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.
To call most DLL functions from managed code, you create a managed definition of the function and then call it. The process is straightforward.
Using a DLL function that requires a callback function has some additional steps. First, you must determine whether the function requires a callback by looking at the documentation for the function. Next, you have to create the callback function in your managed application. Finally, you call the DLL function, passing a pointer to the callback function as an argument.
The following illustration summarizes the callback function and implementation steps:
Callback functions are ideal for use in situations in which a task is performed repeatedly. Another common usage is with enumeration functions, such as EnumFontFamilies, EnumPrinters, and EnumWindows in the Windows API. The EnumWindows function enumerates through all existing windows on your computer, calling the callback function to perform a task on each window. For instructions and an example, see How to: Implement Callback Functions.