A callback function is code within a managed application that helps an unmanaged DLL function complete a task. Calls to a callback function pass indirectly from a managed application, through a DLL function, and back to the managed implementation. Some of the many DLL functions called with platform invoke require a callback function in managed code to run properly.
Generally speaking a
callback is executable code that is passed as an argument to other code
See in the following function's signature:
BOOL EnumWindows(WNDENUMPROC lpEnumFunc, LPARAM lParam)
lp prefix comes from the long pointer term.Thus the
lpEnumFunc is a pointer to a
callback function.
The
EnumWindows function enumerates through all existing windows on your computer, calling the callback function to perform a task on each window. Thiw function belongs to the Win32 API (
user32.dll).
Also,in the following example:
EnumWindows unmanaged DLL function.
CallBack(int hwnd, int lparam) is the managed callback function
hwnd:handle to the window
lparam:application defined parameter
using System;
using System.Runtime.InteropServices;
public delegate bool CallBack(int hwnd, int lParam);
public class EnumReportApp
{
[DllImport("user32")]
public static extern int EnumWindows(CallBack x, int y);
public static void Main()
{
CallBack myCallBack = new CallBack(EnumReportApp.Report);
EnumWindows(myCallBack, 0);
}
public static bool Report(int hwnd, int lParam)
{
Console.Write("Window handle is ");
Console.WriteLine(hwnd);
//Callback functions generally return nonzero values to indicate success //and zero to indicate failure. This example explicitly sets the return v //alue to true to continue the enumeration.
return true;
}
}
Similar enumeration functions are: EnumFontFamilies, EnumPrinters
The usage of a callback is usually in asynchronous logic.
