Merge pull request #3970 from icsharpcode/settings-source-generator

Generate DecompilerSettings boilerplate from [DecompilerSetting] attributes

ICSharpCode.Decompiler.Generators/AnalyzerReleases.Unshipped.md

@@ -6,3 +6,7 @@
 Rule ID | Category | Severity | Notes
 --------|----------|----------|-------
 DSTG001 | DecompilerSyntaxTreeGenerator | Error | Slot kind must map to a single child type
+DSTG002 | DecompilerSettingsGenerator | Error | [DecompilerSetting] target must be a partial instance bool property
+DSTG003 | DecompilerSettingsGenerator | Error | Version-gated setting must not declare [Category]
+DSTG004 | DecompilerSettingsGenerator | Error | Language version has no display category
+DSTG005 | DecompilerSettingsGenerator | Error | Setting must be declared in a non-nested partial class

ICSharpCode.Decompiler.Generators/DecompilerSettingsGenerator.cs

@@ -0,0 +1,369 @@
+// 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.
+
+using System.Collections.Immutable;
+using System.Text;
+
+using Microsoft.CodeAnalysis;
+using Microsoft.CodeAnalysis.CSharp;
+using Microsoft.CodeAnalysis.CSharp.Syntax;
+using Microsoft.CodeAnalysis.Text;
+
+namespace ICSharpCode.Decompiler.Generators;
+
+/// <summary>
+/// Generates the boilerplate behind [DecompilerSetting] partial properties: the backing field,
+/// the accessors with change notification, and - from the per-setting language version - the
+/// [Category] attribute plus the SetLanguageVersion and GetMinimumRequiredVersion methods,
+/// so that a setting's version is declared in exactly one place.
+/// </summary>
+[Generator]
+internal class DecompilerSettingsGenerator : IIncrementalGenerator
+{
+	static readonly DiagnosticDescriptor InvalidSettingProperty = new(
+		id: "DSTG002",
+		title: "[DecompilerSetting] target must be a partial instance bool property",
+		messageFormat: "Setting property '{0}' must be a partial instance bool property with get and set accessors and an uppercase-start name (the generated backing field uses the camelCase form)",
+		category: "DecompilerSettingsGenerator",
+		defaultSeverity: DiagnosticSeverity.Error,
+		isEnabledByDefault: true);
+
+	// The generated [Category] would join a handwritten one on the merged partial property, and
+	// GetCustomAttribute<CategoryAttribute>() (used by the settings UI) throws on duplicates.
+	static readonly DiagnosticDescriptor CategoryOnVersionedSetting = new(
+		id: "DSTG003",
+		title: "Version-gated setting must not declare [Category]",
+		messageFormat: "Setting '{0}' derives its [Category] from the language version; remove the handwritten [Category] attribute",
+		category: "DecompilerSettingsGenerator",
+		defaultSeverity: DiagnosticSeverity.Error,
+		isEnabledByDefault: true);
+
+	static readonly DiagnosticDescriptor UnsupportedLanguageVersion = new(
+		id: "DSTG004",
+		title: "Language version has no display category",
+		messageFormat: "Language version '{0}' has no display category; gate settings on a released C# version, or add the new version to DecompilerSettingsGenerator.CategoryByVersion",
+		category: "DecompilerSettingsGenerator",
+		defaultSeverity: DiagnosticSeverity.Error,
+		isEnabledByDefault: true);
+
+	// The generated implementation is emitted as a top-level partial class; a nested or
