Put a XAML document where its code-behind is

The project exporter wrote every XAML document to the project root under
a fully-qualified name while the code-behind class went into a directory
named after its namespace, so the two halves of one partial class ended
up in different places. WPF tooling pairs MainWindow.xaml with
MainWindow.xaml.cs by name and location; anything else is an unrelated
file to it, and --nested-directories made the split wider still by moving
only the C# half.

Both now go through one function that decides where a type's files live,
so the document lands where the type's own C# file would have, and the
code-behind is named after the document. The BAML writers of the UI and
of the command line had grown their own copies of the naming, which is
how they came to disagree with the C# writer in the first place.

Assisted-by: Claude:claude-opus-5:Claude Code

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs

@@ -387,23 +387,26 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 
 			string GetFileFileNameForHandle(TypeDefinitionHandle h)
 			{
-				var type = metadata.GetTypeDefinition(h);
-				string file = CleanUpFileName(metadata.GetString(type.Name), ".cs");
-				string ns = metadata.GetString(type.Namespace);
-				if (string.IsNullOrEmpty(ns))
-				{
-					return file;
-				}
-				else
+				// A code-behind class belongs to the document it completes: WPF tooling expects
+				// MainWindow.xaml.cs beside MainWindow.xaml, and treats a stray MainWindow.cs
+				// elsewhere in the tree as an unrelated file.
+				foreach (var partialType in partialTypes)
 				{
-					string dir = Settings.UseNestedDirectoriesForNamespaces ? CleanUpPath(ns) : CleanUpDirectoryName(ns);
-					if (directories.Add(dir))
+					if (partialType.DeclaringTypeDefinitionHandle == h && partialType.CompanionFileName != null)
 					{
-						var path = Path.Combine(TargetDirectory, dir);
-						CreateDirectory(path);
+						string companionDirectory = Path.GetDirectoryName(partialType.CompanionFileName)!;
+						if (!string.IsNullOrEmpty(companionDirectory) && directories.Add(companionDirectory))
+							CreateDirectory(Path.Combine(TargetDirectory, companionDirectory));
+						return partialType.CompanionFileName + ".cs";
 					}
-					return Path.Combine(dir, file);
 				}
+
+				var type = metadata.GetTypeDefinition(h);
+				string fileName = GetFileNameForType(metadata.GetString(type.Namespace), metadata.GetString(type.Name), ".cs");
+				string directory = Path.GetDirectoryName(fileName)!;
+				if (!string.IsNullOrEmpty(directory) && directories.Add(directory))
+					CreateDirectory(Path.Combine(TargetDirectory, directory));
+				return fileName;
 			}
 
 			void ProcessFiles(List<IGrouping<string, TypeDefinitionHandle>> files)
@@ -972,6 +975,30 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 		/// Removes invalid characters from file names and reduces their length,
 		/// but keeps file extensions and path structure intact.
 		/// </summary>
+		/// <summary>
+		/// The path of a file belonging to a type: the namespace becomes directories or one
+		/// flattened directory name, depending on
+		/// <see cref="DecompilerSettings.UseNestedDirectoriesForNamespaces"/>. Everything a type
+		/// owns - its C# file and the XAML document it is the code-behind of - goes here, so the
+		/// two end up next to each other.
+		/// </summary>
+		public static string GetFileNameForType(string @namespace, string typeName, string extension,
+			bool useNestedDirectoriesForNamespaces)
+		{
+			string file = CleanUpFileName(typeName, extension);
+			if (string.IsNullOrEmpty(@namespace))
+				return file;
+			string directory = useNestedDirectoriesForNamespaces
+				? CleanUpPath(@namespace)
+				: CleanUpDirectoryName(@namespace);
+			return Path.Combine(directory, file);
+		}
+
+		protected string GetFileNameForType(string @namespace, string typeName, string extension)
+		{
+			return GetFileNameForType(@namespace, typeName, extension, Settings.UseNestedDirectoriesForNamespaces);
+		}
+
 		public static string SanitizeFileName(string fileName)
 		{
 			return CleanUpName(fileName, separateAtDots: false, treatAsFileName: true, treatAsPath: true);

ICSharpCode.Decompiler/PartialTypeInfo.cs

@@ -16,6 +16,8 @@
 // OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
 // DEALINGS IN THE SOFTWARE.
 
+#nullable enable
+
 using System;
 using System.Collections.Generic;
 using System.Diagnostics;
@@ -43,6 +45,13 @@ namespace ICSharpCode.Decompiler
 
 		public TypeDefinitionHandle DeclaringTypeDefinitionHandle { get; }
 
+		/// <summary>
+		/// The document this type is the code-behind of, as a project-relative path
+		/// ("Views/MainWindow.xaml"), where there is one. The project decompiler names the type's
+		/// C# file after it, so that the two sit next to each other the way the tooling expects.
+		/// </summary>
+		public string? CompanionFileName { get; set; }
+
 		public void AddDeclaredMember(IMember member)
 		{
 			declaredMembers.Add(member.MetadataToken);

