Fix nested ResourceDictionaries

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlThemeVariantProviderTransformer.cs

@@ -10,7 +10,8 @@ internal class AvaloniaXamlIlThemeVariantProviderTransformer : IXamlAstTransform
 {
     public IXamlAstNode Transform(AstTransformationContext context, IXamlAstNode node)
     {
-        var type = context.GetAvaloniaTypes().IThemeVariantProvider;
+        var avTypes = context.GetAvaloniaTypes();
+        var type = avTypes.IThemeVariantProvider;
         if (!(node is XamlAstObjectNode on
               && type.IsAssignableFrom(on.Type.GetClrType())))
             return node;
@@ -21,6 +22,13 @@ internal class AvaloniaXamlIlThemeVariantProviderTransformer : IXamlAstTransform
         if (keyDirective is null)
             return node;
 
+        var themeDictionariesColl = avTypes.IDictionaryT.MakeGenericType(avTypes.ThemeVariant, avTypes.IThemeVariantProvider);
+        if (context.ParentNodes().FirstOrDefault() is not XamlAstXamlPropertyValueNode propertyValueNode
+            || !themeDictionariesColl.IsAssignableFrom(propertyValueNode.Property.GetClrProperty().Getter.ReturnType))
+        {
+            return node;
+        }
+        
         var keyProp = type.Properties.First(p => p.Name == "Key");
         on.Children.Add(new XamlAstXamlPropertyValueNode(keyDirective,
             new XamlAstClrProperty(keyDirective, keyProp, context.Configuration),

src/Markup/Avalonia.Markup.Xaml.Loader/CompilerExtensions/Transformers/AvaloniaXamlIlWellKnownTypes.cs

@@ -65,6 +65,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
         public IXamlType Int { get; }
         public IXamlType Long { get; }
         public IXamlType Uri { get; }
+        public IXamlType IDictionaryT { get; }
         public IXamlType FontFamily { get; }
         public IXamlConstructor FontFamilyConstructorUriName { get; }
         public IXamlType Thickness { get; }
@@ -194,6 +195,7 @@ namespace Avalonia.Markup.Xaml.XamlIl.CompilerExtensions.Transformers
             Int = cfg.TypeSystem.GetType("System.Int32");
             Long = cfg.TypeSystem.GetType("System.Int64");
             Uri = cfg.TypeSystem.GetType("System.Uri");
+            IDictionaryT = cfg.TypeSystem.GetType("System.Collections.Generic.IDictionary`2");
             FontFamily = cfg.TypeSystem.GetType("Avalonia.Media.FontFamily");
             FontFamilyConstructorUriName = FontFamily.GetConstructor(new List<IXamlType> { Uri, XamlIlTypes.String });
             ThemeVariant = cfg.TypeSystem.GetType("Avalonia.Styling.ThemeVariant");

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

@@ -385,6 +385,23 @@ namespace Avalonia.Markup.Xaml.UnitTests.Xaml
             Assert.Equal(Colors.Blue, ((ISolidColorBrush)userControl.FindResource("brush")!).Color);
         }
 
+        [Fact]
+        public void ResourceDictionary_Can_Be_Put_Inside_Of_ResourceDictionary()
+        {
+            using (StyledWindow())
+            {
+                var xaml = @"
+<ResourceDictionary xmlns='https://github.com/avaloniaui'
+                    xmlns:x='http://schemas.microsoft.com/winfx/2006/xaml'>
+    <ResourceDictionary x:Key='NotAThemeVariantKey' />
+</ResourceDictionary>";
+                var resources = (ResourceDictionary)AvaloniaRuntimeXamlLoader.Load(xaml);
+                var nested = (ResourceDictionary)resources["NotAThemeVariantKey"];
+
+                Assert.NotNull(nested);
+            }
+        }
+        
         private IDisposable StyledWindow(params (string, string)[] assets)
         {
             var services = TestServices.StyledWindow.With(