[X] Fix XamlCompilation of RD subclasses

src/Controls/src/Build.Tasks/SetPropertiesVisitor.cs

@@ -1464,7 +1464,7 @@ static bool CanAdd(VariableDefinition parent, XmlName propertyName, INode valueN
 		static bool CanAddToResourceDictionary(VariableDefinition parent, TypeReference collectionType, IElementNode node, IXmlLineInfo lineInfo, ILContext context)
 		{
 			if (collectionType.FullName != "Microsoft.Maui.Controls.ResourceDictionary"
-				&& collectionType.ResolveCached(context.Cache).BaseType?.FullName != "Microsoft.Maui.Controls.ResourceDictionary")
+				&& !collectionType.InheritsFromOrImplements(context.Cache, context.Module.ImportReference(context.Cache, ("Microsoft.Maui.Controls", "Microsoft.Maui.Controls", "ResourceDictionary"))))
 				return false;
 
 			if (node.Properties.ContainsKey(XmlName.xKey))

src/Controls/tests/Xaml.UnitTests/Issues/Maui19535.xaml

@@ -0,0 +1,11 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<local:Maui19535CustomThemeDictionary xmlns="http://schemas.microsoft.com/dotnet/2021/maui"
+                    xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml"
+                    xmlns:local="clr-namespace:Microsoft.Maui.Controls.Xaml.UnitTests"
+                    x:Class="Microsoft.Maui.Controls.Xaml.UnitTests.Maui19535">
+
+    <x:String x:Key="CustomTheme">LightTheme</x:String>    
+    <Color x:Key="PrimaryBackground">#6750A4</Color>
+    <Color x:Key="ContentBackground">#FFFBFE</Color>
+
+</local:Maui19535CustomThemeDictionary>

src/Controls/tests/Xaml.UnitTests/Issues/Maui19535.xaml.cs

@@ -0,0 +1,59 @@
+using System;
+using System.Collections.Generic;
+using System.Diagnostics;
+using System.Globalization;
+using System.Linq;
+using Microsoft.Maui.ApplicationModel;
+using Microsoft.Maui.Controls.Core.UnitTests;
+using Microsoft.Maui.Controls.Shapes;
+using Microsoft.Maui.Devices;
+using Microsoft.Maui.Dispatching;
+
+using Microsoft.Maui.Graphics;
+using Microsoft.Maui.UnitTests;
+using NUnit.Framework;
+
+namespace Microsoft.Maui.Controls.Xaml.UnitTests;
+
+public class Maui19535CustomThemeDictionary : ResourceDictionary
+{
+
+}
+
+public partial class Maui19535 : Maui19535CustomThemeDictionary
+{
+    public Maui19535()
+    {
+        InitializeComponent();
+    }
+
+    public Maui19535(bool useCompiledXaml)
+    {
+        //this stub will be replaced at compile time
+    }
+
+    [TestFixture]
+    class Test
+    {
+        [SetUp]
+        public void Setup()
+        {
+            Application.SetCurrentApplication(new MockApplication());
+            DispatcherProvider.SetCurrent(new DispatcherProviderStub());
+        }
+
+        [TearDown] public void TearDown() => AppInfo.SetCurrent(null);
+
+        [Test]
+        public void SubClassOfRDShouldNotThrow([Values(false, true)] bool useCompiledXaml)
+        {
+            if (useCompiledXaml)
+                MockCompiler.Compile(typeof(Maui19535));
+            var rd = new Maui19535(useCompiledXaml);
+            Assert.That(rd.Count, Is.EqualTo(3));
+            Assert.True(rd.TryGetValue("CustomTheme", out var theme));
+            Assert.That(theme, Is.EqualTo("LightTheme"));
+        }
+    }
+
+}