+	// non-partial containing type would make it merge nowhere (or into a stray new type).
+	static readonly DiagnosticDescriptor InvalidContainingType = new(
+		id: "DSTG005",
+		title: "Setting must be declared in a non-nested partial class",
+		messageFormat: "Setting '{0}' must be declared in a partial, non-nested class so the generated implementation merges into it",
+		category: "DecompilerSettingsGenerator",
+		defaultSeverity: DiagnosticSeverity.Error,
+		isEnabledByDefault: true);
+
+	static readonly Dictionary<string, DiagnosticDescriptor> DescriptorsById =
+		new DiagnosticDescriptor[] { InvalidSettingProperty, CategoryOnVersionedSetting, UnsupportedLanguageVersion, InvalidContainingType }
+			.ToDictionary(d => d.Id);
+
+	// Display category per released C# version; the settings UI groups options by these strings.
+	static readonly Dictionary<string, string> CategoryByVersion = new() {
+		["CSharp1"] = "C# 1.0 / VS .NET",
+		["CSharp2"] = "C# 2.0 / VS 2005",
+		["CSharp3"] = "C# 3.0 / VS 2008",
+		["CSharp4"] = "C# 4.0 / VS 2010",
+		["CSharp5"] = "C# 5.0 / VS 2012",
+		["CSharp6"] = "C# 6.0 / VS 2015",
+		["CSharp7"] = "C# 7.0 / VS 2017",
+		["CSharp7_1"] = "C# 7.1 / VS 2017.3",
+		["CSharp7_2"] = "C# 7.2 / VS 2017.4",
+		["CSharp7_3"] = "C# 7.3 / VS 2017.7",
+		["CSharp8_0"] = "C# 8.0 / VS 2019",
+		["CSharp9_0"] = "C# 9.0 / VS 2019.8",
+		["CSharp10_0"] = "C# 10.0 / VS 2022",
+		["CSharp11_0"] = "C# 11.0 / VS 2022.4",
+		["CSharp12_0"] = "C# 12.0 / VS 2022.8",
+		["CSharp13_0"] = "C# 13.0 / VS 2022.12",
+		["CSharp14_0"] = "C# 14.0 / VS 2026",
+	};
+
+	readonly record struct SettingInfo(
+		string Namespace, string ClassName, string Accessibility, string PropertyName, string FieldName,
+		bool DefaultValue, int VersionValue, string? VersionName, string? Category,
+		string FilePath, int SpanStart);
+
+	// A diagnostic captured during the transform; kept as plain values so the pipeline stays cacheable.
+	readonly record struct DiagInfo(string Id, string MessageArg, string FilePath, int SpanStart, int SpanLength,
+		int StartLine, int StartChar, int EndLine, int EndChar);
+
+	readonly record struct SettingResult(SettingInfo? Setting, EquatableArray<DiagInfo>? Diagnostics);
+
+	public void Initialize(IncrementalGeneratorInitializationContext context)
+	{
+		context.RegisterPostInitializationOutput(i => i.AddSource("DecompilerSettingsGeneratorAttributes.g.cs", RoslynHelpers.EmbeddedAttributeSource + @"
+namespace ICSharpCode.Decompiler
+{
+	[global::Microsoft.CodeAnalysis.EmbeddedAttribute]
+	[global::System.AttributeUsage(global::System.AttributeTargets.Property)]
+	sealed class DecompilerSettingAttribute : global::System.Attribute
+	{
+		public DecompilerSettingAttribute() { }
+
+		public DecompilerSettingAttribute(global::ICSharpCode.Decompiler.CSharp.LanguageVersion introducedIn) { }
+
+		/// <summary>Initial value of the setting. Defaults to true.</summary>
+		public bool DefaultValue { get; set; } = true;
+	}
+}
+
+"));
+
+		var settings = context.SyntaxProvider.ForAttributeWithMetadataName(
+			"ICSharpCode.Decompiler.DecompilerSettingAttribute",
+			(n, ct) => n is PropertyDeclarationSyntax,
+			GetSetting);
+
+		context.RegisterSourceOutput(settings.Collect(), WriteSettingsClasses);
+	}
+
