Make DynamicResource use a BindingExpression.

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

@@ -282,7 +282,7 @@ public abstract class UntypedBindingExpressionBase : BindingExpressionBase,
     /// <returns>
     /// True if the value could be written to the binding source; otherwise false.
     /// </returns>
-    internal abstract bool WriteValueToSource(object? value);
+    internal virtual bool WriteValueToSource(object? value) => false;
 
     /// <summary>
     /// Converts a value using a value converter, logging a warning if necessary.
@@ -368,6 +368,27 @@ public abstract class UntypedBindingExpressionBase : BindingExpressionBase,
         }
     }
 
+    /// <summary>
+    /// Logs a binding error.
+    /// </summary>
+    /// <param name="error">The error message.</param>
+    /// <param name="level">The log level.</param>
+    protected void Log(string error, LogEventLevel level = LogEventLevel.Warning)
+    {
+        if (!TryGetTarget(out var target))
+            return;
+
+        if (!Logger.TryGet(level, LogArea.Binding, out var log))
+            return;
+
+        log.Log(
+            target,
+            "An error occurred binding {Property} to {Expression}: {Message}",
+            (object?)TargetProperty ?? "(unknown)",
+            Description,
+            error);
+    }
+
     /// <summary>
     /// Logs a binding error.
     /// </summary>

src/Avalonia.Base/Styling/Setter.cs

@@ -118,7 +118,7 @@ namespace Avalonia.Styling
 
                 if (mode == BindingMode.OneWay || mode == BindingMode.TwoWay)
                 {
-                    if (i.Source is BindingExpression expression)
+                    if (i.Expression is { } expression)
                     {
                         expression.Attach(target.GetValueStore(), target, Property, instance.Priority);
                         return expression;

src/Markup/Avalonia.Markup.Xaml/Avalonia.Markup.Xaml.csproj

@@ -14,6 +14,7 @@
     <Compile Include="Converters\ColorToBrushConverter.cs" />
     <Compile Include="Converters\FontFamilyTypeConverter.cs" />
     <Compile Include="Converters\TimeSpanTypeConverter.cs" />
+    <Compile Include="Data\DynamicResourceExpression.cs" />
     <Compile Include="Extensions.cs" />
     <Compile Include="MarkupExtension.cs" />
     <Compile Include="MarkupExtensions\CompiledBindingExtension.cs" />

