From ab93df3457fc137e9a32f461bcbf718fe5763e78 Mon Sep 17 00:00:00 2001
From: GrahamTheCoder <2490482+GrahamTheCoder@users.noreply.github.com>
Date: Wed, 25 Mar 2026 00:14:11 +0000
Subject: [PATCH] Fix issue 886: Optional ByRef parameters default value
 conversion

Co-authored-by: google-labs-jules[bot] <161369871+google-labs-jules[bot]@users.noreply.github.com>
---
 CodeConverter/CSharp/ArgumentConverter.cs     | 12 +++++-
 CodeConverter/CSharp/ExpressionNodeVisitor.cs | 17 +++++++-
 Tests/CSharp/MemberTests/MemberTests.cs       | 41 ++++++++++++++++++-
 3 files changed, 66 insertions(+), 4 deletions(-)

diff --git a/CodeConverter/CSharp/ArgumentConverter.cs b/CodeConverter/CSharp/ArgumentConverter.cs
index 083801d95..f520f5248 100644
--- a/CodeConverter/CSharp/ArgumentConverter.cs
+++ b/CodeConverter/CSharp/ArgumentConverter.cs
@@ -226,10 +226,18 @@ private CSSyntax.ArgumentSyntax CreateOptionalRefArg(IParameterSymbol p, RefKind
         var type = CommonConversions.GetTypeSyntax(p.Type);
         CSSyntax.ExpressionSyntax initializer;
         if (p.HasExplicitDefaultValue) {
-            initializer = CommonConversions.Literal(p.ExplicitDefaultValue);
+            if (p.ExplicitDefaultValue == null && p.Type.IsValueType && p.Type.OriginalDefinition.SpecialType != SpecialType.System_Nullable_T) {
+                initializer = CS.SyntaxFactory.DefaultExpression(type);
+            } else {
+                initializer = CommonConversions.Literal(p.ExplicitDefaultValue);
+            }
         } else if (HasOptionalAttribute(p)) {
             if (TryGetDefaultParameterValueAttributeValue(p, out var defaultValue)) {
-                initializer = CommonConversions.Literal(defaultValue);
+                if (defaultValue == null && p.Type.IsValueType && p.Type.OriginalDefinition.SpecialType != SpecialType.System_Nullable_T) {
+                    initializer = CS.SyntaxFactory.DefaultExpression(type);
+                } else {
+                    initializer = CommonConversions.Literal(defaultValue);
+                }
             } else {
                 initializer = CS.SyntaxFactory.DefaultExpression(type);
             }
diff --git a/CodeConverter/CSharp/ExpressionNodeVisitor.cs b/CodeConverter/CSharp/ExpressionNodeVisitor.cs
index e95dc0a66..85da37980 100644
--- a/CodeConverter/CSharp/ExpressionNodeVisitor.cs
+++ b/CodeConverter/CSharp/ExpressionNodeVisitor.cs
@@ -471,7 +471,22 @@ public override async Task<CSharpSyntaxNode> VisitParameter(VBSyntax.ParameterSy
                     CS.SyntaxFactory.Attribute(ValidSyntaxFactory.IdentifierName("Optional")),
                 };
                 if (!node.Default.Value.IsKind(VBasic.SyntaxKind.NothingLiteralExpression)) {
-                    optionalAttributes.Add(CS.SyntaxFactory.Attribute(ValidSyntaxFactory.IdentifierName("DefaultParameterValue"), arg));
+                    if (vbSymbol?.Type?.SpecialType == SpecialType.System_Decimal && defaultExpression is CSSyntax.LiteralExpressionSyntax literalExpr) {
+                        var literalValue = literalExpr.Token.Value;
+                        if (literalValue is decimal decimalVal) {
+                            var isInteger = decimalVal == Math.Round(decimalVal);
+                            var newLiteral = isInteger ? CommonConversions.Literal((int)decimalVal) : CommonConversions.Literal((double)decimalVal);
+                            arg = CommonConversions.CreateAttributeArgumentList(CS.SyntaxFactory.AttributeArgument(newLiteral));
+                        }
+                    }
+
+                    if (vbSymbol?.Type?.TypeKind == TypeKind.Struct && vbSymbol?.Type?.SpecialType == SpecialType.None) {
+                        arg = null;
+                    }
+
+                    if (arg != null) {
+                        optionalAttributes.Add(CS.SyntaxFactory.Attribute(ValidSyntaxFactory.IdentifierName("DefaultParameterValue"), arg));
+                    }
                 }
                 attributes.Insert(0,
                     CS.SyntaxFactory.AttributeList(CS.SyntaxFactory.SeparatedList(optionalAttributes)));
diff --git a/Tests/CSharp/MemberTests/MemberTests.cs b/Tests/CSharp/MemberTests/MemberTests.cs
index da604f2dc..2492f4607 100644
--- a/Tests/CSharp/MemberTests/MemberTests.cs
+++ b/Tests/CSharp/MemberTests/MemberTests.cs
@@ -1576,4 +1576,43 @@ 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)'
 ");
     }
-}
\ No newline at end of file
+    [Fact]
+    public async Task OptionalByRefParameterAsync886()
+    {
+        await TestConversionVisualBasicToCSharpAsync(@"Class Issue886
+    Private Shared Sub OptionalParams()
+        FunctionWithOptionalParams()
+    End Sub
+
+    Private Shared Sub FunctionWithOptionalParams(Optional ByRef structParam As TestStruct = Nothing, Optional ByRef decimalParam As Decimal = 0)
+        structParam = New TestStruct
+        decimalParam = 0
+    End Sub
+
+    Friend Structure TestStruct
+        Friend A As Boolean
+    End Structure
+End Class", @"using System.Runtime.InteropServices;
+
+internal partial class Issue886
+{
+    private static void OptionalParams()
+    {
+        TestStruct argstructParam = default;
+        decimal argdecimalParam = 0m;
+        FunctionWithOptionalParams(structParam: ref argstructParam, decimalParam: ref argdecimalParam);
+    }
+
+    private static void FunctionWithOptionalParams([Optional] ref TestStruct structParam, [Optional, DefaultParameterValue(0)] ref decimal decimalParam)
+    {
+        structParam = new TestStruct();
+        decimalParam = 0m;
+    }
+
+    internal partial struct TestStruct
+    {
+        internal bool A;
+    }
+}");
+    }
+}