It turned out that there was a single exception to this rule: the `!` operator can in fact operate on a null value. With this new design we instead have to explicitly check for a null value in every override of `OnSourceChanged ` except in `LogicalNotNode`.

Due to this change, we also have to distinguish between `null` and `(unset)` in `ExpressionNode.SetSource` as well.

Fixes #16071

* Fix comment.

src/Avalonia.Base/Data/BindingNotification.cs

@@ -178,7 +178,7 @@ namespace Avalonia.Data
         /// to <paramref name="value"/>. If <paramref name="value"/> is a
         /// <see cref="BindingNotification"/> then the value will first be extracted.
         /// </remarks>
-        public static object? UpdateValue(object o, object value)
+        public static object? UpdateValue(object? o, object value)
         {
             if (o is BindingNotification n)
             {

src/Avalonia.Base/Data/Core/BindingExpression.cs

@@ -160,7 +160,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
         var source = _nodes[0].Source;
 
         for (var i = 0; i < _nodes.Count; ++i)
-            _nodes[i].SetSource(null, null);
+            _nodes[i].SetSource(AvaloniaProperty.UnsetValue, null);
 
         _nodes[0].SetSource(source, null);
     }
@@ -253,10 +253,6 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
             _nodes[nodeIndex + 1].SetSource(value, dataValidationError);
             WriteTargetValueToSource();
         }
-        else if (value is null)
-        {
-            OnNodeError(nodeIndex, "Value is null.");
-        }
         else
         {
             _nodes[nodeIndex + 1].SetSource(value, dataValidationError);
@@ -273,11 +269,11 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     /// <param name="error">The error message.</param>
     internal void OnNodeError(int nodeIndex, string error)
     {
-        // Set the source of all nodes after the one that errored to null. This needs to be done
-        // for each node individually because setting the source to null will not result in
+        // Set the source of all nodes after the one that errored to unset. This needs to be done
+        // for each node individually because setting the source to unset will not result in
         // OnNodeValueChanged or OnNodeError being called.
         for (var i = nodeIndex + 1; i < _nodes.Count; ++i)
-            _nodes[i].SetSource(null, null);
+            _nodes[i].SetSource(AvaloniaProperty.UnsetValue, null);
 
         if (_mode == BindingMode.OneWayToSource)
             return;
@@ -394,7 +390,7 @@ internal partial class BindingExpression : UntypedBindingExpressionBase, IDescri
     protected override void StopCore()
     {
         foreach (var node in _nodes)
-            node.Reset();
+            node.SetSource(AvaloniaProperty.UnsetValue, null);
 
         if (_mode is BindingMode.TwoWay or BindingMode.OneWayToSource &&
             TryGetTarget(out var target))

src/Avalonia.Base/Data/Core/ExpressionNodes/ArrayIndexerNode.cs

@@ -44,8 +44,11 @@ internal sealed class ArrayIndexerNode : ExpressionNode, ISettableNode
         return false;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is Array array)
             SetValue(array.GetValue(_indexes));
         else

src/Avalonia.Base/Data/Core/ExpressionNodes/AvaloniaPropertyAccessorNode.cs

@@ -38,6 +38,9 @@ internal sealed class AvaloniaPropertyAccessorNode :
 
     protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is AvaloniaObject newObject)
         {
             WeakEvents.AvaloniaPropertyChanged.Subscribe(newObject, this);

src/Avalonia.Base/Data/Core/ExpressionNodes/CollectionNodeBase.cs

@@ -22,8 +22,11 @@ internal abstract class CollectionNodeBase : ExpressionNode,
             UpdateValueOrSetError(sender);
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         Subscribe(source);
         UpdateValue(source);
     }

src/Avalonia.Base/Data/Core/ExpressionNodes/DataContextNode.cs

@@ -4,8 +4,11 @@ namespace Avalonia.Data.Core.ExpressionNodes;
 
 internal sealed class DataContextNode : DataContextNodeBase
 {
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is IDataContextProvider && source is AvaloniaObject ao)
         {
             ao.PropertyChanged += OnPropertyChanged;

src/Avalonia.Base/Data/Core/ExpressionNodes/ExpressionNode.cs

@@ -1,6 +1,7 @@
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
+using System.Diagnostics.CodeAnalysis;
 using System.Reflection;
 using System.Text;
 
