Question

I have a code like this one in a DLL:

int (*callback)(int a, int b);
void mainfunc()
{
    callback(1, 2);
    callback(3, 4);
}

To access this DLL from a C program, I used to do this:

#include <windows.h>

int callback(int a, int b) {return a+b;}

int main()
{
    HANDLE dll = LoadLibrary("test.dll");
    *(void**)GetProcAddress(dll, "callback") = (void*)callback;
    ((void(*))GetProcAddress(dll, "mainfunc"))();
    FreeLibrary(dll);
}

The DLL is still C code, but the main program have switched to C#. How to deal with function pointer? How would that code be in C#?

Answer 1

callback is a global variable in the dll, so you can’t[1] set it from c#.

You shouldn’t be setting it from C either. What you should have, if callback really is shared, is a function like SetCallback( MyCallbackType callback ) to set it.

But probably, you want to pass the function to mainfunc.

In either case, this is easy to do in C#:

// C#
public delegate Int32 Callback( Int32 a, Int32 b );
[DllImport("test.dll")]
static extern void mainfunc( Callback callback );
// Or if you're setting the callback globally, 
[DllImport("test.dll")]
static extern void SetCallback(Callback callback);
[DllImport("test.dll")]
static extern void mainfunc();

The take home message is that DllImport is smart enough to convert a delegate with a reasonably close signature to a C function pointer.

[1] Ok, so you can…dll injection sometimes uses one of a number of hacks along those lines, but it’s all kinds of the wrong way to do it in any other situation.