ILSpy/ILSpy.Tests/Languages/LanguageVersionAffectsDecompilerOutputTests.cs

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using System;
using System.Linq;
using System.Threading.Tasks;

using Avalonia.Headless.NUnit;

using AwesomeAssertions;

using ICSharpCode.ILSpy.AppEnv;
using ICSharpCode.ILSpy.Languages;
using ICSharpCode.ILSpy.TreeNodes;
using ICSharpCode.ILSpy.ViewModels;
using ICSharpCode.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");
	}
}