Generate an 'out' local variable for unmanaged to managed stubs when the value needs to be marshalled out to unmanaged

src/libraries/System.Runtime.InteropServices/gen/ComInterfaceGenerator/UnmanagedToManagedStubGenerator.cs

@@ -65,8 +65,8 @@ public BlockSyntax GenerateStubBody(ExpressionSyntax methodToInvoke)
                 setupStatements.Add(MarshallerHelpers.Declare(PredefinedType(Token(SyntaxKind.BoolKeyword)), InvokeSucceededIdentifier, initializeToDefault: true));
             }
 
-            setupStatements.AddRange(declarations.Initializations);
             setupStatements.AddRange(declarations.Variables);
+            setupStatements.AddRange(declarations.Initializations);
             setupStatements.AddRange(statements.Setup);
 
             List<StatementSyntax> tryStatements = new();

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/GeneratedStatements.cs

@@ -71,7 +71,13 @@ private static ImmutableArray<StatementSyntax> GenerateStatementsForStubContext(
             ImmutableArray<StatementSyntax>.Builder statementsToUpdate = ImmutableArray.CreateBuilder<StatementSyntax>();
             foreach (BoundGenerator marshaller in marshallers.SignatureMarshallers)
             {
-                statementsToUpdate.AddRange(marshaller.Generator.Generate(marshaller.TypeInfo, context));
+                var localContext = context;
+                if (context.CurrentStage is StubCodeContext.Stage.Marshal
+                    && MarshallerHelpers.IsMidlOutBehavior(marshaller.TypeInfo, context))
+                {
+                    localContext = new MarshalToLocalContext(context);
+                }
+                statementsToUpdate.AddRange(marshaller.Generator.Generate(marshaller.TypeInfo, localContext));
             }
 
             if (statementsToUpdate.Count > 0)

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/MarshalToLocalContext.cs

@@ -0,0 +1,32 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System;
+
+namespace Microsoft.Interop
+{
+    internal sealed record MarshalToLocalContext : StubCodeContext
+    {
+        internal StubCodeContext InnerContext { get; init; }
+
+        internal MarshalToLocalContext(StubCodeContext inner)
+        {
+            InnerContext = inner;
+            CurrentStage = inner.CurrentStage;
+            Direction = inner.Direction;
+            ParentContext = inner.ParentContext;
+        }
+
+        public override (TargetFramework framework, Version version) GetTargetFramework() => InnerContext.GetTargetFramework();
+
+        public override bool SingleFrameSpansNativeContext => InnerContext.SingleFrameSpansNativeContext;
+
+        public override bool AdditionalTemporaryStateLivesAcrossStages => InnerContext.AdditionalTemporaryStateLivesAcrossStages;
+
+        public override (string managed, string native) GetIdentifiers(TypePositionInfo info)
+            => InnerContext.GetIdentifiers(info);
+        //=> (inner.GetIdentifiers(info).managed, inner.GetAdditionalIdentifier(info, "out"));
+
+        public override string GetAdditionalIdentifier(TypePositionInfo info, string name) => InnerContext.GetAdditionalIdentifier(info, name);
+    }
+}

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/Marshalling/MarshallerHelpers.cs

@@ -394,5 +394,17 @@ public static MarshalDirection GetMarshalDirection(TypePositionInfo info, StubCo
             }
             throw new UnreachableException("An element is either a return value or passed by value or by ref.");
         }
+
+        /// <summary>
+        /// Returns whether a parameter has MIDL '[out]' behavior should be unmarshalled into a local variable and only assigned to the parameter at the end of the function call.
+        /// </summary>
+        public static bool IsMidlOutBehavior(TypePositionInfo info, StubCodeContext context)
+            => context.Direction is MarshalDirection.UnmanagedToManaged
+                && (info.IsByRef && info.RefKind is RefKind.Out or RefKind.Ref
+                    || info.ByValueContentsMarshalKind.HasFlag(ByValueContentsMarshalKind.Out)
+                    || info.IsManagedReturnPosition);
+
+        internal static StatementSyntax CreateDiscardStatement(string identifier)
+            => ExpressionStatement(AssignmentExpression(SyntaxKind.SimpleAssignmentExpression, IdentifierName("_"), IdentifierName(identifier)));
     }
 }

src/libraries/System.Runtime.InteropServices/gen/Microsoft.Interop.SourceGeneration/VariableDeclarations.cs

