[SG] generated code behind for RD in global xmlns (#31623)

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]