src/Markup/Avalonia.Markup.Xaml/Data/DynamicResourceExpression.cs

@@ -0,0 +1,149 @@
+using System;
+using System.Diagnostics.CodeAnalysis;
+using Avalonia.Controls;
+using Avalonia.Data.Core;
+using Avalonia.Logging;
+using Avalonia.Markup.Xaml.Converters;
+using Avalonia.Media;
+using Avalonia.Styling;
+
+namespace Avalonia.Markup.Xaml.MarkupExtensions
+{
+    internal class DynamicResourceExpression : UntypedBindingExpressionBase
+    {
+        private readonly object _resourceKey;
+        private readonly object? _anchor;
+        private IResourceHost? _host;
+        private IResourceProvider? _provider;
+        private bool _overrideThemeVariant;
+        private bool _targetTypeIsBrush;
+        private ThemeVariant? _themeVariant;
+
+        public DynamicResourceExpression(
+            object resourceKey,
+            object? anchor,
+            ThemeVariant? themeVariant)
+        {
+            _resourceKey = resourceKey;
+            _anchor = anchor;
+            _themeVariant = themeVariant;
+        }
+
+        public override string Description => $"DynamicResource {_resourceKey}";
+
+        protected override void StartCore()
+        {
+            if (!TryGetResourceHost(out _host))
+            {
+                // The target is not an IResourceHost, so we need to find one from the anchor.
+                if (_anchor is IResourceProvider provider)
+                {
+                    _provider = provider;
+                    _host = provider.Owner;
+                    _overrideThemeVariant = _themeVariant is not null;
+                }
+            }
+
+            // If we wouldn't find a host or provider then log an error: we can't do anything.
+            if (_host is null && _provider is null)
+            {
+                Log("Unable to find IResourceHost or IResourceProvider from which to lookup " +
+                    $"DynamicResource {_resourceKey}.", LogEventLevel.Error);
+                return;
+            }
+
+            // Hook up events.
+            if (_provider is not null)
+                _provider.OwnerChanged += OnResourceProviderOwnerChanged;
+            Subscribe(_host);
+
+            // And publish the initial value.
+            _targetTypeIsBrush = TargetType == typeof(IBrush);
+            PublishValue();
+        }
+
+        protected override void StopCore()
+        {
+            if (_provider is not null)
+                _provider.OwnerChanged -= OnResourceProviderOwnerChanged;
+            Unsubscribe(_host);
+            _host = null;
+            _provider = null;
+        }
+
+        private void OnResourceProviderOwnerChanged(object? sender, EventArgs e)
+        {
+            Unsubscribe(_host);
+            _host = _provider?.Owner;
+            Subscribe(_host);
+            PublishValue();
+        }
+
+        private void ResourcesChanged(object? sender, ResourcesChangedEventArgs e) => PublishValue();
+
+        private void ActualThemeVariantChanged(object? sender, EventArgs e)
+        {
+            _themeVariant = ((IThemeVariantHost)sender!).ActualThemeVariant;
+            PublishValue();
+        }
+
+        private void PublishValue()
+        {
+            if (_host is not null)
+            {
+                var theme = _themeVariant;
+                var value = _host.FindResource(theme, _resourceKey) ?? AvaloniaProperty.UnsetValue;
+                var convertedValue = _targetTypeIsBrush ?
+                    ColorToBrushConverter.Convert(value, typeof(IBrush)) :
+                    value;
+                PublishValue(convertedValue);
+            }
+            else
+            {
+                PublishValue(AvaloniaProperty.UnsetValue);
+            }
+        }
+
+        private bool TryGetResourceHost([NotNullWhen(true)] out IResourceHost? host)
+        {
+            if (TryGetTarget(out var target) && target is IResourceHost targetHost)
+            {
+                host = targetHost;
+                return true;
+            }
+
+            if (_anchor is IResourceHost anchorHost)
+            {
+                host = anchorHost;
+                return host is not null;
+            }
+
+            host = null;
+            return false;
+        }
+
+        private void Subscribe(IResourceHost? host)
+        {
+            if (host is not null)
+            {
+                host.ResourcesChanged += ResourcesChanged;
+                
+                if (!_overrideThemeVariant && _host is IThemeVariantHost themeVariantHost)
+                {
+                    _themeVariant = themeVariantHost.ActualThemeVariant;
+                    themeVariantHost.ActualThemeVariantChanged += ActualThemeVariantChanged;
+                }
+            }
+        }
+
+        private void Unsubscribe(IResourceHost? host)
+        {
+            if (host is not null)
+            {
+                host.ResourcesChanged -= ResourcesChanged;
+                if (!_overrideThemeVariant && _host is IThemeVariantHost themeVariantHost)
+                    themeVariantHost.ActualThemeVariantChanged -= ActualThemeVariantChanged;
+            }
+        }
+    }
+}

src/Markup/Avalonia.Markup.Xaml/MarkupExtensions/DynamicResourceExtension.cs

@@ -1,6 +1,9 @@
 using System;
+using System.Diagnostics.CodeAnalysis;
 using Avalonia.Controls;
 using Avalonia.Data;
+using Avalonia.Data.Core;
+using Avalonia.Logging;
 using Avalonia.Markup.Xaml.Converters;
 using Avalonia.Media;
 using Avalonia.Styling;
@@ -11,7 +14,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
     {
         private object? _anchor;
         private BindingPriority _priority;
-        private ThemeVariant? _currentThemeVariant;
+        private ThemeVariant? _themeVariant;
 
         public DynamicResourceExtension()
         {
@@ -38,7 +41,7 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
                     (object?)serviceProvider.GetFirstParent<IResourceHost>();
             }
 
-            _currentThemeVariant = StaticResourceExtension.GetDictionaryVariant(serviceProvider);
+            _themeVariant = StaticResourceExtension.GetDictionaryVariant(serviceProvider);
 
             return this;
         }
@@ -50,34 +53,9 @@ namespace Avalonia.Markup.Xaml.MarkupExtensions
             bool enableDataValidation)
         {
             if (ResourceKey is null)
-            {
                 return null;
-            }
-
-            var control = target as IResourceHost ?? _anchor as IResourceHost;
-
-            if (control != null)
-            {
-                var source = control.GetResourceObservable(ResourceKey, GetConverter(targetProperty));
-                return InstancedBinding.OneWay(source, _priority);
-            }
-            else if (_anchor is IResourceProvider resourceProvider)
-            {
-                var source = resourceProvider.GetResourceObservable(ResourceKey, _currentThemeVariant, GetConverter(targetProperty));
-                return InstancedBinding.OneWay(source, _priority);
-            }
-
-            return null;
-        }
-
-        private static Func<object?, object?>? GetConverter(AvaloniaProperty? targetProperty)
-        {
-            if (targetProperty?.PropertyType == typeof(IBrush))
-            {
-                return x => ColorToBrushConverter.Convert(x, typeof(IBrush));
-            }
-
-            return null;
+            var expression = new DynamicResourceExpression(ResourceKey, _anchor, _themeVariant);
+            return new InstancedBinding(target, expression, BindingMode.OneWay, _priority);
         }
     }
 }