@@ -83,17 +83,51 @@ public static VariableDeclarations GenerateDeclarationsForUnmanagedToManaged(Bou
             foreach (BoundGenerator marshaller in marshallers.NativeParameterMarshallers)
             {
                 TypePositionInfo info = marshaller.TypeInfo;
-                if (info.IsNativeReturnPosition || info.IsManagedReturnPosition)
+                if (info.IsNativeReturnPosition)
+                    continue;
+
+                if (info.IsManagedReturnPosition)
                     continue;
 
                 // Declare variables for parameters
                 AppendVariableDeclarations(variables, marshaller, context, initializeToDefault: initializeDeclarations);
+
+                {
+                    // We need to use the 'out' value - This should be removed once the ownership behavior is fixed
+                    var boundaryBehavior = marshaller.Generator.GetValueBoundaryBehavior(info, context);
+                    if (marshaller.Generator.UsesNativeIdentifier(marshaller.TypeInfo, context)
+                        && boundaryBehavior is not
+                            (ValueBoundaryBehavior.NativeIdentifier or ValueBoundaryBehavior.CastNativeIdentifier))
+                    {
+                        if (MarshallerHelpers.IsMidlOutBehavior(marshaller.TypeInfo, context))
+                        {
+                            string outlocal = context.GetAdditionalIdentifier(info, "out");
+                            initializations.Add(MarshallerHelpers.CreateDiscardStatement(outlocal));
+                        }
+                    }
+                }
             }
 
             if (!marshallers.IsManagedVoidReturn)
             {
                 // Declare variables for stub return value
                 AppendVariableDeclarations(variables, marshallers.ManagedReturnMarshaller, context, initializeToDefault: initializeDeclarations);
+                {
+                    var marshaller = marshallers.ManagedReturnMarshaller;
+                    var info = marshaller.TypeInfo;
+                    // We need to use the 'out' value - This should be removed once the ownership behavior is fixed
+                    var boundaryBehavior = marshaller.Generator.GetValueBoundaryBehavior(info, context);
+                    if (marshaller.Generator.UsesNativeIdentifier(marshaller.TypeInfo, context)
+                        && boundaryBehavior is not
+                            (ValueBoundaryBehavior.NativeIdentifier or ValueBoundaryBehavior.CastNativeIdentifier))
+                    {
+                        if (MarshallerHelpers.IsMidlOutBehavior(marshaller.TypeInfo, context))
+                        {
+                            string outlocal = context.GetAdditionalIdentifier(info, "out");
+                            initializations.Add(MarshallerHelpers.CreateDiscardStatement(outlocal));
+                        }
+                    }
+                }
             }
 
             if (!marshallers.IsUnmanagedVoidReturn && !marshallers.ManagedNativeSameReturn)
@@ -112,6 +146,7 @@ static void AppendVariableDeclarations(ImmutableArray<LocalDeclarationStatementS
             {
                 (string managed, string native) = context.GetIdentifiers(marshaller.TypeInfo);
 
+                var info = marshaller.TypeInfo;
                 // Declare variable for return value
                 if (marshaller.TypeInfo.IsNativeReturnPosition)
                 {
@@ -143,14 +178,17 @@ static void AppendVariableDeclarations(ImmutableArray<LocalDeclarationStatementS
                             (ValueBoundaryBehavior.NativeIdentifier or ValueBoundaryBehavior.CastNativeIdentifier))
                     {
                         TypeSyntax localType = marshaller.Generator.AsNativeType(marshaller.TypeInfo).Syntax;
-                        if (boundaryBehavior != ValueBoundaryBehavior.AddressOfNativeIdentifier)
+                        if (MarshallerHelpers.IsMidlOutBehavior(marshaller.TypeInfo, context))
                         {
+                            string outlocal = context.GetAdditionalIdentifier(info, "out");
+                            // <nativeType> __param_native_out;
                             statementsToUpdate.Add(MarshallerHelpers.Declare(
-                                localType,
-                                native,
-                                false));
+                                marshaller.Generator.AsNativeType(info).Syntax,
+                                outlocal,
+                                true));
                         }
-                        else
+
+                        if (boundaryBehavior is ValueBoundaryBehavior.AddressOfNativeIdentifier)
                         {
                             // To simplify propogating back the value to the "byref" parameter,
                             // we'll just declare the native identifier as a ref to its type.