@@ -60,16 +61,6 @@ internal abstract class ExpressionNode
         BuildString(builder);
     }
 
-    /// <summary>
-    /// Resets the node to its uninitialized state when the <see cref="Owner"/> is unsubscribed.
-    /// </summary>
-    public void Reset()
-    {
-        SetSource(null, null);
-        _source = null;
-        _value = AvaloniaProperty.UnsetValue;
-    }
-
     /// <summary>
     /// Sets the owner binding.
     /// </summary>
@@ -101,28 +92,26 @@ internal abstract class ExpressionNode
     /// </param>
     public void SetSource(object? source, Exception? dataValidationError)
     {
-        var oldSource = Source;
-
-        if (source == AvaloniaProperty.UnsetValue)
-            source = null;
+        if (_source?.TryGetTarget(out var oldSource) != true)
+            oldSource = AvaloniaProperty.UnsetValue;
 
         if (source == oldSource)
             return;
 
-        if (oldSource is not null)
+        if (oldSource is not null && oldSource != AvaloniaProperty.UnsetValue)
             Unsubscribe(oldSource);
 
-        _source = new(source);
-
-        if (source is null)
+        if (source == AvaloniaProperty.UnsetValue)
         {
-            // If the source is null then the value is null. We explicitly do not want to call
+            // If the source is unset then the value is unset. We explicitly do not want to call
             // OnSourceChanged as we don't want to raise errors for subsequent nodes in the
             // binding change.
+            _source = null;
             _value = AvaloniaProperty.UnsetValue;
         }
         else
         {
+            _source = new(source);
             try { OnSourceChanged(source, dataValidationError); }
             catch (Exception e) { SetError(e); }
         }
@@ -242,6 +231,26 @@ internal abstract class ExpressionNode
         }
     }
 
+    /// <summary>
+    /// Called from <see cref="OnSourceChanged(object?, Exception?)"/> to validate that the source
+    /// is non-null and raise a node error if it is not.
+    /// </summary>
+    /// <param name="source">The expression node source.</param>
+    /// <returns>
+    /// True if the source is non-null; otherwise, false.
+    /// </returns>
+    protected bool ValidateNonNullSource([NotNullWhen(true)] object? source)
+    {
+        if (source is null)
+        {
+            Owner?.OnNodeError(Index - 1, "Value is null.");
+            _value = null;
+            return false;
+        }
+
+        return true;
+    }
+
     /// <summary>
     /// When implemented in a derived class, subscribes to the new source, and updates the current 
     /// <see cref="Value"/>.
@@ -250,7 +259,7 @@ internal abstract class ExpressionNode
     /// <param name="dataValidationError">
     /// Any data validation error reported by the previous expression node.
     /// </param>
-    protected abstract void OnSourceChanged(object source, Exception? dataValidationError);
+    protected abstract void OnSourceChanged(object? source, Exception? dataValidationError);
 
     /// <summary>
     /// When implemented in a derived class, unsubscribes from the previous source.

src/Avalonia.Base/Data/Core/ExpressionNodes/FuncTransformNode.cs

@@ -21,8 +21,11 @@ internal sealed class FuncTransformNode : ExpressionNode
         // We don't have enough information to add anything here.
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         SetValue(_transform(source));
     }
 }

src/Avalonia.Base/Data/Core/ExpressionNodes/LogicalAncestorElementNode.cs

@@ -56,8 +56,11 @@ internal sealed class LogicalAncestorElementNode : SourceNode
         return target is ILogical logical && logical.IsAttachedToLogicalTree;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is ILogical logical)
         {
             var locator = ControlLocator.Track(logical, _ancestorLevel, _ancestorType);

src/Avalonia.Base/Data/Core/ExpressionNodes/LogicalNotNode.cs

@@ -28,14 +28,12 @@ internal sealed class LogicalNotNode : ExpressionNode, ISettableNode
         return false;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
         var v = BindingNotification.ExtractValue(source);
 
