[SG] generated code behind for RD in global xmlns

we weren't generating the boiler plate for ResourceDictionary using the
global xmlns for XamlC compilation or sourcegen.

fixing that, and getting back the #performance

- fies #31602

Author: StephaneDelcroix

src/Controls/src/SourceGen/CodeBehindCodeWriter.cs

@@ -312,15 +312,29 @@ public static bool TryParseXaml(XamlProjectItemForCB parseResult, string uid, Co
 			XmlnsHelper.ParseXmlns(rootClass.Value, out rootType, out rootClrNamespace, out _, out _);
 		}
 #if _MAUIXAML_SOURCEGEN_BACKCOMPAT
-		else if (hasXamlCompilationProcessingInstruction && root.NamespaceURI == XamlParser.MauiUri)
+		else if (hasXamlCompilationProcessingInstruction
+				&& (root.NamespaceURI == XamlParser.MauiUri || root.NamespaceURI == XamlParser.MauiGlobalUri))
+#else
+		else if (root.NamespaceURI == XamlParser.MauiUri || root.NamespaceURI == XamlParser.MauiGlobalUri)
+#endif
 		{
+			//make sure the base type can be resolved. if not, don't consider this as xaml, and move away
+			var typeArgs = GetAttributeValue(root, "TypeArguments", XamlParser.X2006Uri, XamlParser.X2009Uri);
+			try
+			{
+				var basetype = new XmlType(root.NamespaceURI, root.LocalName, typeArgs != null ? TypeArgumentsParser.ParseExpression(typeArgs, nsmgr, null) : null).GetTypeSymbol(null, compilation, xmlnsCache);
+			}
+			catch
+			{
+				return false;
+			}
+
 			rootClrNamespace = "__XamlGeneratedCode__";
 			rootType = $"__Type{uid}";
 			generateDefaultCtor = true;
 			addXamlCompilationAttribute = true;
 			hideFromIntellisense = true;
 		}
-#endif
 		else if (parseResult?.ProjectItem?.ManifestResourceName != null && parseResult.ProjectItem.TargetPath != null)
 		{ // rootClass == null && !hasXamlCompilationProcessingInstruction) {
 			xamlResourceIdOnly = true; //only generate the XamlResourceId assembly attribute

src/Controls/src/SourceGen/ISymbolExtensions.cs

@@ -1,11 +1,6 @@
-using System.Collections.Generic;
 using System.Collections.Immutable;
-using System.ComponentModel;
 using System.Linq;
 using Microsoft.CodeAnalysis;
-using Microsoft.CodeAnalysis.CSharp;
-using Microsoft.CodeAnalysis.CSharp.Syntax;
-using Microsoft.CodeAnalysis.Text;
 
 namespace Microsoft.Maui.Controls.SourceGen;
 

src/Controls/tests/SourceGen.UnitTests/SourceGenXamlCodeBehindTests.cs

@@ -92,6 +92,35 @@ public void TestCodeBehindGenerator_AggregatedXmlns()
 		Assert.IsTrue(generated.Contains("global::Microsoft.Maui.Controls.Label label", StringComparison.Ordinal));
 	}
 
+	[Test]
+	public void TestCodeBehindGenerator_AggregatedXmlnsOnRD()
+	{
+		var xaml =
+"""
+<?xml version="1.0" encoding="UTF-8"?>
+<ResourceDictionary
+	xmlns="http://schemas.microsoft.com/dotnet/maui/global"
+	xmlns:x="http://schemas.microsoft.com/winfx/2009/xaml">
+	<x:String x:Key="MyString">Hello MAUI!</x:String>
+</ResourceDictionary>
+""";
+
+		var code =
+		"""
+using Microsoft.Maui.Controls;
+[assembly: XmlnsDefinition("http://schemas.microsoft.com/dotnet/maui/global", "http://schemas.microsoft.com/dotnet/2021/maui")]
+""";
+		var compilation = CreateMauiCompilation();
+		compilation = compilation.AddSyntaxTrees(CSharpSyntaxTree.ParseText(code));
+		var result = RunGenerator<CodeBehindGenerator>(compilation, new AdditionalXamlFile("Test.xaml", xaml));
+
+		Assert.IsFalse(result.Diagnostics.Any());
+
+		var generated = result.Results.Single().GeneratedSources.Single(gs => gs.HintName.EndsWith(".sg.cs")).SourceText.ToString();
+
+		Assert.IsTrue(generated.Contains("public partial class __Type", StringComparison.Ordinal));
+	}
+
 	public void TestCodeBehindGenerator_LocalXaml([Values] bool resolvedType)
 	{
 		var xaml =
@@ -305,9 +334,11 @@ public void TestCodeBehindGenerator_NotXaml()
 </foo>
 """;
 		var compilation = SourceGeneratorDriver.CreateMauiCompilation();
+		compilation = compilation.AddSyntaxTrees(CSharpSyntaxTree.ParseText("[assembly: global::Microsoft.Maui.Controls.Xaml.Internals.AllowImplicitXmlnsDeclaration]"));
 		var result = SourceGeneratorDriver.RunGenerator<CodeBehindGenerator>(compilation, new AdditionalXamlFile("Test.xaml", xaml));
 
-		Assert.That(result.Diagnostics.Any());
+		var generated = result.Results.Single().GeneratedSources.Single(gs => gs.HintName.EndsWith(".sg.cs")).SourceText.ToString();
+		Assert.That(result.Diagnostics.Any() || string.IsNullOrWhiteSpace(generated));
 	}
 
 	[Test]