How to implement the COM interface of C # in C++

Question

I have a .NET Framework binary that implements two COM interface classes, where one serves as a callback function. Now, I want to use the implemented C# COM interface in a C++ project. I know how to create an object of type IMyDotNetInterface2, but I'm not sure how to implement a COM interface of type IMyDotNetInterface.

using System;
using System.Runtime.InteropServices;
using System.Windows.Forms;

namespace MyInterop
{
    [Guid("03AD5D2D-2AFD-439f-8713-A4EC0705B4D9")]
    public interface IMyDotNetInterface
    {
        void Send(string message);
    }
	[Guid("04AD5D2D-2AFD-439f-8713-A4EC0705B4D9")]
    public interface IMyDotNetInterface2
    {
        void SetCallback(IMyDotNetInterface callback);
		string GetStatus();
    }
	[Guid("05AD5D2D-2AFD-439f-8713-A4EC0705B4D9")]
    public class MyDotNetInterface : IMyDotNetInterface2
    {
		private IMyDotNetInterface _callback;
        public void SetCallback(IMyDotNetInterface callback)
		{
			_callback = callback;
		}
		public string GetStatus()
		{
			return "OK";
		}
    }
}

#import "<Full Path>\com.MyInterop.tlb" named_guids raw_interfaces_only
void DotNetCom()
{
	CoInitialize(NULL);   //Initialize all COM Components
		
	// <namespace>::<InterfaceName>
	MyInterop::IMyDotNetInterface2Ptr pDotNetCOMPtr;

	// CreateInstance parameters
	HRESULT hRes = pDotNetCOMPtr.CreateInstance(MyInterop::CLSID_MyDotNetClass);
	if (hRes == S_OK)
	{
		BSTR str;
		pDotNetCOMPtr->ShowCOMDialog ();
	}

	CoUninitialize ();   //DeInitialize all COM Components
}

Answer 1

Hi, @Aspire
You could refer to example code link in the similar thread's question: How does implementing multiple COM interfaces work in C++? Below is a quick test

// ConsoleApplication1.cpp : This file contains the 'main' function. Program execution begins and ends there.
//


#import "......\bin\Debug\MyInterop.tlb" named_guids raw_interfaces_only
#include <iostream>
#include<combaseapi.h>
#include"./Debug/MyInterop.tlh"


struct MyClass:public  MyInterop::IMyDotNetInterface
{
public:
    virtual ULONG  __stdcall AddRef(void) override { return S_OK; }
    virtual ULONG  __stdcall  Release(void) override { return S_OK; }
    MyClass()
    {

  }
    virtual HRESULT  __stdcall  QueryInterface(REFIID riid, void** ppvObject) override
    {
        if (riid == __uuidof(IUnknown))
        {
            *ppvObject = (IUnknown*)this;
            return S_OK;
        }
        IID uid;
        IIDFromString(L"{C# interface GUID/IID}", &uid);
        if (riid == uid)
        {
            *ppvObject = (IDispatch*)this;// (IUnknown*)this;
            return S_OK;
        }
        if (riid == __uuidof(IDispatch))
        {
            *ppvObject = (IDispatch*)this;
            return S_OK;
        }
        return E_NOTIMPL;
    }
    virtual HRESULT  __stdcall GetTypeInfoCount(UINT* pctinfo) override { return S_OK; }
    virtual HRESULT  __stdcall GetTypeInfo(UINT iTInfo, LCID lcid, ITypeInfo** ppTInfo) override { return S_OK; }
    virtual HRESULT  __stdcall  GetIDsOfNames(REFIID riid, LPOLESTR* rgszNames, UINT cNames, LCID lcid, DISPID* rgDispId) override
    {
        *rgDispId = 1;
        return S_OK;
    }

    virtual HRESULT  __stdcall Invoke(DISPID dispIdMember, REFIID riid, LCID lcid, WORD wFlags, DISPPARAMS* pDispParams, VARIANT* pVarResult, EXCEPINFO* pExcepInfo, UINT* puArgErr) override
    {
       
        return S_OK;
    }
    virtual  HRESULT  __stdcall  ShowCOMDialog()
    {
        std::cout << "hello";
        return S_OK;
    }
};



void DotNetCom()
{
    CoInitialize(NULL);   //Initialize all COM Components

    
    MyInterop::IMyDotNetInterfacePtr pDotNetCOMPtr;

    // CreateInstance parameters
    // e.g. CreateInstance (<namespace::CLSID_<ClassName>)
    HRESULT hRes =
        pDotNetCOMPtr.CreateInstance(MyInterop::CLSID_MyDotNetClass);
    if (hRes == S_OK)
    {
        BSTR str;
        pDotNetCOMPtr->ShowCOMDialog();
        //call .NET COM exported function ShowDialog ()
    }

    CoUninitialize();   //DeInitialize all COM Components
}
int main()
{
    MyClass test;
    test.ShowCOMDialog();
   
}

Best regards,

Minxin Yu

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Answer 2

I have created two sample solutions using VS2022 17.8.3. Each solution contains projects for a C# Com Server, a C++ Com Server and a C++ test program. One solution (CppComCallback) defines and implements the C++ Com callback in the C++ COM server. The other (CppComCallback2) defines the callback interface in the C# Com Server and imports the type library into the C++ Com Server which implements the related interface. I thought that providing concrete examples would be simpler than trying to walk through the details of creating the C++ COM servers. If you have any additional questions, I'd be happy to answer them. Don't forget to unblock the zip files from Onedrive before you extract their contents. Also, run Visual Studio as Administrator when building the sample solutions so that COM registration will succeed.

CppComCallback.zip

CppComCallback2.zip