Honor MarshalAs.IidParameterIndex for out object in source-generated COM stubs

[Copilot]

Source-generated COM accepted [MarshalAs(UnmanagedType.Interface)] out object but ignored IidParameterIndex, always returning IUnknown* to native callers. This change enables activation-style behavior by using the runtime IID for marshal-out and preserving unsupported-shape diagnostics.

• Description
  • Parser + marshalling metadata
    • Added IidParameterIndex use-site propagation through marshalling info.
    • Added explicit supported-shape gating: only [MarshalAs(UnmanagedType.Interface)] out object.
    • Tracked IID parameter as an element dependency so codegen ordering remains valid.
  • Managed→unmanaged COM stub generation
    • Added a dedicated resolver for IID-indexed COM marshal-out.
    • Generated stub now:
      1. gets IUnknown* via ComInterfaceMarshaller<object>,
      2. calls Marshal.QueryInterface with the runtime IID parameter,
      3. writes the QI result to void**,
      4. releases the intermediate IUnknown*,
      5. throws COMException(hr) on QI failure (HRESULT propagation path).
    • Included failure cleanup for non-null QI out pointer on error.
  • Diagnostics
    • Preserved unsupported configuration diagnostics for non-qualifying shapes.
    • Updated message text to document the supported pattern.
  • Test updates
    • Added compile-pass coverage for the supported shape.
    • Added compile-fail coverage for non-supported shapes (ref/in usage path).
    • Added runtime coverage validating:
      • requested IID is honored,
      • unsupported IID returns E_NOINTERFACE,
      • supported IID returns the requested interface pointer.

[GeneratedComInterface]
[Guid("...")]
partial interface IActivationFactory
{
    void GetActivationFactory(
        in Guid iid,
        [MarshalAs(UnmanagedType.Interface, IidParameterIndex = 0)] out object factory);
}

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[Copilot]

@copilot The unresolved (outdated-by-line-drift, but still applicable) comment about isArrayType in MarshalAsParser.cs was not addressed by the subsequent commits. The current code at HEAD still s...

Fixed in 3d9dc1c.

• MarshalAsParser now reports ConfigurationNotSupported when IidParameterIndex is used on array marshal shapes (LPArray / ByValArray) instead of silently ignoring it.
• Added compile-fail coverage: MarshalAsIidParameterIndexOnArrayUnmanagedType_ReportsDiagnostic for [MarshalAs(UnmanagedType.LPArray, IidParameterIndex = 0)] out object[].
• The test asserts the same IidParameterIndexConfigurationNameWithSupportedShape diagnostic message as the other unsupported-shape tests.

[jtschuster]

I had CCA emit the compiler generated code and copied it here to review. I don't see any issues.

Details

// <auto-generated />
#pragma warning disable CS0612, CS0618, CS0649, CS1591

[global::System.Runtime.InteropServices.StructLayoutAttribute(global::System.Runtime.InteropServices.LayoutKind.Sequential)]
file unsafe struct InterfaceImplementationVtable
{
    public delegate* unmanaged[MemberFunction]<void*, global::System.Guid*, void**, int> QueryInterface_0;
    public delegate* unmanaged[MemberFunction]<void*, uint> AddRef_1;
    public delegate* unmanaged[MemberFunction]<void*, uint> Release_2;
    public delegate* unmanaged[MemberFunction]<global::System.Runtime.InteropServices.ComWrappers.ComInterfaceDispatch*, global::System.Guid*, void**, int> GetActivationFactory_3;
}

file unsafe sealed class InterfaceInformation : global::System.Runtime.InteropServices.Marshalling.IIUnknownInterfaceType
{
    public static global::System.Guid Iid { get; } = new([194, 86, 30, 120, 48, 165, 143, 74, 144, 254, 1, 36, 68, 38, 224, 204]);
    public static void** ManagedVirtualMethodTable => (void**)global::System.Runtime.CompilerServices.Unsafe.AsPointer(in InterfaceImplementation.Vtable);
}

[global::System.Runtime.InteropServices.DynamicInterfaceCastableImplementationAttribute]
file unsafe interface InterfaceImplementation : global::ComInterfaceGenerator.Tests.IActivationFactory
{
    [global::System.Runtime.CompilerServices.FixedAddressValueTypeAttribute]
    public static readonly InterfaceImplementationVtable Vtable;

    static InterfaceImplementation()
    {
        global::System.Runtime.InteropServices.ComWrappers.GetIUnknownImpl(
            out *(nint*)&((InterfaceImplementationVtable*)global::System.Runtime.CompilerServices.Unsafe.AsPointer(ref Vtable))->QueryInterface_0,
            out *(nint*)&((InterfaceImplementationVtable*)global::System.Runtime.CompilerServices.Unsafe.AsPointer(ref Vtable))->AddRef_1,
            out *(nint*)&((InterfaceImplementationVtable*)global::System.Runtime.CompilerServices.Unsafe.AsPointer(ref Vtable))->Release_2);