+	static SettingResult GetSetting(GeneratorAttributeSyntaxContext context, CancellationToken cancellationToken)
+	{
+		var property = (IPropertySymbol)context.TargetSymbol;
+		var node = (PropertyDeclarationSyntax)context.TargetNode;
+		var diagnostics = new List<DiagInfo>();
+
+		if (property.Type.SpecialType != SpecialType.System_Boolean || property.IsStatic
+			|| property.GetMethod == null || property.SetMethod == null || property.SetMethod.IsInitOnly
+			|| !char.IsUpper(property.Name[0])
+			|| !node.Modifiers.Any(m => m.IsKind(SyntaxKind.PartialKeyword)))
+		{
+			diagnostics.Add(MakeDiagInfo(InvalidSettingProperty.Id, property.Name, node));
+			return new SettingResult(null, diagnostics.ToEquatableArray());
+		}
+
+		if (property.ContainingType.ContainingType != null
+			|| node.Parent is not ClassDeclarationSyntax containingClass
+			|| !containingClass.Modifiers.Any(m => m.IsKind(SyntaxKind.PartialKeyword)))
+		{
+			diagnostics.Add(MakeDiagInfo(InvalidContainingType.Id, property.Name, node));
+			return new SettingResult(null, diagnostics.ToEquatableArray());
+		}
+
+		var attribute = context.Attributes[0];
+		int versionValue = 0;
+		string? versionName = null;
+		string? category = null;
+		if (attribute.ConstructorArguments.Length == 1)
+		{
+			var versionArgument = attribute.ConstructorArguments[0];
+			if (versionArgument.Kind == TypedConstantKind.Error || versionArgument.Value is not int boundVersion || versionArgument.Type is null)
+			{
+				// The argument did not bind (e.g. a typo'd enum member); the compiler already
+				// reports that error at the argument, so just skip the setting instead of
+				// crashing the whole generator.
+				return new SettingResult(null, null);
+			}
+			versionValue = boundVersion;
+			versionName = VersionNameFromSyntax(attribute)
+				?? versionArgument.Type.GetMembers()
+					.OfType<IFieldSymbol>()
+					.FirstOrDefault(f => f.HasConstantValue && Equals(f.ConstantValue, versionValue))?.Name
+				?? versionValue.ToString();
+			if (!CategoryByVersion.TryGetValue(versionName, out category))
+			{
+				diagnostics.Add(MakeDiagInfo(UnsupportedLanguageVersion.Id, versionName, node));
+				return new SettingResult(null, diagnostics.ToEquatableArray());
+			}
+			if (property.GetAttributes().Any(a => a.AttributeClass?.ToDisplayString() == "System.ComponentModel.CategoryAttribute"))
+			{
+				diagnostics.Add(MakeDiagInfo(CategoryOnVersionedSetting.Id, property.Name, node));
+				// The compiler would otherwise also flag the generated [Category] as a duplicate;
+				// suppress it so the mistake surfaces as the single DSTG003.
+				category = null;
+			}
+		}
+
+		bool defaultValue = true;
+		foreach (var named in attribute.NamedArguments)
+		{
+			// A named argument that failed to bind is already a compiler error; ignore it here.
+			if (named.Value.Value is not bool namedValue)
+				continue;
+			if (named.Key == "DefaultValue")
+				defaultValue = namedValue;
+		}
+
+		string fieldName = char.ToLowerInvariant(property.Name[0]) + property.Name.Substring(1);
+		if (SyntaxFacts.GetKeywordKind(fieldName) != SyntaxKind.None)
+			fieldName = "@" + fieldName;
+
+		var setting = new SettingInfo(
+			property.ContainingNamespace.IsGlobalNamespace ? "" : property.ContainingNamespace.ToDisplayString(),
+			property.ContainingType.Name,
