Question

I’m trying to encapsulate some older win32 code in a C++/CLI ref class to make it better accessible from .NET code. This class needs to start a Win32 thread and pass a pointer to the class as a thread parameter. The code looks something like this:

ref class MmePlayer
{
    int StartPlayback()
    {
        hPlayThread = CreateThread(NULL, 0, PlayThread, this, 0, &PlayThreadId);
    }
};

static DWORD WINAPI PlayThread(LPVOID pThreadParam)
{
    // Get a pointer to the object that started the thread
    MmePlayer^ Me = pThreadParam;
}

The thread really needs to be a Win32 thread because it receives messages from the MME subsystem. I’ve tried wrapping the PlayThread function pointer in an interior_ptr, but the compiler wouldn’t allow that.
Also, I’ve tried to make the thread function a class method, but the compiler does not allow the _stdcall modifier on ref class methods.
Do you know of a way to handle this?

Answer 1

Managed classes are passed around using ‘handles’ instead of references. You can’t treat a handle to a managed class like a pointer. What you’re going to want to do is create a native helper class that holds a handle to the managed class. Then you pass a pointer to the native helper into the thread start function. Like this:

#include <msclr/auto_gcroot.h>
using msclr::auto_gcroot;

ref class MmePlayer;

class MmeHelper 
{
     auto_gcroot<MmePlayer^> myPlayer;
};

ref class MmePlayer
{
    int StartPlayback()
    {
        myHelper = new MmeHelper();
        myHelper->myPlayer = this;
        hPlayThread = CreateThread(NULL, 0, PlayThread, myHelper, 0, &PlayThreadId);
    }

    MmeHelper * myHelper;
};

static DWORD WINAPI PlayThread(LPVOID pThreadParam)
{
    // Get a pointer to the object that started the thread
    MmeHelper* helper = pThreadParam;
}