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);    } }