ICSharpCode.ILSpyCmd.Tests/BamlFixtureTypes.cs

@@ -0,0 +1,28 @@
+// Copyright (c) 2026 Siegfried Pammer
+//
+// Permission is hereby granted, free of charge, to any person obtaining a copy of this
+// software and associated documentation files (the "Software"), to deal in the Software
+// without restriction, including without limitation the rights to use, copy, modify, merge,
+// publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons
+// to whom the Software is furnished to do so, subject to the following conditions:
+//
+// The above copyright notice and this permission notice shall be included in all copies or
+// substantial portions of the Software.
+//
+// THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED,
+// INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR
+// PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE
+// FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR
+// OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER
+// DEALINGS IN THE SOFTWARE.
+
+namespace ICSharpCode.ILSpyCmd.Tests.Views
+{
+	/// <summary>
+	/// Plays the role of a WPF code-behind class: the BAML fixture "views/deeppage.baml" names it
+	/// as its root, which is how the BAML decompiler recognises an x:Class type.
+	/// </summary>
+	public class DeepPage
+	{
+	}
+}

ICSharpCode.ILSpyCmd.Tests/ProjectExportBamlTests.cs

@@ -80,6 +80,32 @@ namespace ICSharpCode.ILSpyCmd.Tests
 			});
 		}
 
+		[Test]
+		public async Task TheCodeBehindSitsNextToItsDocument()
+		{
+			// WPF tooling pairs MainWindow.xaml with MainWindow.xaml.cs by name and location; a
+			// code-behind anywhere else is an unrelated file as far as the project is concerned.
+			await RunAsync(testAssemblyPath, "--disable-updatecheck", "-p", "-o", outputDirectory);
+
+			string documentDirectory = Path.Combine(outputDirectory, "ICSharpCode.ILSpyCmd.Tests.Views");
+			Assert.Multiple(() => {
+				Assert.That(File.Exists(Path.Combine(documentDirectory, "DeepPage.xaml")), Is.True, "the document");
+				Assert.That(File.Exists(Path.Combine(documentDirectory, "DeepPage.xaml.cs")), Is.True, "its code-behind");
+			});
+		}
+
+		[Test]
+		public async Task DocumentsFollowTheNamespaceDirectoriesToo()
+		{
+			await RunAsync(testAssemblyPath, "--disable-updatecheck", "-p", "--nested-directories", "-o", outputDirectory);
+
+			string documentDirectory = Path.Combine(outputDirectory, "ICSharpCode", "ILSpyCmd", "Tests", "Views");
+			Assert.Multiple(() => {
+				Assert.That(File.Exists(Path.Combine(documentDirectory, "DeepPage.xaml")), Is.True, "the document");
+				Assert.That(File.Exists(Path.Combine(documentDirectory, "DeepPage.xaml.cs")), Is.True, "its code-behind");
+			});
+		}
+
 		[Test]
 		public async Task TheOldOptInFlagStillWorks()
 		{

ICSharpCode.ILSpyCmd/BamlAwareWholeProjectDecompiler.cs

@@ -67,8 +67,10 @@ namespace ICSharpCode.ILSpyCmd
 				: null;
 			if (typeDefinition != null)
 			{
-				xamlFileName = SanitizeFileName(typeDefinition.ReflectionName + ".xaml");
-				partialTypeInfo = new PartialTypeInfo(typeDefinition);
+				// Next to where the type's own C# file goes, so that the code-behind can be named
+				// after the document and land beside it.
+				xamlFileName = GetFileNameForType(typeDefinition.Namespace, typeDefinition.Name, ".xaml");
+				partialTypeInfo = new PartialTypeInfo(typeDefinition) { CompanionFileName = xamlFileName };
 				foreach (var member in result.GeneratedMembers)
 					partialTypeInfo.AddDeclaredMember(member);
 			}

ILSpy/TreeNodes/BamlResourceNodeFactory.cs

@@ -79,16 +79,17 @@ namespace ICSharpCode.ILSpy.Baml
 				CancellationToken = context.DecompilationOptions.CancellationToken,
 			};
 			var result = decompiler.Decompile(stream);
-			// If the BAML root names a CLR partial-class type, prefer the type's reflection name
-			// for the .xaml file so it lines up with the matching .xaml.cs the C# project writer
-			// emits. Otherwise just swap extensions on the existing resource name.
+			// If the BAML root names a CLR partial-class type, the document goes where that type's
+			// own C# file goes, and the code-behind is then named after the document and lands
+			// beside it. Otherwise just swap extensions on the existing resource name.
 			var typeDefinition = result.TypeName.HasValue
 				? typeSystem.MainModule.GetTypeDefinition(result.TypeName.Value.TopLevelTypeName)
 				: null;
 			if (typeDefinition != null)
 			{
-				fileName = WholeProjectDecompiler.SanitizeFileName(typeDefinition.ReflectionName + ".xaml");
-				var partialTypeInfo = new PartialTypeInfo(typeDefinition);
+				fileName = WholeProjectDecompiler.GetFileNameForType(typeDefinition.Namespace, typeDefinition.Name, ".xaml",
+					context.DecompilationOptions.DecompilerSettings.UseNestedDirectoriesForNamespaces);
+				var partialTypeInfo = new PartialTypeInfo(typeDefinition) { CompanionFileName = fileName };
 				foreach (var member in result.GeneratedMembers)
 					partialTypeInfo.AddDeclaredMember(member);
 				context.AddPartialTypeInfo(partialTypeInfo);