+			SyntaxFacts.GetText(property.DeclaredAccessibility),
+			property.Name,
+			fieldName,
+			defaultValue,
+			versionValue,
+			versionName,
+			category,
+			node.SyntaxTree.FilePath,
+			node.SpanStart);
+		return new SettingResult(setting, diagnostics.Count == 0 ? null : diagnostics.ToEquatableArray());
+	}
+
+	// Prefer the enum member name as spelled at the use site: constant values are not unique in
+	// LanguageVersion (CSharp15_0 and Preview share a value), so a value-based reverse lookup can
+	// name an alias the user never wrote.
+	static string? VersionNameFromSyntax(AttributeData attribute)
+	{
+		if (attribute.ApplicationSyntaxReference?.GetSyntax() is not AttributeSyntax { ArgumentList.Arguments: { Count: >= 1 } arguments })
+			return null;
+		if (arguments[0].NameEquals != null)
+			return null;
+		return arguments[0].Expression switch {
+			MemberAccessExpressionSyntax memberAccess => memberAccess.Name.Identifier.Text,
+			IdentifierNameSyntax identifier => identifier.Identifier.Text,
+			_ => null,
+		};
+	}
+
+	static DiagInfo MakeDiagInfo(string id, string messageArg, SyntaxNode node)
+	{
+		var lineSpan = node.GetLocation().GetLineSpan();
+		return new DiagInfo(id, messageArg, node.SyntaxTree.FilePath, node.Span.Start, node.Span.Length,
+			lineSpan.StartLinePosition.Line, lineSpan.StartLinePosition.Character,
+			lineSpan.EndLinePosition.Line, lineSpan.EndLinePosition.Character);
+	}
+
+	static void WriteSettingsClasses(SourceProductionContext context, ImmutableArray<SettingResult> results)
+	{
+		foreach (var result in results)
+		{
+			if (result.Diagnostics is not { } resultDiagnostics)
+				continue;
+			foreach (var diag in resultDiagnostics)
+			{
+				// Indexer lookup so a diagnostic id missing from the map fails loudly instead of
+				// being reported under an unrelated descriptor.
+				var descriptor = DescriptorsById[diag.Id];
+				var location = Location.Create(diag.FilePath, new TextSpan(diag.SpanStart, diag.SpanLength),
+					new LinePositionSpan(new LinePosition(diag.StartLine, diag.StartChar), new LinePosition(diag.EndLine, diag.EndChar)));
+				context.ReportDiagnostic(Diagnostic.Create(descriptor, location, diag.MessageArg));
+			}
+		}
+
+		var settings = results
+			.Where(r => r.Setting != null)
+			.Select(r => r.Setting!.Value)
+			.OrderBy(s => s.FilePath, StringComparer.Ordinal)
+			.ThenBy(s => s.SpanStart);
+
+		foreach (var settingsClass in settings.GroupBy(s => (s.Namespace, s.ClassName)))
+		{
+			WriteSettingsClass(context, settingsClass.Key.Namespace, settingsClass.Key.ClassName, settingsClass.ToArray());
+		}
+	}
+
+	static void WriteSettingsClass(SourceProductionContext context, string ns, string className, SettingInfo[] settings)
+	{
+		var builder = new StringBuilder();
+		builder.AppendLine("// <auto-generated/>");
+		builder.AppendLine("#nullable enable");
+		builder.AppendLine();
+		if (ns.Length > 0)
+		{
+			builder.AppendLine($"namespace {ns}");
+			builder.AppendLine("{");
+		}
+		builder.AppendLine($"\tpartial class {className}");
+		builder.AppendLine("\t{");
+
+		foreach (var setting in settings)
+		{
+			builder.AppendLine($"\t\tbool {setting.FieldName} = {(setting.DefaultValue ? "true" : "false")};");
+			builder.AppendLine();
+			if (setting.Category != null)
+			{
