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 +675,93 @@ 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.
+    /// VisitParameter in ExpressionNodeVisitor sets the default to null for these cases; this method
+    /// prepends a null-coalescing assignment to restore the char default at runtime.
+    /// </summary>
+    private static (BaseMethodDeclarationSyntax MethodBlock, BlockSyntax ConvertedStatements) FixCharDefaultsForStringParams(
+        IMethodSymbol declaredSymbol, BaseMethodDeclarationSyntax methodBlock, BlockSyntax convertedStatements, SemanticModel semanticModel)
+    {
+        var prependedStatements = new List<StatementSyntax>();
+        var vbParams = declaredSymbol?.Parameters ?? ImmutableArray<IParameterSymbol>.Empty;
+        var csParams = methodBlock.ParameterList.Parameters;
+
+        for (int i = 0; i < csParams.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 = csParams[i];
+            // The default was set to null at point of creation in VisitParameter (ExpressionNodeVisitor).
+            // Reconstruct the char expression from VB syntax to avoid depending on the already-converted value.
+            var charExpr = BuildCharExpressionFromVbSyntax(defaultValueNode, semanticModel);
+
+            // Build: paramName = paramName ?? charExpr.ToString();
+            var paramId = ValidSyntaxFactory.IdentifierName(csParam.Identifier.ValueText);
+            var toStringCall = CS.SyntaxFactory.InvocationExpression(
+                CS.SyntaxFactory.MemberAccessExpression(
+                    CS.SyntaxKind.SimpleMemberAccessExpression,
+                    charExpr,
+                    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);
+
+        return (methodBlock, convertedStatements.WithStatements(CS.SyntaxFactory.List(prependedStatements.Concat(convertedStatements.Statements))));
+    }
+
+    private static ExpressionSyntax BuildCharExpressionFromVbSyntax(VBSyntax.ExpressionSyntax defaultValueNode, SemanticModel semanticModel)
+    {
+        // For char literal expressions (e.g. "^"c), use the constant value directly
+        if (defaultValueNode.IsKind(VBasic.SyntaxKind.CharacterLiteralExpression)) {
+            var constant = semanticModel.GetConstantValue(defaultValueNode);
+            if (constant.HasValue && constant.Value is char c)
+                return CS.SyntaxFactory.LiteralExpression(CS.SyntaxKind.CharacterLiteralExpression, CS.SyntaxFactory.Literal(c));
+        }
+        return VbNameExprToCsExpr(defaultValueNode);
+    }
+
+    private static ExpressionSyntax VbNameExprToCsExpr(VBSyntax.ExpressionSyntax vbExpr)
+    {
+        switch (vbExpr) {
+            case VBSyntax.IdentifierNameSyntax identifier:
+                return CS.SyntaxFactory.IdentifierName(identifier.Identifier.Text);
+            case VBSyntax.MemberAccessExpressionSyntax memberAccess:
+                // Skip VB's "Global." qualifier — it's VB's global namespace prefix, has no direct C# identifier equivalent
+                if (memberAccess.Expression.IsKind(VBasic.SyntaxKind.GlobalName))
+                    return CS.SyntaxFactory.IdentifierName(memberAccess.Name.Identifier.Text);
+                return CS.SyntaxFactory.MemberAccessExpression(CS.SyntaxKind.SimpleMemberAccessExpression,
+                    VbNameExprToCsExpr(memberAccess.Expression),
+                    CS.SyntaxFactory.IdentifierName(memberAccess.Name.Identifier.Text));
+            case VBSyntax.QualifiedNameSyntax qualifiedName:
+                // Qualified names (e.g. Global.TestModule) appear in name/type context within default values
+                if (qualifiedName.Left.IsKind(VBasic.SyntaxKind.GlobalName))
+                    return CS.SyntaxFactory.IdentifierName(qualifiedName.Right.Identifier.Text);
+                return CS.SyntaxFactory.MemberAccessExpression(CS.SyntaxKind.SimpleMemberAccessExpression,
+                    VbNameExprToCsExpr(qualifiedName.Left),
+                    CS.SyntaxFactory.IdentifierName(qualifiedName.Right.Identifier.Text));
+            default:
+                throw new NotSupportedException($"Unexpected VB expression in char default value: {vbExpr}");
+        }
+    }
+
     private static bool IsThisResumeLayoutInvocation(StatementSyntax s)
     {
         return s is ExpressionStatementSyntax ess && ess.Expression is InvocationExpressionSyntax ies && ies.Expression.ToString().Equals("this.ResumeLayout", StringComparison.Ordinal);

CodeConverter/CSharp/ExpressionNodeVisitor.cs

@@ -476,8 +476,16 @@ public override async Task<CSharpSyntaxNode> VisitParameter(VBSyntax.ParameterSy
                 attributes.Insert(0,
                     CS.SyntaxFactory.AttributeList(CS.SyntaxFactory.SeparatedList(optionalAttributes)));
             } else {
-                @default = CS.SyntaxFactory.EqualsValueClause(
-                    await node.Default.Value.AcceptAsync<ExpressionSyntax>(TriviaConvertingExpressionVisitor));
+                var paramSymbol = _semanticModel.GetDeclaredSymbol(node) as IParameterSymbol;
+                if (paramSymbol?.Type?.SpecialType == SpecialType.System_String &&
+                    _semanticModel.GetTypeInfo(defaultValue).Type?.SpecialType == SpecialType.System_Char) {
+                    // VB allows a Char constant as default for a String parameter; C# does not.
+                    // Set null here; FixCharDefaultsForStringParams (DeclarationNodeVisitor) adds the null-coalesce assignment.
+                    @default = CS.SyntaxFactory.EqualsValueClause(ValidSyntaxFactory.NullExpression);
+                } else {
+                    @default = CS.SyntaxFactory.EqualsValueClause(
+                        await node.Default.Value.AcceptAsync<ExpressionSyntax>(TriviaConvertingExpressionVisitor));
+                }
             }
         }
 

Tests/CSharp/MemberTests/MemberTests.cs

@@ -1576,4 +1576,92 @@ 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;
+    }
+}");
+    }
+
+    [Fact]
+    public async Task TestCharConstInSameClassDefaultValueForStringParameterAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Class TestClass
+    Friend Const Sep As Char = "",""c
+
+    Friend Function Join(Optional s As String = Sep) As String
+        Return s
+    End Function
+End Class", @"
+internal partial class TestClass
+{
+    internal const char Sep = ',';
+
+    internal string Join(string s = null)
+    {
+        s = s ?? Sep.ToString();
+        return s;
+    }
+}");
+    }
+
+    [Fact]
+    public async Task TestMultipleCharDefaultValuesForStringParametersAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(
+            @"Class TestClass
+    Friend Function Format(Optional prefix As String = ""[""c, Optional suffix As String = ""]""c) As String
+        Return prefix & suffix
+    End Function
+End Class", @"
+internal partial class TestClass
+{
+    internal string Format(string prefix = null, string suffix = null)
+    {
+        prefix = prefix ?? '['.ToString();
+        suffix = suffix ?? ']'.ToString();
+        return prefix + suffix;
+    }
+}");
+    }
 }