src/libraries/System.Runtime.InteropServices/tests/ComInterfaceGenerator.Tests/RcwAroundCcwTests.cs

@@ -181,17 +181,8 @@
             public void Set(int value) => _data = value;
         }
 
-        [GeneratedComInterface]
-        [Guid("5A9D3ED6-CC17-4FB9-8F82-0070489B7213")]
-        internal partial interface IBool
-        {
-            [return: MarshalAs(UnmanagedType.I1)]
-            bool Get();
-            void Set([MarshalAs(UnmanagedType.I1)] bool value);
-        }
-
         [GeneratedComClass]
         internal partial class IBoolImpl : IBool
         {
             bool _data;
             public bool Get() => _data;

src/libraries/System.Runtime.InteropServices/tests/TestAssets/NativeExports/NativeExports.csproj

@@ -18,6 +18,13 @@
       '$(TargetOS)' == 'tvossimulator'">true</_TargetsAppleOS>
   </PropertyGroup>
 
+<Target Name="DisableComInterfaceGenerator"
+        BeforeTargets="CoreCompile">
+  <ItemGroup>
+  <Analyzer Remove="@(Analyzer)" Condition="'%(FileName)' == 'Microsoft.Interop.ComInterfaceGenerator'" />
+  </ItemGroup>
+</Target>
+
   <ItemGroup>
     <Compile Include="..\..\TestAssets\SharedTypes\ComInterfaces\**\*.cs" Link="ComInterfaceGenerator\ComInterfaces\%(RecursiveDir)\%(FileName).cs" />
     <Compile Include="$(CommonPath)DisableRuntimeMarshalling.cs" Link="Common\DisableRuntimeMarshalling.cs" />

src/libraries/System.Runtime.InteropServices/tests/TestAssets/SharedTypes/ComInterfaces/IRefStrings.cs

@@ -0,0 +1,18 @@
+// Licensed to the .NET Foundation under one or more agreements.
+// The .NET Foundation licenses this file to you under the MIT license.
+
+using System.Runtime.InteropServices;
+using System.Runtime.InteropServices.Marshalling;
+
+namespace SharedTypes.ComInterfaces
+{
+    [GeneratedComInterface(StringMarshalling = System.Runtime.InteropServices.StringMarshalling.Utf8)]
+    [Guid(_guid)]
+    internal partial interface IRefStrings
+    {
+        public const string _guid = "5146B7DB-0588-469B-B8E5-B38090A2FC15";
+        void RefString(ref string value);
+        void InString(in string value);
+        void OutString(out string value);
+    }
+}

src/libraries/System.Runtime.InteropServices/tests/TestAssets/SharedTypes/SharedTypes.csproj

@@ -9,6 +9,13 @@
     <Compile Include="$(CommonPath)DisableRuntimeMarshalling.cs" Link="Common\DisableRuntimeMarshalling.cs" />
   </ItemGroup>
 
+<Target Name="DisableComInterfaceGenerator"
+        BeforeTargets="CoreCompile">
+  <ItemGroup>
+  <Analyzer Remove="@(Analyzer)" Condition="'%(FileName)' == 'Microsoft.Interop.ComInterfaceGenerator'" />
+  </ItemGroup>
+</Target>
+
   <ItemGroup>
     <ProjectReference Include="..\..\Ancillary.Interop\Ancillary.Interop.csproj" />
   </ItemGroup>