Use ILSpyHelper_AsRefReadOnly to ensure that overload resolution can pick the correct overload using 'in'.

ICSharpCode.Decompiler/CSharp/CallBuilder.cs

@@ -971,7 +971,11 @@ namespace ICSharpCode.Decompiler.CSharp
 			RequireTarget = 1,
 			RequireTypeArguments = 2,
 			NoOptionalArgumentAllowed = 4,
-			All = 7
+			/// <summary>
+			/// Add calls to AsRefReadOnly for in parameters that did not have an explicit DirectionExpression yet.
+			/// </summary>
+			EnforceExplicitIn = 8,
+			All = 0xf,
 		}
 
 		private CallTransformation GetRequiredTransformationsForCall(ExpectedTargetDetails expectedTargetDetails, IMethod method,
@@ -1122,6 +1126,11 @@ namespace ICSharpCode.Decompiler.CSharp
 							requireTypeArguments = true;
 							typeArguments = method.TypeArguments.ToArray();
 						}
+						else if ((allowedTransforms & CallTransformation.EnforceExplicitIn) != 0)
+						{
+							EnforceExplicitIn(argumentList.Arguments, argumentList.ExpectedParameters);
+							allowedTransforms &= ~CallTransformation.EnforceExplicitIn;
+						}
 						else
 						{
 							break;
@@ -1141,6 +1150,32 @@ namespace ICSharpCode.Decompiler.CSharp
 			return transform;
 		}
 
+		private void EnforceExplicitIn(TranslatedExpression[] arguments, IParameter[] expectedParameters)
+		{
+			for (int i = 0; i < arguments.Length; i++)
+			{
+				if (expectedParameters[i].ReferenceKind != ReferenceKind.In)
+					continue;
+				if (arguments[i].Expression is DirectionExpression)
+					continue;
+
+				arguments[i] = WrapInAsRefReadOnly(arguments[i]);
+				expressionBuilder.statementBuilder.EmitAsRefReadOnly = true;
+			}
+		}
+
+		private TranslatedExpression WrapInAsRefReadOnly(TranslatedExpression arg)
+		{
+			return new DirectionExpression(
+				FieldDirection.In,
+				new InvocationExpression {
+					Target = new IdentifierExpression("ILSpyHelper_AsRefReadOnly"),
+					Arguments = { arg.Expression }
+				}
+			).WithRR(new ByReferenceResolveResult(arg.Type, ReferenceKind.In))
+			.WithoutILInstruction();
+		}
+
 		private bool IsPossibleExtensionMethodCallOnNull(IMethod method, IList<TranslatedExpression> arguments)
 		{
 			return method.IsExtensionMethod && arguments.Count > 0 && arguments[0].Expression is NullReferenceExpression;

ICSharpCode.Decompiler/CSharp/StatementBuilder.cs

@@ -47,6 +47,8 @@ namespace ICSharpCode.Decompiler.CSharp
 		internal IType currentResultType;
 		internal bool currentIsIterator;
 
+		internal bool EmitAsRefReadOnly;
+
 		public StatementBuilder(IDecompilerTypeSystem typeSystem, ITypeResolveContext decompilationContext,
 			ILFunction currentFunction, DecompilerSettings settings, DecompileRun decompileRun,
 			CancellationToken cancellationToken)
@@ -1367,6 +1369,32 @@ namespace ICSharpCode.Decompiler.CSharp
 		{
 			var blockStatement = ConvertBlockContainer(new BlockStatement(), container, container.Blocks, isLoop);
 			DeclareLocalFunctions(currentFunction, container, blockStatement);
+			if (currentFunction.Body == container)
+			{
+				if (EmitAsRefReadOnly)
+				{
+					var methodDecl = new MethodDeclaration();
+					if (settings.StaticLocalFunctions)
+					{
+						methodDecl.Modifiers = Modifiers.Static;
+					}
+
+					methodDecl.ReturnType = new ComposedType() { HasReadOnlySpecifier = true, HasRefSpecifier = true, BaseType = new SimpleType("T") };
+					methodDecl.Name = "ILSpyHelper_AsRefReadOnly";
+					methodDecl.TypeParameters.Add(new TypeParameterDeclaration("T"));
+					methodDecl.Parameters.Add(new ParameterDeclaration { ParameterModifier = ReferenceKind.In, Type = new SimpleType("T"), Name = "temp" });
+
+					methodDecl.Body = new BlockStatement();
+					methodDecl.Body.AddChild(new Comment(
+						"ILSpy generated this function to help ensure overload resolution can pick the overload using 'in'"),
+											 Roles.Comment);
+					methodDecl.Body.Add(new ReturnStatement(new DirectionExpression(FieldDirection.Ref, new IdentifierExpression("temp"))));
+
+					blockStatement.Statements.Add(
+						new LocalFunctionDeclarationStatement(methodDecl)
+					);
+				}
+			}
 			return blockStatement;
 		}