17.5 Callbacks from Unmanaged Code

The P/Invoke layer does its best to present a natural programming model on both sides of the boundary, mapping between the relevant constructs where possible. Since C# not only can call out to C functions but can also be called back from the C functions (via function pointers), the P/Invoke layer needs to map unmanaged function pointers onto something natural for the managed world. The managed equivalent of a function pointer is a delegate, so the P/Invoke layer automatically maps between delegates (in C#) and function pointers (in C). To call a function in a DLL that takes a callback function pointer as a parameter, declare a delegate with the correct signature for the callback function and pass an instance of the delegate when calling the function that performs the callback, as follows:

using System;
using System.Runtime.InteropServices;
  
class CallbackFun {
   delegate bool CallBack(int hWnd, int lParam);
   [DllImport("user32.dll")]
   static extern int EnumWindows(CallBack hWnd, int lParam);
   static bool PrintWindow(int hWnd, int lParam) {
      Console.WriteLine(hWnd);
      return true;
   }
   static void Main( ) {
      CallBack e = new CallBack(PrintWindow);
      EnumWindows(e, 0);
   }
}