Handle C# ref return when converting VB->C#

CodeConverter/CSharp/ExpressionNodeVisitor.cs

@@ -1833,7 +1833,9 @@ private RefConversion NeedsVariableForArgument(VBasic.Syntax.ArgumentSyntax node
         RefConversion GetRefConversion(VBSyntax.ExpressionSyntax expression)
         {
             var symbolInfo = GetSymbolInfoInDocument<ISymbol>(expression);
-            if (symbolInfo is IPropertySymbol propertySymbol) {
+            if (symbolInfo is IPropertySymbol propertySymbol
+            // a property in VB.NET code can be ReturnsByRef if it's defined in a C# assembly the VB.NET code references
+            && !propertySymbol.ReturnsByRef && !propertySymbol.ReturnsByRefReadonly) {
                 return propertySymbol.IsReadOnly ? RefConversion.PreAssigment : RefConversion.PreAndPostAssignment;
             }
 
@@ -1858,7 +1860,16 @@ RefConversion GetRefConversion(VBSyntax.ExpressionSyntax expression)
         bool IsRefArrayAcces(VBSyntax.ExpressionSyntax expression)
         {
             if (!(expression is VBSyntax.InvocationExpressionSyntax ies)) return false;
-            return _semanticModel.GetOperation(ies).IsArrayElementAccess() && GetRefConversion(ies.Expression) == RefConversion.Inline;
+            var op = _semanticModel.GetOperation(ies);
+            return (op.IsArrayElementAccess() || IsReturnsByRefPropertyElementAccess(op))
+                && GetRefConversion(ies.Expression) == RefConversion.Inline;
+
+            static bool IsReturnsByRefPropertyElementAccess(IOperation op)
+            {
+                return op.IsPropertyElementAccess()
+                 && op is IPropertyReferenceOperation { Property: { } prop }
+                 && (prop.ReturnsByRef || prop.ReturnsByRefReadonly);
+            }
         }
     }
 

CodeConverter/CSharp/OperationExtensions.cs

@@ -56,9 +56,10 @@ public static bool IsAssignableExpression(this IOperation operation)
             case OperationKind.DynamicMemberReference:
                 return true;
 
-            //Just documenting since it's the only one mentioning reference that can't necessarily be assigned to AFAIK
             case OperationKind.PropertyReference:
-                return false;
+                //a property might be RefReturn, if it's defined in a referenced C# assembly
+                var prop = ((IPropertyReferenceOperation)operation).Property;
+                return prop.ReturnsByRef || prop.ReturnsByRefReadonly;
         }
 
         return false;

Tests/CSharp/MultiFileSolutionAndProjectTests.cs

@@ -18,16 +18,22 @@ public MultiFileSolutionAndProjectTests(MultiFileTestFixture multiFileTestFixtur
         _multiFileTestFixture = multiFileTestFixture;
     }
 
-    [Fact] /* enable for executing locally */
+    [Fact]
     public async Task ConvertWholeSolutionAsync()
     {
 
         await _multiFileTestFixture.ConvertProjectsWhereAsync(p => true, Language.CS);
     }
 
-    [Fact] /* enable for executing locally */
+    [Fact]
     public async Task ConvertVbLibraryOnlyAsync()
     {
         await _multiFileTestFixture.ConvertProjectsWhereAsync(p => p.Name == "VbLibrary", Language.CS);
     }
+
+    [Fact(Skip= "Roslyn bugs mean we can't run this test: https://github.com/icsharpcode/CodeConverter/pull/1116#issuecomment-2242645546")]
+    public async Task ConvertVbUsingCSharpRefReturnOnlyAsync()
+    {
+        await _multiFileTestFixture.ConvertProjectsWhereAsync(p => p.Name == "VisualBasicUsingCSharpRefReturn", Language.CS);
+    }
 }

Tests/TestData/MultiFileCharacterization/SourceFiles/CSharpRefReturn/CSharpRefReturn.csproj

@@ -0,0 +1,48 @@
+<?xml version="1.0" encoding="utf-8"?>
+<Project ToolsVersion="15.0" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
+  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
+  <PropertyGroup>
+    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
+    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
+    <ProjectGuid>{8B843547-F49D-40A2-8C4E-1B81D8C5D589}</ProjectGuid>
+    <OutputType>Library</OutputType>
+    <AppDesignerFolder>Properties</AppDesignerFolder>
+    <RootNamespace>CSharpRefReturn</RootNamespace>
+    <AssemblyName>CSharpRefReturn</AssemblyName>
+    <TargetFrameworkVersion>v4.8</TargetFrameworkVersion>
+    <FileAlignment>512</FileAlignment>
+    <Deterministic>true</Deterministic>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
+    <DebugSymbols>true</DebugSymbols>
+    <DebugType>full</DebugType>
+    <Optimize>false</Optimize>
+    <OutputPath>bin\Debug\</OutputPath>
+    <DefineConstants>DEBUG;TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
+    <DebugType>pdbonly</DebugType>
+    <Optimize>true</Optimize>
+    <OutputPath>bin\Release\</OutputPath>
+    <DefineConstants>TRACE</DefineConstants>
+    <ErrorReport>prompt</ErrorReport>
+    <WarningLevel>4</WarningLevel>
+  </PropertyGroup>
+  <ItemGroup>
+    <Reference Include="System" />
+    <Reference Include="System.Core" />
+    <Reference Include="System.Xml.Linq" />
+    <Reference Include="System.Data.DataSetExtensions" />
+    <Reference Include="Microsoft.CSharp" />
+    <Reference Include="System.Data" />
+    <Reference Include="System.Net.Http" />
+    <Reference Include="System.Xml" />
+  </ItemGroup>
+  <ItemGroup>
+    <Compile Include="RefReturnList.cs" />
+    <Compile Include="Properties\AssemblyInfo.cs" />
+  </ItemGroup>
+  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
+</Project>