         if (TryConvert(v, out var value))
-        {
             SetValue(BindingNotification.UpdateValue(source, !value), dataValidationError);
-        }
         else
             SetError($"Unable to convert '{source}' to bool.");
     }

src/Avalonia.Base/Data/Core/ExpressionNodes/MethodCommandNode.cs

@@ -39,8 +39,11 @@ internal sealed class MethodCommandNode : ExpressionNode, IWeakEventSubscriber<P
         builder.Append("()");
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is INotifyPropertyChanged newInpc)
             WeakEvents.ThreadSafePropertyChanged.Subscribe(newInpc, this);
 

src/Avalonia.Base/Data/Core/ExpressionNodes/NamedElementNode.cs

@@ -30,8 +30,11 @@ internal sealed class NamedElementNode : SourceNode
         return target is not ILogical logical || logical.IsAttachedToLogicalTree;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (_nameScope.TryGetTarget(out var scope))
             _subscription = NameScopeLocator.Track(scope, _name).Subscribe(SetValue);
         else

src/Avalonia.Base/Data/Core/ExpressionNodes/ParentDataContextNode.cs

@@ -11,8 +11,11 @@ internal sealed class ParentDataContextNode : DataContextNodeBase
     private static readonly AvaloniaObject s_unset = new();
     private AvaloniaObject? _parent = s_unset;
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is AvaloniaObject newElement)
             newElement.PropertyChanged += OnPropertyChanged;
 

src/Avalonia.Base/Data/Core/ExpressionNodes/PropertyAccessorNode.cs

@@ -48,8 +48,11 @@ internal sealed class PropertyAccessorNode : ExpressionNode, IPropertyAccessorNo
     }
 
     [UnconditionalSuppressMessage("Trimming", "IL2026:Members annotated with 'RequiresUnreferencedCodeAttribute' require dynamic access otherwise can break functionality when trimming application code", Justification = "<Pending>")]
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         var reference = new WeakReference<object?>(source);
 
         if (_plugin.Start(reference, PropertyName) is { } accessor)

src/Avalonia.Base/Data/Core/ExpressionNodes/Reflection/DynamicPluginPropertyAccessorNode.cs

@@ -42,8 +42,11 @@ internal sealed class DynamicPluginPropertyAccessorNode : ExpressionNode, IPrope
         return _accessor?.SetValue(value, BindingPriority.LocalValue) ?? false;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         var reference = new WeakReference<object?>(source);
 
         if (GetPlugin(source) is { } plugin &&

src/Avalonia.Base/Data/Core/ExpressionNodes/Reflection/DynamicPluginStreamNode.cs

@@ -16,8 +16,11 @@ internal sealed class DynamicPluginStreamNode : ExpressionNode
         builder.Append('^');
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         var reference = new WeakReference<object?>(source);
 
         if (GetPlugin(reference) is { } plugin &&

src/Avalonia.Base/Data/Core/ExpressionNodes/Reflection/ReflectionIndexerNode.cs

@@ -52,8 +52,11 @@ internal sealed class ReflectionIndexerNode : CollectionNodeBase, ISettableNode
         return true;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         _indexes = null;
 
         if (GetIndexer(source.GetType(), out _getter, out _setter))

src/Avalonia.Base/Data/Core/ExpressionNodes/Reflection/ReflectionTypeCastNode.cs

@@ -19,8 +19,11 @@ internal sealed class ReflectionTypeCastNode : ExpressionNode
         builder.Append(')');
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (_targetType.IsInstanceOfType(source))
             SetValue(source);
         else

src/Avalonia.Base/Data/Core/ExpressionNodes/StreamNode.cs

@@ -23,8 +23,11 @@ internal sealed class StreamNode : ExpressionNode, IObserver<object?>
     void IObserver<object?>.OnError(Exception error) { }
     void IObserver<object?>.OnNext(object? value) => SetValue(value);
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (_plugin.Start(new(source)) is { } accessor)
         {
             _subscription = accessor.Subscribe(this);

src/Avalonia.Base/Data/Core/ExpressionNodes/TemplatedParentNode.cs

@@ -22,8 +22,11 @@ internal sealed class TemplatedParentNode : SourceNode
         throw new InvalidOperationException("Cannot find a StyledElement to get a TemplatedParent.");
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is StyledElement newElement)
         {
             newElement.PropertyChanged += OnPropertyChanged;

src/Avalonia.Base/Data/Core/ExpressionNodes/VisualAncestorElementNode.cs

@@ -56,8 +56,11 @@ internal sealed class VisualAncestorElementNode : SourceNode
         return target is Visual visual && visual.IsAttachedToVisualTree;
     }
 