+				builder.AppendLine($"\t\t[global::System.ComponentModel.Category(\"{setting.Category}\")]");
+			}
+			builder.AppendLine($"\t\t{setting.Accessibility} partial bool {setting.PropertyName} {{");
+			builder.AppendLine($"\t\t\tget {{ return {setting.FieldName}; }}");
+			builder.AppendLine("\t\t\tset {");
+			builder.AppendLine($"\t\t\t\tif ({setting.FieldName} != value)");
+			builder.AppendLine("\t\t\t\t{");
+			builder.AppendLine($"\t\t\t\t\t{setting.FieldName} = value;");
+			builder.AppendLine("\t\t\t\t\tOnPropertyChanged();");
+			builder.AppendLine("\t\t\t\t}");
+			builder.AppendLine("\t\t\t}");
+			builder.AppendLine("\t\t}");
+			builder.AppendLine();
+		}
+
+		var versionBuckets = settings
+			.Where(s => s.VersionName != null)
+			.GroupBy(s => s.VersionValue)
+			.OrderBy(g => g.Key)
+			.ToArray();
+		if (versionBuckets.Length > 0)
+		{
+			WriteSetLanguageVersion(builder, versionBuckets);
+			builder.AppendLine();
+			WriteGetMinimumRequiredVersion(builder, versionBuckets);
+		}
+
+		builder.AppendLine("\t}");
+		if (ns.Length > 0)
+		{
+			builder.AppendLine("}");
+		}
+		// The hint name must carry the full grouping key: two same-named settings classes in
+		// different namespaces would otherwise collide in AddSource and kill the generator.
+		string hintName = ns.Length == 0 ? $"{className}.Settings.g.cs" : $"{ns}.{className}.Settings.g.cs";
+		context.AddSource(hintName, SourceText.From(builder.ToString().Replace("\r\n", "\n"), Encoding.UTF8));
+	}
+
+	// Emitted as partial implementing declarations: the containing class supplies the defining
+	// stubs, which is where the XML documentation lives (the docs on a partial method's defining
+	// declaration apply as long as the implementation carries none).
+	static void WriteSetLanguageVersion(StringBuilder builder, IGrouping<int, SettingInfo>[] versionBuckets)
+	{
+		builder.AppendLine("\t\tpublic partial void SetLanguageVersion(global::ICSharpCode.Decompiler.CSharp.LanguageVersion languageVersion)");
+		builder.AppendLine("\t\t{");
+		builder.AppendLine("\t\t\t// By default, all decompiler features are enabled.");
+		builder.AppendLine("\t\t\t// Disable some of them based on language version:");
+		foreach (var bucket in versionBuckets)
+		{
+			builder.AppendLine($"\t\t\tif (languageVersion < global::ICSharpCode.Decompiler.CSharp.LanguageVersion.{bucket.First().VersionName})");
+			builder.AppendLine("\t\t\t{");
+			foreach (var setting in bucket)
+			{
+				builder.AppendLine($"\t\t\t\t{setting.FieldName} = false;");
+			}
+			builder.AppendLine("\t\t\t}");
+		}
+		builder.AppendLine("\t\t}");
+	}
+
+	static void WriteGetMinimumRequiredVersion(StringBuilder builder, IGrouping<int, SettingInfo>[] versionBuckets)
+	{
+		builder.AppendLine("\t\tpublic partial global::ICSharpCode.Decompiler.CSharp.LanguageVersion GetMinimumRequiredVersion()");
+		builder.AppendLine("\t\t{");
+		foreach (var bucket in versionBuckets.Reverse())
+		{
+			builder.AppendLine($"\t\t\tif ({string.Join(" || ", bucket.Select(s => s.FieldName))})");
+			builder.AppendLine($"\t\t\t\treturn global::ICSharpCode.Decompiler.CSharp.LanguageVersion.{bucket.First().VersionName};");
+		}
+		builder.AppendLine("\t\t\treturn global::ICSharpCode.Decompiler.CSharp.LanguageVersion.CSharp1;");
+		builder.AppendLine("\t\t}");
+	}
+}

