Question

I have a legacy application written in C , and i would like to gradually move some code to c#. But before being able to rewrite everything i will need to have just few components writen in c# first that are going to be used from C.

Answer 1

I assume your C# class is a static class. You need to create an interop layer in C++/CLI before you can use it in pure C. Create a C++/CLI class to wrap your C# class. Once that is done use the export function to export the specific C functions. C++/CLI will manage the interop on your behalf. The rule of thumb is if you class/function has any CLI it WILL BE CLI. So your extern functions should only return native datatypes.

extern "C" __declspec( dllexport ) int MyFunc(long parm1);

Here is an article to help you get started. It converts C++ to C# but the process is reversed in your case. CodeProject Unfortunately there is no convenient reverse PInvoke for pure C.

Unfortunately I have never gone from C# to C. It sounds like an interesting project. Good luck!

Ok If you have not figured it out yet i have a quick sample for you.

C# CSLibrary.Math.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace CSLibrary
{
    static public class Math
    {
        static public int Add(int a, int b)
        {
            int c = a + b;
            return c;
        }
    }
}

Cpp/C++ Project CPPCLibrary.h (Compiled with C++/CLI Option with project dependencies)

#pragma once

using namespace System;

extern "C" __declspec( dllexport ) int MathAdd(int a, int b)
{
    return CSLibrary::Math::Add(a, b);
}

C Project CTest.c (Compiled as C Code)

#include "stdafx.h"
#pragma comment(lib, "../Debug/CPPCLILibrary.lib") 

extern __declspec( dllimport ) int MathAdd(int a, int b);

int _tmain(int argc, _TCHAR* argv[])
{
    int answer = MathAdd(10, 32);
    _tprintf(_T("%d\n"), answer);
    return 0;
}

All files are in the same solution but different projects. I have confirmed this has worked. I hope this helps anyone who comes across it.

Cheers!