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.NET: Use Callback Function with Windows API

A delegate is a type that safely encapsulates a method, similar to a function pointer in C/C++. However, delegates are type-safe and secure. They are used to pass methods as arguments to other methods, handle or raise events, and call multiple methods (multicasting).

using System;
using System.Runtime.InteropServices;

namespace Callback
{
	internal class Program
	{
		// NOTE: this code will attempt to prevent the Ctrl+C key combination from terminating the console application
		[DllImport("Kernel32.dll", CallingConvention = CallingConvention.StdCall)]
		private static extern int SetConsoleCtrlHandler(ConsoleCtrlDelegate HandlerRoutine, int Add);

		// NOTE: define the delegate
		delegate int ConsoleCtrlDelegate(CtrlTypes CtrlType);

		enum CtrlTypes : uint
		{
			CTRL_C_EVENT = 0,
			CTRL_BREAK_EVENT = 1,
			CTRL_CLOSE_EVENT = 2,
			CTRL_LOGOFF_EVENT = 5,
			CTRL_SHUTDOWN_EVENT = 6
		}

		// NOTE: the callback function
		static int ConsoleCtrlCheck(CtrlTypes ctrlType)
		{
			// Handle the CTRL-CLOSE event
			if (ctrlType == CtrlTypes.CTRL_CLOSE_EVENT || ctrlType == CtrlTypes.CTRL_C_EVENT)
			{
				return 1; // Return true to indicate that the event is handled
			}
			return 0; // Other events can proceed as normal
		}

		static void Main(string[] args)
		{
			int add = 1;
			// NOTE: set callback function
			int result = SetConsoleCtrlHandler(new ConsoleCtrlDelegate(ConsoleCtrlCheck), add);

		}
	}
}

Here is an example of calling a function from a custom DLL that calls a callback function in the application.

using System;
using System.Runtime.InteropServices;

namespace Callback
{
	internal class Program
	{
		[DllImport("CallbackDll.dll", CallingConvention = CallingConvention.StdCall)]
		private static extern void MessageIt(MessageItDelegate routine);

		// NOTE: define the delegate
		delegate string MessageItDelegate();

		static string MessageItCallback()
		{
			return "HEY, MAN!"; // NOTE: a Message Box will pop up with this message
		}

		static void Main(string[] args)
		{
			MessageIt(new MessageItDelegate(MessageItCallback));
		}
	}
}

This is the message that pops up.

Here is the C/C++ code for the above DLL.

// dllmain.cpp : Defines the entry point for the DLL application.
#include <windows.h>

BOOL APIENTRY DllMain( HMODULE hModule,
                       DWORD  ul_reason_for_call,
                       LPVOID lpReserved
                     )
{
    switch (ul_reason_for_call)
    {
    case DLL_PROCESS_ATTACH:
    case DLL_THREAD_ATTACH:
    case DLL_THREAD_DETACH:
    case DLL_PROCESS_DETACH:
        break;
    }
    return TRUE;
}

extern "C" // NOTE: must use extern "C" because the C++ compiler will mangle the function name otherwise
{
    void __stdcall MessageIt(char* (__stdcall* callback)())
    {
        MessageBoxA(NULL, callback(), "Hello, World", MB_OK);
    }
}

Here is the module definition file for the custom DLL.

LIBRARY CallbackDll.dll
EXPORTS
	MessageIt @1

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