I'm trying to figure out how some stuff can be done when calling
functions in a native DLL from managed C++ and the called functions
have callbacks. What I want is to be able to use an opaque type as a
handle for the user of the DLL, in this case it's called Test and the
user can create/delete instances of this type, set the callback
function and call a function that, in turn, will call the callback
function. However I can't get it to compile and I suspect that it
might have something to do with the calling convention of the
functions of the DLL. Code as follows.

/*
* The DLL, two files in a standard Win32 project
*/
/* MyTest.h */

namespace MyTest
{
    struct Test;
    typedef void (__cdecl *CallBackFunc)(int);
    __declspec(dllexport) Test* newTest(int i);
    __declspec(dllexport) void delTest(Test* t);
    __declspec(dllexport) void setCallback(Test* t, CallBackFunc);
    __declspec(dllexport) void useCallBack(Test* t);
}

/* MyTest.cpp */

#include "MyTest.h"

namespace MyTest
{
    struct Test {
        int i_;
        CallBackFunc ptr_;
        Test(int i) : i_(i) { }
    };

    Test* newTest(int i) {
        return new Test(i);
    }

    void delTest(Test* t) {
        delete t;
    }

    void setCallback(Test* t, CallBackFunc p) {
        t->ptr_ = (CallBackFunc)p;
    }

    void useCallBack(Test* t) {
        t->ptr_(1);
    }
}

/*
* The app using the DLL, a .Net console app
*/
/* App.cpp */
#include "MyTest.h"

using namespace System;
using namespace System::Runtime::InteropServices;

ref class Tester
{
public:
    void called(int i)
    {
        Console::WriteLine(i);
    }
};

delegate void CallBack(int);

int main()
{
    Tester^ test = gcnew Tester();
    CallBack^ cb = gcnew CallBack(test, &Tester::called);

    MyTest::Test* t = MyTest::newTest(7);
    MyTest::setCallback(t,
   
((MyTest::CallBackFunc)Marshal::GetFunctionPointerForDelegate(cb).ToPointer));

    MyTest::useCallBack(t);
    MyTest::delTest(t);

    return 0;
}

I've added the DLL-project as a reference and added the directory
containing MyTest.h as an additional include directory, and that
should make it all work (except for the compilation, error I get). The
error is on the MyTest::setCallBack(...)-line and has error code C2440
[1].

If anyone can help figure out what's needed to fix this I'd be very
grateful!

[1] http://msdn2.microsoft.com/en-us/library/sy5tsf8z.aspx

--
Erik Wikström