Asynchronous communication between C++ and C# RRS feed

  • Question

  • I have a console application in C++ that when loaded load a library in C#, which, in turn, when loaded shows a modal form with several buttons.

    I need to implement a call back function from C# to C++ to indicate, in the C++ console output, every time an action is made in the modal form.

    I have treated the following code C++:

    #include "pch.h"
    #include <iostream>
    #include <Windows.h>
    #include <string>
    #include <tchar.h>
    #import ".\CSharpDll.tlb" raw_interfaces_only
    int wmain(int argc, char* argv[]) {
    	CoInitialize(0); // Init COM
    	BSTR thing_to_send = ::SysAllocString(L"10 20");
    	BSTR returned_thing;
    	CSharpDll::_TheClassPtr obj(__uuidof(CSharpDll::TheClass));
    	HRESULT hResult = obj->GetTheThing(thing_to_send, &returned_thing);
    	if (hResult == S_OK) {
    		std::wcout << returned_thing << std::endl;
    		return 0;
    	return 1;
    //Call back function to test
    typedef void(*CallbackFunction)(const char*);
    void setCallback(CallbackFunction delegate)
    	std::cout << &delegate << std::endl;

    La aplicacion anterior se comunica con la libreria en C# con el siguiente codigo C#:

    using System;
    using System.Runtime.InteropServices;
    using System.Windows.Forms;
    using System.Runtime.CompilerServices;
    using System.Threading.Tasks;
    using System.IO.Pipes;
    using System.IO;
    using System.Text;
    namespace BimObjectCSharpClass
        public class SendData
            public delegate void CallbackDelegate(string message);
            public static extern void SetCallback(IntPtr aCallback);
            private CallbackDelegate del;
            public void TestCallbacks()
                System.Console.Write("Registering C# callback...\n");
                del = new CallbackDelegate(Callback01);
                System.Console.Write("Calling passed C++ callback...\n");
            public void Callback01(string message)
                System.Console.Write("callback 01 called. Message: " + message + "\n");
        [ComVisible(true)] // Don't forget 
        [ClassInterface(ClassInterfaceType.AutoDual)] // these two lines
        [Guid("485B98AF-53D4-4148-B2BD-CC3920BF0ADF")] // or this GUID
        public class TheClass
            int Num1;
            int Num2;
            public String GetTheThing(String arg) // Make sure this is public
                string s = arg;
                string[] words = s.Split(' ');
                Num1 = Int32.Parse(words[0]);
                Num2 = Int32.Parse(words[1]);
                ShowForm(Num1, Num2);
                int retVal = 0;
                return retVal.ToString();// arg + "the thing";
            static void ShowForm(int a, int b)
                Form Form4 = new Form1(a, b);

    When the C ++ application is executed, the form is not displayed and the application ends without an answer after a few seconds (15 mm).

    If the section indicated as //Return call function statement is commented (on C# side), the application is executed and the form is displayed. This indicates that something is not declared as it should be on the C# side.

    For me it is totally new to make C++ converge with C# during a modal function like that of the form.

    I need to know how to correct the statement so that the callback function is recognized.

    • Edited by Fisad Saturday, November 17, 2018 4:10 PM
    Friday, November 16, 2018 1:44 PM

All replies

  • In the code you have posted, the C# is calling SetCallback, but the C++ function is setCallback.  Those are two different things.

    Tim Roberts | Driver MVP Emeritus | Providenza &amp; Boekelheide, Inc.

    Friday, November 16, 2018 10:42 PM
  • in this case it is irrelevant since the C # code is never loaded, something in the position of the statement is bad
    Saturday, November 17, 2018 9:08 AM
  • If the form is shown when you comment some lines out, you may get a not shown exception if the lines are not commented. Can you add eg a messagebox to show exceptions?
    Saturday, November 17, 2018 11:13 AM
  • I have corrected the code but when I run the callback function it gives me an ECall security error
    Saturday, November 17, 2018 4:08 PM
  • Hi Fisad,

    Could you please share the detailed error message, and complete sample code and steps to reproduce the issue.

    Best regards,


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    Monday, November 19, 2018 7:04 AM