        Vtable.GetActivationFactory_3 = &ABI_GetActivationFactory;
    }

    [global::System.CodeDom.Compiler.GeneratedCodeAttribute("Microsoft.Interop.ComInterfaceGenerator", "42.42.42.42")]
    [global::System.Runtime.CompilerServices.SkipLocalsInitAttribute]
    void global::ComInterfaceGenerator.Tests.IActivationFactory.GetActivationFactory(in global::System.Guid iid, out object factory)
    {
        var(__this, __vtable) = ((global::System.Runtime.InteropServices.Marshalling.IUnmanagedVirtualMethodTableProvider)this).GetVirtualMethodTableInfoForKey(typeof(global::ComInterfaceGenerator.Tests.IActivationFactory));
        var __target = ((delegate* unmanaged[MemberFunction]<void*, global::System.Guid*, void**, int> )__vtable[3]);
        {
            bool __invokeSucceeded = default;
            factory = default;
            void* __factory_native = default;
            int __invokeRetVal = default;
            try
            {
                // Pin - Pin data in preparation for calling the P/Invoke.
                fixed (global::System.Guid* __iid_native = &iid)
                {
                    __invokeRetVal = __target(__this, __iid_native, &__factory_native);
                }
    
                // NotifyForSuccessfulInvoke - Keep alive any managed objects that need to stay alive across the call.
                global::System.Runtime.InteropServices.Marshal.ThrowExceptionForHR(__invokeRetVal, new([194, 86, 30, 120, 48, 165, 143, 74, 144, 254, 1, 36, 68, 38, 224, 204]), (global::System.IntPtr)__this);
                global::System.GC.KeepAlive(this);
                __invokeSucceeded = true;
                // Unmarshal - Convert native data to managed data.
                factory = global::System.Runtime.InteropServices.Marshalling.ComInterfaceMarshaller<object>.ConvertToManaged(__factory_native);
            }
            finally
            {
                if (__invokeSucceeded)
                {
                    // CleanupCalleeAllocated - Perform cleanup of callee allocated resources.
                    global::System.Runtime.InteropServices.Marshalling.ComInterfaceMarshaller<object>.Free(__factory_native);
                }
            }
        }
    }

    [global::System.Runtime.InteropServices.UnmanagedCallersOnlyAttribute(CallConvs = new[] { typeof(global::System.Runtime.CompilerServices.CallConvMemberFunction) })]
    internal static int ABI_GetActivationFactory(global::System.Runtime.InteropServices.ComWrappers.ComInterfaceDispatch* __this_native, global::System.Guid* __iid_native__param, void** __factory_native__param)
    {
        global::ComInterfaceGenerator.Tests.IActivationFactory @this = default;
        ref global::System.Guid __iid_native = ref *__iid_native__param;
        global::System.Guid iid = default;
        ref void* __factory_native = ref *__factory_native__param;
        object factory = default;
        int __retVal = default;
        try
        {
            // Unmarshal - Convert native data to managed data.
            iid = __iid_native;
            @this = global::System.Runtime.InteropServices.ComWrappers.ComInterfaceDispatch.GetInstance<global::ComInterfaceGenerator.Tests.IActivationFactory>(__this_native);
            @this.GetActivationFactory(in iid, out factory);
            // NotifyForSuccessfulInvoke - Keep alive any managed objects that need to stay alive across the call.
            __retVal = 0; // S_OK
            // Marshal - Convert managed data to native data.
            void* __factory_native__unknown = (void*)global::System.Runtime.InteropServices.Marshalling.ComInterfaceMarshaller<object>.ConvertToUnmanaged(factory);
            if (__factory_native__unknown != null)
            {
                global::System.IntPtr __factory_native__queriedInterface = 0;
                int __factory_native__queryInterfaceHResult = global::System.Runtime.InteropServices.Marshal.QueryInterface((global::System.IntPtr)__factory_native__unknown, in iid, out __factory_native__queriedInterface);
                global::System.Runtime.InteropServices.Marshal.Release((global::System.IntPtr)__factory_native__unknown);
                if (__factory_native__queryInterfaceHResult != 0)
                {
                    if (__factory_native__queriedInterface != 0)
                        global::System.Runtime.InteropServices.Marshal.Release(__factory_native__queriedInterface);
                    __factory_native = null;
                    return __factory_native__queryInterfaceHResult;
                }
    
                __factory_native = (void*)__factory_native__queriedInterface;
            }
            else
            {
                __factory_native = null;
            }
        }
        catch (global::System.Exception __exception)
        {
            __retVal = global::System.Runtime.InteropServices.Marshalling.ExceptionAsHResultMarshaller<int>.ConvertToUnmanaged(__exception);
        }
    
        return __retVal;
    }
}

namespace ComInterfaceGenerator.Tests
{
    [global::System.Runtime.InteropServices.Marshalling.IUnknownDerivedAttribute<InterfaceInformation, InterfaceImplementation>]
    unsafe partial interface IActivationFactory
    {
    }
}

[AaronRobinsonMSFT]

@jtschuster Please take another look.

[Copilot]

Copilot's findings

• Files reviewed: 26/26 changed files
• Comments generated: 2

[jtschuster]

LGTM

[Copilot]

Copilot's findings

• Files reviewed: 26/26 changed files
• Comments generated: 1

[Copilot]

Copilot's findings

• Files reviewed: 26/26 changed files
• Comments generated: 1