Fix #827: VB auto-property backing field access maps to MyClassProp for virtual properties

When a VB.NET virtual auto-property (e.g. `Overridable Property Prop As Integer = 5`)
is accessed via its backing field (`_Prop`) in the same class, this should map to the
non-virtual `MyClassProp` backing property in C#, not the virtual `Prop` property.

Also handles the edge case where `MyClass._Prop` is written explicitly: in that case,
`NameExpressionNodeVisitor` already adds the `MyClass` prefix, so we return the bare
property name to avoid double-prefixing.

Additionally, extends `GetMyClassAccessedNames` to detect backing field accesses so
that the `MyClassProp` property is generated when needed.

https://claude.ai/code/session_01AkwUvu3XuCdj3D4axoX4UX

Author: claude

CodeConverter/CSharp/CommonConversions.cs

@@ -293,7 +293,12 @@ public SyntaxToken ConvertIdentifier(SyntaxToken id, bool isAttribute = false, S
                     // AND the first explicitly declared parameter is this symbol, we need to replace it with value.
                     text = "value";
                 } else if (normalizedText.StartsWith("_", StringComparison.OrdinalIgnoreCase) && idSymbol is IFieldSymbol propertyFieldSymbol && propertyFieldSymbol.AssociatedSymbol?.IsKind(SymbolKind.Property) == true) {
-                    text = propertyFieldSymbol.AssociatedSymbol.Name;
+                    // For virtual auto-properties, VB backing field _Prop maps to the C# MyClassProp backing property (bypasses virtual dispatch).
+                    // Exception: when accessed as MyClass._Prop, NameExpressionNodeVisitor adds the "MyClass" prefix itself, so we just return the property name.
+                    var isAccessedViaMyClass = id.Parent?.Parent is VBSyntax.MemberAccessExpressionSyntax { Expression: VBSyntax.MyClassExpressionSyntax };
+                    text = !isAccessedViaMyClass && propertyFieldSymbol.IsImplicitlyDeclared && propertyFieldSymbol.AssociatedSymbol is IPropertySymbol { IsVirtual: true, IsAbstract: false } vProp
+                        ? "MyClass" + vProp.Name
+                        : propertyFieldSymbol.AssociatedSymbol.Name;
                 } else if (normalizedText.EndsWith("Event", StringComparison.OrdinalIgnoreCase) && idSymbol is IFieldSymbol eventFieldSymbol && eventFieldSymbol.AssociatedSymbol?.IsKind(SymbolKind.Event) == true) {
                     text = eventFieldSymbol.AssociatedSymbol.Name;
                 } else if (WinformsConversions.MayNeedToInlinePropertyAccess(id.Parent, idSymbol) && _typeContext.HandledEventsAnalysis.ShouldGeneratePropertyFor(idSymbol.Name)) {

CodeConverter/CSharp/DeclarationNodeVisitor.cs

@@ -689,12 +689,25 @@ private static async Task<BlockSyntax> ConvertStatementsAsync(SyntaxList<VBSynta
         return CS.SyntaxFactory.Block(await statements.SelectManyAsync(async s => (IEnumerable<StatementSyntax>) await s.Accept(methodBodyVisitor)));
     }
 
-    private static HashSet<string> GetMyClassAccessedNames(VBSyntax.ClassBlockSyntax classBlock)
+    private HashSet<string> GetMyClassAccessedNames(VBSyntax.ClassBlockSyntax classBlock)
     {
         var memberAccesses = classBlock.DescendantNodes().OfType<VBSyntax.MemberAccessExpressionSyntax>();
         var accessedTextNames = new HashSet<string>(memberAccesses
             .Where(mae => mae.Expression is VBSyntax.MyClassExpressionSyntax)
             .Select(mae => mae.Name.Identifier.Text), StringComparer.OrdinalIgnoreCase);
+
+        // Also treat direct backing field access (_Prop) as MyClass access for virtual auto-properties.
+        // In VB, writing _Prop directly accesses the backing field, bypassing virtual dispatch -
+        // the same semantics as MyClass.Prop. In C#, these virtual properties get a MyClassProp
+        // backing property, so _Prop must map to MyClassProp.
+        var backingFieldIdentifiers = classBlock.DescendantNodes().OfType<VBSyntax.IdentifierNameSyntax>()
+            .Where(id => id.Identifier.ValueText.StartsWith("_", StringComparison.OrdinalIgnoreCase));
+        foreach (var id in backingFieldIdentifiers) {
+            if (_semanticModel.GetSymbolInfo(id).Symbol is IFieldSymbol { IsImplicitlyDeclared: true, AssociatedSymbol: IPropertySymbol { IsVirtual: true, IsAbstract: false } associatedProp }) {
+                accessedTextNames.Add(associatedProp.Name);
+            }
+        }
+
         return accessedTextNames;
     }
 

Tests/CSharp/MemberTests/PropertyMemberTests.cs

@@ -872,6 +872,52 @@ public static IEnumerable<object[]> SomeObjects
             yield return new object[3];
         }
     }
+}");
+    }
+
+    /// <summary>Issue #827: VB auto-property backing field access (_Prop) should map to MyClassProp for overridable properties</summary>
+    [Fact]
+    public async Task TestOverridableAutoPropertyBackingFieldAccessAsync()
+    {
+        await TestConversionVisualBasicToCSharpAsync(@"Class Foo
+    Overridable Property Prop As Integer = 5
+
+    Sub Test()
+        _Prop = 10
+        Dim isCorrect = MyClass.Prop = 10
+    End Sub
+End Class
+Class Child
+    Inherits Foo
+    Overrides Property Prop As Integer = 20
+End Class", @"
+internal partial class Foo
+{
+    public int MyClassProp { get; set; } = 5;
+
+    public virtual int Prop
+    {
+        get
+        {
+            return MyClassProp;
+        }
+
+        set
+        {
+            MyClassProp = value;
+        }
+    }
+
+    public void Test()
+    {
+        MyClassProp = 10;
+        bool isCorrect = MyClassProp == 10;
+    }
+}
+
+internal partial class Child : Foo
+{
+    public override int Prop { get; set; } = 20;
 }");
     }
 }