Toolbar language-version dropdown affects decompile output

Two halves to the fix:

Assisted-by: Claude:claude-opus-4-7:Claude Code

ILSpy.Tests/Languages/LanguageVersionAffectsDecompilerOutputTests.cs

@@ -0,0 +1,92 @@
+// Copyright (c) 2026 AlphaSierraPapa for the SharpDevelop Team
+//
+// 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.
+
+using System;
+using System.Linq;
+using System.Threading.Tasks;
+
+using Avalonia.Headless.NUnit;
+
+using AwesomeAssertions;
+
+using ILSpy.AppEnv;
+using ILSpy.Languages;
+using ILSpy.TreeNodes;
+using ILSpy.ViewModels;
+using ILSpy.Views;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.ILSpy.Tests;
+
+[TestFixture]
+public class LanguageVersionAffectsDecompilerOutputTests
+{
+	[AvaloniaTest]
+	public async Task Changing_Language_Version_Triggers_Redecompile_And_Output_Reflects_New_Version()
+	{
+		// Picking System.Linq.Enumerable.Select (the IEnumerable+Func overload) — its body uses
+		// `is`-pattern-matching with declaration (C# 7+), so dropping to C# 6 forces the decompiler
+		// to fall back to explicit cast + null-check + variable assignment. Output text therefore
+		// differs sharply between C# 14 and C# 6.
+		//
+		// Without the fix the test fails two ways at once:
+		//   1. Setting LanguageService.CurrentVersion never triggers a re-decompile (the
+		//      DockWorkspace listener watches CurrentLanguage only) so the wait for "text differs"
+		//      times out.
+		//   2. Even if a re-decompile fired, TryGetLiveDecompilerSettings never reads
+		//      CurrentVersion, so the produced DecompilerSettings still targets Latest and the
+		//      output text would not change.
+
+		var window = AppComposition.Current.GetExport<MainWindow>();
+		window.Show();
+		var vm = (MainWindowViewModel)window.DataContext!;
+		await vm.AssemblyTreeModel.WaitForAssembliesAsync(minimumCount: 3);
+
+		var typeNode = vm.AssemblyTreeModel.FindNode<TypeTreeNode>(
+			"System.Linq", "System.Linq", "System.Linq.Enumerable");
+		typeNode.IsExpanded = true;
+		// Two `Select` overloads (with and without index); the IEnumerable + Func<TSource,TResult>
+		// one is the version-sensitive target.
+		var selectNode = typeNode.Children.OfType<MethodTreeNode>()
+			.First(m => m.MethodDefinition.Name == "Select"
+				&& m.MethodDefinition.Parameters.Count == 2);
+
+		var languageService = AppComposition.Current.GetExport<LanguageService>();
+		// Baseline: latest C# version so `is`-pattern-matching is used.
+		languageService.CurrentVersion = languageService.CurrentLanguage.LanguageVersions.Last();
+
+		vm.AssemblyTreeModel.SelectNode(selectNode);
+		var tab = await vm.DockWorkspace.WaitForDecompiledTextAsync();
+		var latestText = tab.Text;
+
+		// Act — drop to C# 6 (no `is`-pattern-matching declarations) and wait for the next
+		// decompile to land.
+		var cs6 = languageService.CurrentLanguage.LanguageVersions
+			.First(v => v.DisplayName.StartsWith("C# 6.0"));
+		languageService.CurrentVersion = cs6;
+
+		await Waiters.WaitForAsync(
+			() => !tab.IsDecompiling && tab.Text != latestText,
+			TimeSpan.FromSeconds(15),
+			"re-decompile to fire after CurrentVersion change AND emit different output");
+
+		tab.Text.Should().NotBe(latestText,
+			"switching the language version must visibly change the decompiled output");
+	}
+}

ILSpy/Docking/DockWorkspace.cs

@@ -461,12 +461,14 @@ namespace ILSpy.Docking
 
 		void OnLanguagePropertyChanged(object? sender, PropertyChangedEventArgs e)
 		{
-			if (e.PropertyName == nameof(LanguageService.CurrentLanguage))
+			if (e.PropertyName is nameof(LanguageService.CurrentLanguage) or nameof(LanguageService.CurrentVersion))
 			{
 				if (ActiveDecompilerTab is { } tab)
 				{
 					tab.Language = languageService.CurrentLanguage;
-					// Re-decompile by re-assigning the same node so the tab refreshes for the new language.
+					// Re-decompile by re-assigning the same node so the tab refreshes for the new
+					// language or language version (the version is read inside
+					// TryGetLiveDecompilerSettings, which builds the per-run DecompilerSettings).
 					var node = tab.CurrentNode;
 					tab.CurrentNode = null;
 					tab.CurrentNode = node;

ILSpy/TextView/DecompilerTabPageModel.cs

@@ -578,14 +578,31 @@ namespace ILSpy.TextView
 			Title = string.IsNullOrEmpty(output.Title) ? "(report)" : output.Title;
 		}
 
-		// Pulls the live DecompilerSettings via MEF and returns a clone for this run. MEF
-		// resolve is wrapped in try/catch so design-time / minimal test hosts that bypass
-		// composition fall back to default settings rather than throwing.
+		// Pulls the live DecompilerSettings via MEF and returns a clone for this run. Also
+		// bakes the active LanguageService.CurrentVersion into the clone — without this the
+		// toolbar's Language-Version dropdown writes-through to LanguageSettings but never
+		// reaches the decompiler. MEF resolves are wrapped in try/catch so design-time /
+		// minimal test hosts that bypass composition fall back to default settings rather
+		// than throwing.
 		static ICSharpCode.Decompiler.DecompilerSettings? TryGetLiveDecompilerSettings()
 		{
 			try
 			{
-				return AppEnv.AppComposition.Current.GetExport<SettingsService>().DecompilerSettings.Clone();
+				var settings = AppEnv.AppComposition.Current.GetExport<SettingsService>().DecompilerSettings.Clone();
+				try
+				{
+					var version = AppEnv.AppComposition.Current.GetExport<Languages.LanguageService>().CurrentVersion;
+					if (Enum.TryParse<ICSharpCode.Decompiler.CSharp.LanguageVersion>(version?.Version, out var languageVersion))
+						settings.SetLanguageVersion(languageVersion);
+					else
+						settings.SetLanguageVersion(ICSharpCode.Decompiler.CSharp.LanguageVersion.Latest);
+				}
+				catch
+				{
+					// No LanguageService available (non-C# language or minimal host) — leave the
+					// settings at whatever default the source DecompilerSettings carried.
+				}
+				return settings;
 			}
 			catch
 			{