Calling managed code from unmanaged code, VC++

One option might be to post a message to the C# application's window.

Alternatively you can have the C# application call an exported function in your C++ app, and provide a function pointer/delegate. Then, once the C# application has provided the C++ application a function pointer, the C++ application can just call the function pointer.

Since C# code is compiled at run-time, the location of a given function is not known in advance, therefore native C++ cannot "directly" execute code in a C# application (without the C# app initiating the process by providing a delegate). Of course, the exception to that is managed C++.Net - maybe a mixed-mode C++ module would work for you.

I've still got some example code handy from when I was tinkering with this myself...

Note that the C# application must run first, because the location of FnPtrProc() isn't known until the C# program starts (i.e. is compiled).  The C# program calls a function in the C++ DLL, CallbackTest(), and passes in a pointer (or delegate, in C# terms) to FnPtrProc(). The C++ code can then make a call to the C# function.

Apparently I was messing with this example a little bit, so I can't say for certain it'll run as-is - e.g. you might need to change which C# line is commented...

C# Code:

using System;
using System.Runtime.InteropServices;

class Program
{
	[DllImport("MyDll.dll")]
	private static extern void CallbackTest(string addr);
	
	delegate void FnPtr(string message, int number);

	static void Main(string[] args)
	{
		FnPtr proc = new FnPtr(FnPtrProc);

		CallbackTest(Marshal.GetFunctionPointerForDelegate(proc).ToString());
		//CallbackTest(Marshal.GetFunctionPointerForDelegate(proc).ToString());
		//CallbackTest(Marshal.GetFunctionPointerForDelegate(proc).ToString());
		Console.ReadKey();
	}

	static void FnPtrProc(string message, int number)
	{
		Console.WriteLine("Message: {0}", message);
		Console.WriteLine("Number: {0}", number);
	}
}

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C++ Code:

// MyDll.cpp : Defines the exported functions for the DLL application.
//

#include "stdafx.h"
#include "MyDll.h"

using namespace std;

typedef void(*CallbackPtr)(const char*,int);

extern "C" __declspec(dllexport)
void WINAPI CallbackTest(const char* callbackAddr)
{
	stringstream ss(callbackAddr);
	INT_PTR pFn;
	ss >> pFn;

	CallbackPtr fn;
	fn = (CallbackPtr)pFn;

	for (int i = 0; i< 3; i++)
		fn("Hello World", i);
}

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Thanks tgerbert!

The legacy application has to provide data/parameters to C# application which is internally calling webservice and get back the results. So do you think if the C# application calling the exported function in C++ will work?or rather this has to be C++ calling C# application, please let me know!

Honestly, I'm a little curious why I used a char* and stringstream instead of a void*?

...because I was working on a question where the asker specifically wanted to pass the function pointer value as a numeric string (just remembered). ;)