C#: How to receive a double pointer to struct from a C++ unmanaged function callback

Question

I will be laconic. I have a library written in C++ using the openCV lib. One of my functions is:

EXTERN_HEADER HWND createHandle(FListener fl);

where FListener is a callback function pointer defined as:

typedef void (__stdcall *FListener)
(int fSN, int fC, byte* fSData, IplImage **fS);

IplImage is an openCV struct.

I am trying to use this library and these functions in C# so i am DllImport-ing as such:

[UnmanagedFunctionPointer(CallingConvention.StdCall)]
delegate void FListener(int fSN, int fC, ref byte fSData, ref IntPtr fS);

[DllImport("FLib.dll", CharSet = CharSet.Auto, 
CallingConvention = CallingConvention.Cdecl)]
static extern int createHandle(FListener fl);

and finally i declare the method that will be called back to my c# program as:

private void test(int fSN, int fC, ref byte fSD, ref IntPtr fS)
{
    //how -for the love of God- do I access that openCV double pointer struct 
    //inside "fS"?
}

Naturally fS is a pointer to an array of pointers pointing to IplImages. Do I have to declare the IplImage struct again inside my C# code? I dont want to use any C# wrapper for openCV. I want to keep things "clean" and "simple" but i am totally stuck with the marshalling part... Any help would be appreciated.

UPDATE: If i pass the fS array as an IntPtr* it works like a charm. The elements are retrieved as fS[0], fS[1] etc. If I pass it as a "ref IntPtr" then the first element can be retrieved as fS but where can i find the second one etc? I tried fS + Marshal.SizeOf(typeof(IplImage)) with no luck... any ideas?

No ideas at all?

Answer 1

It's been a while since I've done Platform Invoke, but I think your callback will have to be unsafe, eg,

unsafe delegate void FListener(int fSN, int fC, ref byte fSData, IplImage **fS);

This may not be the exact syntax, but it should get you going in the right direction.

Answer 2

As for your second question, you can access the other elements using the IntPtr.ToPointer() method in unsafe code, so that you will have a pointer to the base of the structure/array of pointers or whatever, and then you can simply use the indexer the access them, for example:

int* p = (int*)fS.ToPointer();
p[3] = ...;

if it's a pointer to array of IntPtrs do the same, ToPointer once again.