Callback function from C# to C++

Question

Hello,

I have searched on the net, but I haven't found an answer to my problem.

I have a C++ program (an exe file) and a C# dll. The exe file contains a callback function. I need to send the pointer to the C# dll, in order to call the function back from C#.

Here is a scenario:

In the C++ file:

struct MSG_INFO
{
    void* caller;        
    char* info;
    long errnum;
};

void callback(MSG_INFO* info)
{
  // do something
}

The C# dll should receive the pointer to the callback function from the exe and call it through this pointer. If I use managed C++ instead of C#, everything works perfect. I use .NET 2.0 and Visual Studio 2005.

A short C# sample of how to receive the pointer to callback function and how to call the function using the pointer would be great.
Thank you for any help/advice.

Answer 1

This is fairly simple to do. Just use the Marshal class to generate a delegate for the function. Y ou need to do only two things to get this to work.

1.  Make sure that the structure has been correctly marshaled in your C# code. You should have a class that looks like this (I'm assuming that "caller" is a pointer to a object here; if this doesn't work, then it should after a little tweaking):

using System.Runtime.InteropServices;

[StructLayout(LayoutKind.Sequential)]

class MSG_INFO

{

    [MarshalAs(UnmanagedType.LPStruct)]

    public IntPtr caller;

    [MarshalAs(UnmanagedType.LPStr)]

    public string info;

    [MarshalAs(UnmanagedType.I4)]

    public int errnum;

}

2.  You need to create a delegate that takes a "pointer" to the structure you just created. You should be able to get away with this.

delegate void CallbackFunction(MSG_INFO msgInfo);

3.  The final step is to Marshal the pointer to a Delegate. This is fairly easy. I'm going to assume that you're getting a void pointer and start from there.

    unsafe void InvokeCallback(void* callback, MSG_INFO msgInfo)
    {
        IntPtr ptr = new IntPtr(callback);
        CallbackFunction cbFunc = (CallbackFunction)Marshal.GetDelegateForFunctionPointer(ptr, typeof(CallbackFunction));
        cbFunc(msgInfo);
    }

That should work fine for you. If it doesn't, we can tweak it a bit. If you've got alot of these callbacks going on, I would suggest that you make/use a generic method for calling the callbacks.

Answer 2

I hate it when people give half ass solution. It is either they don't know how to do it or they assume people can read their mind.