Handle Managed(C#) Event in Managed(C++/CLI)

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Introduction

While working with one project, we had a requiremnet in which an event had to be raised from managed application and had to handle that event in managed C++ application. We read many articles and blogs but did not get a proper example. After spending two days, we managed to handle the event in managed C++. So, I added this tip which might help others.

Implementing a Library for Event in C#

As a first step, implement managed library that raises an event.

Create a new C# Class Library project from Visual Studio and add the below code in its class file.

using System;

namespace CSharpWithEvent
{
     /// <summary>
    /// Packet that will be used as part of argument in event.
    /// </summary>
    public class UpdateInfo
    {
        public string PropertyName { get; set; }
        public int NewValue { get; set; }
    }
    /// <summary>
    /// Class derived from EventArgs that is used by our event.
    /// </summary>
    public class ValueChangeEventArgs : EventArgs
    {
        private UpdateInfo _updatePacket;

        public UpdateInfo UpdatePacket
        {
            get { return _updatePacket; }
            set { _updatePacket = value; }
        }
        public ValueChangeEventArgs(UpdateInfo updatePacket)
        {
            _updatePacket = updatePacket;
        }
    }
    /// <summary>
    /// Main class that manages events and objects.
    /// </summary>
    public class ImageManager
    {
        private EventHandler<valuechangeeventargs> _onValueChanged;
        public event EventHandler<valuechangeeventargs> OnValueChanged
        {
            add { _onValueChanged += value; }
            remove { _onValueChanged -= value; }
        }

        /// <summary>
        /// Method that is called to update Image.
        /// <summary>
        public void UpdateImage(UpdateInfo updatePacket)
        {
            /*Raised Event*/
            if (_onValueChanged != null)
            {
                _onValueChanged(this, new ValueChangeEventArgs(updatePacket));
            }
        }
        /// <summary>
        /// A method to raise an event from user code itself.
        /// <summary>
        public void RaiseUpdateEvent()
        {
            UpdateInfo updatePacket = new UpdateInfo();
            UpdateImage(updatePacket);
        }
    }
}

Implementing Client/User Application in Managed C++

This is the C++ project that we need to handle event that will be raised from the above C# project.

1. Create a new C++/CLI CLR Console Application.
2. To handle an event in native class, we need to write managed wrapper/helper class that will intercept the event and redirect to native object, so here is the simple helper class written in managed C++.
   C++

   //Helper.h
   
   #include "stdafx.h"
   #include <vcclr.h>
   #include "ImageUserClass.h"
   using namespace System;
   
   ref class Helper
   {
   public:
   	Helper();
       ImageUserClass* mPtr;
       void OnImageValueChange( Object^ sender, ValueChangeEventArgs^ updatePacket );
   };
   
   //Helper.cpp
   
   #include "stdafx.h";
   #include "Helper.h"
   #include "ImageUserClass.h"
   
   Helper::Helper()
   {
   	
   }
   void Helper::OnImageValueChange( Object^ sender, ValueChangeEventArgs^ updatePacket )
   {
       mPtr->OnImageValueChange( sender, updatePacket );
   }

3. At last, we only need a native C++ class which implements event handler. Below is the code for the same.
   C++

   //ImageUserClass.h
   #pragma once
   #include "stdafx.h"
   #include <vcclr.h>
   using namespace std;
   using namespace System;
   using namespace CSharpWithEvent;
   
   class ImageUserClass
   {
   public:
   	gcroot<imagemanager^> m_imageManager;
   	ImageUserClass();
   	void OnImageValueChange(Object^ sender, ValueChangeEventArgs^ updatePacket);
   };
   
   //ImageUserClass.cpp
   
   #include "stdafx.h";
   #include "Helper.h";
   using namespace std;
   using namespace System;
   ImageUserClass::ImageUserClass()
   {
   	Helper^ h = gcnew Helper;
   	h->mPtr = this;
   	m_imageManager = gcnew ImageManager();
   	m_imageManager->OnValueChanged += gcnew EventHandler<valuechangeeventargs^>
                                                (h, &Helper::OnImageValueChange );
   	<valuechangeeventargs^>m_imageManager->RaiseUpdateEvent();
   }
   void ImageUserClass::OnImageValueChange(Object^ sender, ValueChangeEventArgs^ updatePacket)
   {
   	Console::WriteLine("Update packet received.");
   }