ICSharpCode.Decompiler.Generators/DecompilerSyntaxTreeGenerator.cs

@@ -843,17 +843,7 @@ internal class DecompilerSyntaxTreeGenerator : IIncrementalGenerator
 		var visitorMembers = astNodeAdditions.Collect();
 
 		context
-			.RegisterPostInitializationOutput(i => i.AddSource("DecompilerSyntaxTreeGeneratorAttributes.g.cs", @"
+			.RegisterPostInitializationOutput(i => i.AddSource("DecompilerSyntaxTreeGeneratorAttributes.g.cs", RoslynHelpers.EmbeddedAttributeSource + @"
-
-using System;
-
-namespace Microsoft.CodeAnalysis
-{
-    internal sealed partial class EmbeddedAttribute : global::System.Attribute
-    {
-    }
-}
-
 namespace ICSharpCode.Decompiler.CSharp.Syntax
 {
 	[global::Microsoft.CodeAnalysis.EmbeddedAttribute]

ICSharpCode.Decompiler.Generators/RoslynHelpers.cs

@@ -22,6 +22,21 @@ namespace ICSharpCode.Decompiler.Generators;
 
 public static class RoslynHelpers
 {
+	/// <summary>
+	/// Post-init source declaring Microsoft.CodeAnalysis.EmbeddedAttribute. Every generator that
+	/// emits embedded attribute types has to ship its own copy in its own post-init output; the
+	/// declarations merge only while they stay partial and textually compatible, so all generators
+	/// must build their copy from this single constant.
+	/// </summary>
+	public const string EmbeddedAttributeSource = @"
+namespace Microsoft.CodeAnalysis
+{
+    internal sealed partial class EmbeddedAttribute : global::System.Attribute
+    {
+    }
+}
+";
+
 	public static bool IsDerivedFrom(this INamedTypeSymbol type, INamedTypeSymbol baseType)
 	{
 		INamedTypeSymbol? t = type;

ICSharpCode.Decompiler.Tests/DecompilerSettingsTests.cs

@@ -0,0 +1,53 @@
+// 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.
+
+using ICSharpCode.Decompiler.CSharp;
+
+using NUnit.Framework;
+
+namespace ICSharpCode.Decompiler.Tests
+{
+	[TestFixture]
+	class DecompilerSettingsTests
+	{
+		[Test]
+		public void GetMinimumRequiredVersionReturnsTheHighestEnabledFeatureVersion()
+		{
+			var settings = new DecompilerSettings(LanguageVersion.CSharp1);
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp1));
+
+			settings.AnonymousMethods = true;
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp2));
+
+			// Syntax-preference settings participate too: enabled, they emit syntax of their version.
+			settings.UseEnhancedUsing = true;
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp8_0));
+
+			settings.SwitchOnReadOnlySpanChar = true;
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp11_0));
+
+			// The scan must pick the highest enabled feature, not the first match bottom-up.
+			settings.ParamsCollections = true;
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp13_0));
+
+			settings.AnonymousMethods = false;
+			settings.ParamsCollections = false;
+			Assert.That(settings.GetMinimumRequiredVersion(), Is.EqualTo(LanguageVersion.CSharp11_0));
+		}
+	}
+}

ICSharpCode.Decompiler/CSharp/ProjectDecompiler/WholeProjectDecompiler.cs

@@ -60,17 +60,31 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 
 		LanguageVersion? languageVersion;
 
+		/// <summary>
+		/// The C# language version written into the exported project file as LangVersion.
+		/// This is an export parameter, not decompiler state: when not set explicitly, it defaults
+		/// to <see cref="DecompilerSettings.GetMinimumRequiredVersion"/> of the current settings,
+		/// and an explicit value below that minimum is rejected (here and again when the export
+		/// starts) because the emitted code could not compile under it.
+		/// </summary>
 		public LanguageVersion LanguageVersion {
 			get { return languageVersion ?? Settings.GetMinimumRequiredVersion(); }
 			set {
-				var minVersion = Settings.GetMinimumRequiredVersion();
+				ValidateLanguageVersion(value);
-				if (value < minVersion)
-					throw new InvalidOperationException($"The chosen settings require at least {minVersion}." +
-						$" Please change the DecompilerSettings accordingly.");
 				languageVersion = value;
 			}
 		}
 
+		void ValidateLanguageVersion(LanguageVersion version)
+		{
+			var minVersion = Settings.GetMinimumRequiredVersion();
+			if (version < minVersion)
+			{
+				throw new InvalidOperationException($"The chosen settings require at least {minVersion}." +
+					" Please change the DecompilerSettings accordingly.");
+			}
+		}
+
 		bool IProjectInfoProvider.CheckForOverflowUnderflow => Settings.CheckForOverflowUnderflow;
 
 		public IAssemblyResolver AssemblyResolver { get; }
@@ -154,6 +168,14 @@ namespace ICSharpCode.Decompiler.CSharp.ProjectDecompiler
 			{
 				throw new InvalidOperationException("Must set TargetDirectory");
 			}
+			// The LanguageVersion setter already rejects a version below what the settings require,
+			// but Settings is mutable and shared, so re-validate against the settings actually in
+			// effect now - otherwise the exported project would carry a LangVersion under which the
+			// emitted code cannot compile.
+			if (languageVersion is { } explicitVersion)
+			{
+				ValidateLanguageVersion(explicitVersion);
+			}
 			DecompilerEventSource.Log.ProjectDecompilationStart(file.Name);
 			int codeFileCount = 0, resourceFileCount = 0;
 			try