Converting between 2 different libraries using the same COM interface in C#

Question

I have a pair of libraries that both use the same COM interface. In one library I have a class that implements that interface. The other library requires an object that implements the interface.

However both libraries have their own definition of the interface. Both are slightly different but essentially the same interface.

So I try to case between them as follows:

 Library2.Interface intf = (Library2.Interface)impl;

but this raises as an exception. If I do the following:

 Library1.Interface intf = (Library1.Interface)impl;

Then it casts without problem but I am no longer able to pass the class to Library2.

I naively assumed that both interfaces having the same GUID would prevent this being a problem but I appear to be wrong on that. Does anyone have any idea how I can convert between the 2 libraries? Perhaps via a Marshal of some sort?

Answer 1

This is a very interesting problem, and I think I may have an interesting solution for it. So, although Library1.Interface and Library2.Interface are the two binary compatible ComImport interfaces, they're still two different .NET interfaces and cannot be cast to each other.

To make the casting possible, we need to somehow hide the identity of the managed Library1.Interface .NET object behind a COM-callable wrapper (CCW), then make sure this CCW doesn't get marshaled to the same .NET object. So that the .NET marshaller would create a separate RCW proxy which then could be cast to Library2.Interface as a plain vanilla COM object.

Besides using separate COM apartments for Library1.Interface and Library2.Interface objects, I can only think of one other way of doing this: COM aggregation. Any .NET object can be aggregated via Marshal.CreateAggregatedObject as an inner object. The trick is to construct the unmanaged IUnknown COM identity object to serve as an outer (parent) object for aggregation. Such outer object will be given a separate RCW proxy when accessed from .NET.

Below is my take on this:

var server = ComWrapper.Create<Library2.Interface>(() => new Library1.Server());
var client = new Library2.Client();
client.CallMethod(server);

The whole logic as a console app (certain knowledge of COM binary protocols is required to understand this code):

using System;
using System.Runtime.InteropServices;

namespace Library1
{
    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("4C08A691-5D61-4E9A-B16D-75BAD2834BAE")]
    public interface Interface
    {
        void TestMethod();
    }

    [ComVisible(true)]
    public class Server : Interface
    {
        public Server() { }

        public void TestMethod()
        {
            Console.WriteLine("TestMethod called");
        }
    }
}

namespace Library2
{
    [ComImport, InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
    [Guid("4C08A691-5D61-4E9A-B16D-75BAD2834BAE")]
    public interface Interface
    {
        void TestMethod();
    }

    public class Client
    {
        public void CallMethod(Library2.Interface server)
        {
            server.TestMethod();
        }
    }
}

namespace TestApp
{
    class Program
    {
        static void Main(string[] args)
        {
            // convert Library1.Server to Library2.Interface 
            var server = ComWrapper.Create<Library2.Interface>(() => new Library1.Server());
            var client = new Library2.Client();
            client.CallMethod(server);

            Marshal.ReleaseComObject(server);
            Console.ReadLine();
        }
    }

    /// <summary>
    /// ComWrapper - http://stackoverflow.com/q/26758316/1768303
    /// by Noseratio
    /// </summary>
    public class ComWrapper
    {
        readonly Guid IID_IUnknown = new Guid("00000000-0000-0000-C000-000000000046");
        const int S_OK = 0;
        const int E_FAIL = unchecked((int)0x80004005);

        delegate int QueryInterfaceMethod(IntPtr pUnk, ref Guid iid, out IntPtr ppv);
        delegate int AddRefMethod(IntPtr pUnk);
        delegate int ReleaseMethod(IntPtr pUnk);

        [StructLayout(LayoutKind.Sequential)]
        struct UnkObject
        {
            public IntPtr pVtable;
        }

        [StructLayout(LayoutKind.Sequential)]
        struct UnkVtable
        {
            public IntPtr pQueryInterface;
            public IntPtr pAddRef;
            public IntPtr pRelease;
        }

        int _refCount = 0;
        IntPtr _pVtable;
        IntPtr _outerObject;
        IntPtr _aggregatedObject;
        GCHandle _gcHandle;

        QueryInterfaceMethod _queryInterfaceMethod;
        AddRefMethod _addRefMethod;
        ReleaseMethod _releaseMethod;

        private ComWrapper()
        {
        }

        ~ComWrapper()
        {
            Console.WriteLine("~ComWrapper");
            Free();
        }

        private IntPtr Initialize(Func<object> createInnerObject)
        {
            try
            {
                // implement IUnknown methods
                _queryInterfaceMethod = delegate(IntPtr pUnk, ref Guid iid, out IntPtr ppv)
                {
                    lock (this)
                    {
                        // delegate anything but IID_IUnknown to the aggregated object
                        if (IID_IUnknown == iid)
                        {
                            ppv = _outerObject;
                            Marshal.AddRef(_outerObject);
                            return S_OK;
                        }
                        return Marshal.QueryInterface(_aggregatedObject, ref iid, out ppv);
                    }
                };

                _addRefMethod = delegate(IntPtr pUnk)
                {
                    lock (this)
                    {
                        return ++_refCount;
                    }
                };

                _releaseMethod = delegate(IntPtr pUnk)
                {
                    lock (this)
                    {
                        if (0 == --_refCount)
                        {
                            Free();
                        }
                        return _refCount;
                    }
                };

                // create the IUnknown vtable
                var vtable = new UnkVtable();
                vtable.pQueryInterface = Marshal.GetFunctionPointerForDelegate(_queryInterfaceMethod);
                vtable.pAddRef = Marshal.GetFunctionPointerForDelegate(_addRefMethod);
                vtable.pRelease = Marshal.GetFunctionPointerForDelegate(_releaseMethod);

                _pVtable = Marshal.AllocCoTaskMem(Marshal.SizeOf(vtable));
                Marshal.StructureToPtr(vtable, _pVtable, false);

                // create the IUnknown object
                var unkObject = new UnkObject();
                unkObject.pVtable = _pVtable;
                _outerObject = Marshal.AllocCoTaskMem(Marshal.SizeOf(unkObject));
                Marshal.StructureToPtr(unkObject, _outerObject, false);

                // pin the managed ComWrapper instance
                _gcHandle = GCHandle.Alloc(this, GCHandleType.Normal);

                // create and aggregate the inner object
                _aggregatedObject = Marshal.CreateAggregatedObject(_outerObject, createInnerObject());

                return _outerObject;
            }
            catch
            {
                Free();
                throw;
            }
        }

        private void Free()
        {
            Console.WriteLine("Free");
            if (_aggregatedObject != IntPtr.Zero)
            {
                Marshal.Release(_aggregatedObject);
                _aggregatedObject = IntPtr.Zero;
            }
            if (_pVtable != IntPtr.Zero)
            {
                Marshal.FreeCoTaskMem(_pVtable);
                _pVtable = IntPtr.Zero;
            }
            if (_outerObject != IntPtr.Zero)
            {
                Marshal.FreeCoTaskMem(_outerObject);
                _outerObject = IntPtr.Zero;
            }
            if (_gcHandle.IsAllocated)
            {
                _gcHandle.Free();
            }
        }

        public static T Create<T>(Func<object> createInnerObject)
        {
            var wrapper = new ComWrapper();
            var unk = wrapper.Initialize(createInnerObject);
            Marshal.AddRef(unk);
            try
            {
                var comObject = Marshal.GetObjectForIUnknown(unk);
                return (T)comObject;
            }
            finally
            {
                Marshal.Release(unk);
            }
        }
    }
}