-    protected override void OnSourceChanged(object source, Exception? dataValidationError)
+    protected override void OnSourceChanged(object? source, Exception? dataValidationError)
     {
+        if (!ValidateNonNullSource(source))
+            return;
+
         if (source is Visual visual)
         {
             var locator = VisualLocator.Track(visual, _ancestorLevel, _ancestorType);

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

@@ -1,4 +1,6 @@
 using System;
+using System.Collections;
+using System.Collections.Generic;
 using System.Globalization;
 using System.Reactive.Subjects;
 using Avalonia.Controls;
@@ -426,11 +428,120 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             }
         }
 
+        [Theory]
+        [MemberData(nameof(NegationData))]
+        public void Negating_Object_Returns_Correct_Value(object value, bool? expected)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        Tag='{Binding !Object}'>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var viewModel = new WindowViewModel { Object = value };
+
+                window.DataContext = viewModel;
+                window.ApplyTemplate();
+
+                Assert.Equal(expected, window.Tag);
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(NegationData))]
+        public void Double_Negating_Object_Returns_Correct_Value(object value, bool? negated)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        Tag='{Binding !!Object}'>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var viewModel = new WindowViewModel { Object = value };
+
+                window.DataContext = viewModel;
+                window.ApplyTemplate();
+
+                var expected = negated.HasValue ? !negated : null;
+                Assert.Equal(expected, window.Tag);
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(NegationData))]
+        public void Negating_Object_Returns_Correct_Value_When_Bound_To_Bool(object value, bool? expected)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        IsVisible='{Binding !Object}'>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var viewModel = new WindowViewModel { Object = value };
+
+                window.DataContext = viewModel;
+                window.ApplyTemplate();
+
+                Assert.Equal(expected ?? false, window.IsVisible);
+            }
+        }
+
+        [Theory]
+        [MemberData(nameof(NegationData))]
+        public void Double_Negating_Object_Returns_Correct_Value_When_Bound_To_Bool(object value, bool? negated)
+        {
+            using (UnitTestApplication.Start(TestServices.StyledWindow))
+            {
+                var xaml = @"
+<Window xmlns='https://github.com/avaloniaui'
+        xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'
+        IsVisible='{Binding !!Object}'>
+</Window>";
+                var window = (Window)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var viewModel = new WindowViewModel { Object = value };
+
+                window.DataContext = viewModel;
+                window.ApplyTemplate();
+
+                var expected = negated.HasValue ? !negated : false;
+                Assert.Equal(expected, window.IsVisible);
+            }
+        }
+
+        public static IEnumerable<object[]> NegationData()
+        {
+            yield return new object[] { true, false };
+            yield return new object[] { false, true };
+            yield return new object[] { null, true };
+            yield return new object[] { new object(), null };
+            yield return new object[] { "foo", null };
+            yield return new object[] { "true", false };
+            yield return new object[] { "false", true };
+            yield return new object[] { 0, true };
+            yield return new object[] { 1, false };
+            yield return new object[] { 2, false };
+            yield return new object[] { -1, false };
+            yield return new object[] { 0.0, true };
+            yield return new object[] { 1.0, false };
+            yield return new object[] { 2.0, false };
+            yield return new object[] { -1.0, false };
+            yield return new object[] { double.NaN, false };
+            yield return new object[] { double.PositiveInfinity, false };
+            yield return new object[] { double.NegativeInfinity, false };
+        }
+
         private class WindowViewModel
         {
             public bool ShowInTaskbar { get; set; }
             public string Greeting1 { get; set; } = "Hello";
             public string Greeting2 { get; set; } = "World";
+            public object Object { get; set; }
         }
 
         public class CultureAppender : IValueConverter