Claude/fix issue 557 char string default param

CodeConverter/CSharp/DeclarationNodeVisitor.cs

@@ -1,3 +1,4 @@
+using System.Collections.Immutable;
 using System.Runtime.InteropServices;
 using Microsoft.CodeAnalysis.Classification;
 using Microsoft.CodeAnalysis.CSharp;
@@ -674,11 +675,64 @@ public override async Task<CSharpSyntaxNode> VisitMethodBlock(VBSyntax.MethodBlo
             convertedStatements = convertedStatements.InsertNodesBefore(firstResumeLayout, _typeContext.HandledEventsAnalysis.GetInitializeComponentClassEventHandlers());
         }
 
+        (methodBlock, convertedStatements) = FixCharDefaultsForStringParams(declaredSymbol, methodBlock, convertedStatements, _semanticModel);
+
         var body = _accessorDeclarationNodeConverter.WithImplicitReturnStatements(node, convertedStatements, csReturnVariableOrNull);
 
         return methodBlock.WithBody(body);
     }
 
+    /// <summary>
+    /// In VB, a Char constant can be the default value of a String parameter. In C#, this is invalid.
+    /// Fix: replace the default with null and prepend a null-coalescing assignment in the method body.
+    /// </summary>
+    private static (BaseMethodDeclarationSyntax MethodBlock, BlockSyntax ConvertedStatements) FixCharDefaultsForStringParams(
+        IMethodSymbol declaredSymbol, BaseMethodDeclarationSyntax methodBlock, BlockSyntax convertedStatements, SemanticModel semanticModel)
+    {
+        var prependedStatements = new List<StatementSyntax>();
+        var updatedParams = methodBlock.ParameterList.Parameters.ToList();
+        var vbParams = declaredSymbol?.Parameters ?? ImmutableArray<IParameterSymbol>.Empty;
+
+        for (int i = 0; i < updatedParams.Count && i < vbParams.Length; i++) {
+            var vbParam = vbParams[i];
+            if (vbParam.Type.SpecialType != SpecialType.System_String
+                || !vbParam.HasExplicitDefaultValue) continue;
+            // ExplicitDefaultValue is normalized to the parameter's declared type (String), so we
+            // must inspect the VB syntax to detect when the original expression is Char-typed.
+            var vbSyntaxParam = vbParam.DeclaringSyntaxReferences.FirstOrDefault()?.GetSyntax() as VBSyntax.ParameterSyntax;
+            var defaultValueNode = vbSyntaxParam?.Default?.Value;
+            if (defaultValueNode == null) continue;
+            if (semanticModel.GetTypeInfo(defaultValueNode).Type?.SpecialType != SpecialType.System_Char) continue;
+
+            var csParam = updatedParams[i];
+            var defaultExpr = csParam.Default?.Value;
+            if (defaultExpr is null) continue;
+
+            // Replace the default value with null
+            updatedParams[i] = csParam.WithDefault(
+                CS.SyntaxFactory.EqualsValueClause(ValidSyntaxFactory.NullExpression));
+
+            // Build: paramName = paramName ?? existingDefaultExpr.ToString();
+            var paramId = ValidSyntaxFactory.IdentifierName(csParam.Identifier.ValueText);
+            var toStringCall = CS.SyntaxFactory.InvocationExpression(
+                CS.SyntaxFactory.MemberAccessExpression(
+                    CS.SyntaxKind.SimpleMemberAccessExpression,
+                    defaultExpr.WithoutTrivia(),
+                    CS.SyntaxFactory.IdentifierName("ToString")));
+            var coalesce = CS.SyntaxFactory.BinaryExpression(CS.SyntaxKind.CoalesceExpression, paramId, toStringCall);
+            var assignment = CS.SyntaxFactory.AssignmentExpression(CS.SyntaxKind.SimpleAssignmentExpression, paramId, coalesce);
+            prependedStatements.Add(CS.SyntaxFactory.ExpressionStatement(assignment));
+        }
+
+        if (prependedStatements.Count == 0) return (methodBlock, convertedStatements);
+
+        var newParamList = methodBlock.ParameterList.WithParameters(CS.SyntaxFactory.SeparatedList(updatedParams, methodBlock.ParameterList.Parameters.GetSeparators()));
+        methodBlock = methodBlock.WithParameterList(newParamList);
+        convertedStatements = convertedStatements.WithStatements(CS.SyntaxFactory.List(prependedStatements.Concat(convertedStatements.Statements)));
+
+        return (methodBlock, convertedStatements);
+    }
+
     private static bool IsThisResumeLayoutInvocation(StatementSyntax s)
     {
         return s is ExpressionStatementSyntax ess && ess.Expression is InvocationExpressionSyntax ies && ies.Expression.ToString().Equals("this.ResumeLayout", StringComparison.Ordinal);

Tests/CSharp/MemberTests/MemberTests.cs

@@ -1576,4 +1576,49 @@ private void OptionalByRefWithDefault([Optional][DefaultParameterValue(""a"")] r
 CS7036: There is no argument given that corresponds to the required parameter 'str1' of 'MissingByRefArgumentWithNoExplicitDefaultValue.ByRefNoDefault(ref string)'
 ");
     }
+
+    [Fact]
+    public async Task TestCharConstDefaultValueForStringParameterAsync()
+    {
+        // Issue #557: VB allows a Char constant as a default value for a String parameter, but C# does not.
+        // Replace the default with null and prepend a null-coalescing assignment in the method body.
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Module TestModule
+    Friend Const DlM As Char = ""^""c
+
+    Friend Function LeftSideOf(Optional ByVal strDlM As String = DlM) As String
+        Return strDlM
+    End Function
+End Module", @"
+internal static partial class TestModule
+{
+    internal const char DlM = '^';
+
+    internal static string LeftSideOf(string strDlM = null)
+    {
+        strDlM = strDlM ?? DlM.ToString();
+        return strDlM;
+    }
+}");
+    }
+
+    [Fact]
+    public async Task TestCharLiteralDefaultValueForStringParameterAsync()
+    {
+        // Issue #557: inline char literal as default value for a String parameter.
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Class TestClass
+    Friend Function Foo(Optional s As String = ""^""c) As String
+        Return s
+    End Function
+End Class", @"
+internal partial class TestClass
+{
+    internal string Foo(string s = null)
+    {
+        s = s ?? '^'.ToString();
+        return s;
+    }
+}");
+    }
 }

Tests/CSharp/MemberTests/PropertyMemberTests.cs

@@ -874,4 +874,5 @@ public static IEnumerable<object[]> SomeObjects
     }
 }");
     }
+
 }