Tests/TestData/MultiFileCharacterization/SourceFiles/CSharpRefReturn/Properties/AssemblyInfo.cs

@@ -0,0 +1,36 @@
+using System.Reflection;
+using System.Runtime.CompilerServices;
+using System.Runtime.InteropServices;
+
+// General Information about an assembly is controlled through the following
+// set of attributes. Change these attribute values to modify the information
+// associated with an assembly.
+[assembly: AssemblyTitle("CSharpRefReturn")]
+[assembly: AssemblyDescription("")]
+[assembly: AssemblyConfiguration("")]
+[assembly: AssemblyCompany("")]
+[assembly: AssemblyProduct("CSharpRefReturn")]
+[assembly: AssemblyCopyright("Copyright ©  2024")]
+[assembly: AssemblyTrademark("")]
+[assembly: AssemblyCulture("")]
+
+// Setting ComVisible to false makes the types in this assembly not visible
+// to COM components.  If you need to access a type in this assembly from
+// COM, set the ComVisible attribute to true on that type.
+[assembly: ComVisible(false)]
+
+// The following GUID is for the ID of the typelib if this project is exposed to COM
+[assembly: Guid("8b843547-f49d-40a2-8c4e-1b81d8c5d589")]
+
+// Version information for an assembly consists of the following four values:
+//
+//      Major Version
+//      Minor Version
+//      Build Number
+//      Revision
+//
+// You can specify all the values or you can default the Build and Revision Numbers
+// by using the '*' as shown below:
+// [assembly: AssemblyVersion("1.0.*")]
+[assembly: AssemblyVersion("1.0.0.0")]
+[assembly: AssemblyFileVersion("1.0.0.0")]

Tests/TestData/MultiFileCharacterization/SourceFiles/CSharpRefReturn/RefReturnList.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace CSharpRefReturn
+{
+    public class RefReturnList<T>
+    {
+        private T dummy;
+        public ref T this[int i] {
+            get {
+                return ref dummy;
+            }
+        }
+
+        public ref T RefProperty
+        {
+            get
+            {
+                return ref dummy;
+            }
+        }
+    }
+}

Tests/TestData/MultiFileCharacterization/SourceFiles/VisualBasicUsesCSharpRefReturn/ByRefArgument.vb

@@ -0,0 +1,22 @@
+Public Class ByRefArgument
+    Sub UseArr()
+        Dim arrObj() As Object
+        Modify(arrObj(0))
+
+        Dim arrInt() As Integer
+        Modify(arrInt(0))
+    End Sub
+
+    Sub UseRefReturn()
+        Dim lstObj As CSharpRefReturn.RefReturnList(Of Object)
+        Modify(lstObj(0))
+        Modify(lstObj.RefProperty)
+
+        Dim lstInt As CSharpRefReturn.RefReturnList(Of Integer)
+        Modify(lstInt(0))
+        Modify(lstInt.RefProperty)
+    End Sub
+
+    Sub Modify(ByRef o As Object)
+    End Sub
+End Class

Tests/TestData/MultiFileCharacterization/SourceFiles/VisualBasicUsesCSharpRefReturn/My Project/Application.myapp

@@ -0,0 +1,10 @@
+<?xml version="1.0" encoding="utf-8"?>
+<MyApplicationData xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema">
+  <MySubMain>false</MySubMain>
+  <SingleInstance>false</SingleInstance>
+  <ShutdownMode>0</ShutdownMode>
+  <EnableVisualStyles>true</EnableVisualStyles>
+  <AuthenticationMode>0</AuthenticationMode>
+  <ApplicationType>1</ApplicationType>
+  <SaveMySettingsOnExit>true</SaveMySettingsOnExit>
+</MyApplicationData>

Tests/TestData/MultiFileCharacterization/SourceFiles/VisualBasicUsesCSharpRefReturn/My Project/AssemblyInfo.vb

@@ -0,0 +1,35 @@
+Imports System
+Imports System.Reflection
+Imports System.Runtime.InteropServices
+
+' General Information about an assembly is controlled through the following
+' set of attributes. Change these attribute values to modify the information
+' associated with an assembly.
+
+' Review the values of the assembly attributes
+
+<Assembly: AssemblyTitle("VisualBasicUsesCSharpRefReturn")>
+<Assembly: AssemblyDescription("")>
+<Assembly: AssemblyCompany("")>
+<Assembly: AssemblyProduct("VisualBasicUsesCSharpRefReturn")>
+<Assembly: AssemblyCopyright("Copyright ©  2024")>
+<Assembly: AssemblyTrademark("")>
+
+<Assembly: ComVisible(False)>
+
+'The following GUID is for the ID of the typelib if this project is exposed to COM
+<Assembly: Guid("47520bc8-6837-4f63-9aef-0a252d368f05")>
+
+' Version information for an assembly consists of the following four values:
+'
+'      Major Version
+'      Minor Version
+'      Build Number
+'      Revision
+'
+' You can specify all the values or you can default the Build and Revision Numbers
+' by using the '*' as shown below:
+' <Assembly: AssemblyVersion("1.0.*")>
+
+<Assembly: AssemblyVersion("1.0.0.0")>
+<Assembly: AssemblyFileVersion("1.0.0.0")>