MC++ TCP server talking to MFC & MC++ TCP clients

• MC++ TCP server - 5 Kb
• MFC TCP client - 21 Kb
• MC++ TCP client - 4 Kb

Introduction

The original article proposed to demonstrate elementary network programming by designing a very simple SMTP server class. It had no support for sending attachments and was quite uncomplicated in nature. Since there are much better classes available within the BCL for sending emails, the exercise seemed to be quite futile. Therefore I have updated the article so that it provides a decent introduction to network programming. There are already several introductions to network programming on the web and I wanted this to be slightly different from all of them. Whether I was successful in that endeavor is up to the end-readers to judge.

Basically with the advent of .NET, remoting and web services are the preferred means of network communication. Both mechanisms are object oriented and allows the programmer to work from a high level of abstraction. But there are often situations, where you might want to communicate with native programs that were written prior to .NET, or for some reason you cannot expect all clients to be running .NET enabled programs. That's where we can use the slightly lower level network classes for our network-based programs. There is a Socket class available but I prefer to use the TcpListener and TcpClient classes which offer a higher level of abstraction.

In this article we'll design a multi-threaded TCP server using Managed C++. This server is very simple in functionality. It accepts a line of text and returns the reversed version of the same. We'll write an MFC dialog based application that will use the MFC CSocket class to chat with our Managed C++ TCP server. Since we have used basic TCP protocol, it does not matter what language the client program is written in. Just to complete the wheel, I also include a Managed C++ version of the TCP client, which also serves to demonstrate the TcpClient class.

TCP server - MC++

We write two classes here - SpookServer which basically starts our TCP server and listens for connections, and SpookClient which is a class for handling individual client connections. The SpookServer class itself spawns off a new thread in which it waits for connections. And for each connection, it instantiates a SpookClient object which immediately spawns off a new thread to handle the particular client connection. This system of one thread per client is quite acceptable in our case because each TCP chat involves so little process flow. It's just a matter of accepting a line of text, reversing it and sending it back.

The SpookServer class

__gc class SpookServer
{
private:
    TcpListener* lis;
    Thread* t;
    ManualResetEvent* mre;
public:
    SpookServer(int port)
    {   
        mre = new ManualResetEvent(true);
        lis = new TcpListener(port);
        t = new Thread(new ThreadStart(this,&SpookServer::ServerThread));
        t->Start();     
    }
    void CloseServer()
    {
        mre->Reset();
        lis->Stop();
    }
private:
    void ServerThread()
    {
        lis->Start();
        while(mre->WaitOne(100,false))
        {
            try
            {
                TcpClient* c = lis->AcceptTcpClient();
                SpookClient* sc = new SpookClient(c);
            }
            catch(Exception* e)
            {               
            }
                    
        }
    }
};

Well, the class is rather simple, but I'll still give a quick run-through of how it works. We use a ManualResetEvent object for synchronization. This is basically so that the calling class or method has a means of stopping the server anytime, in our case this is _tmain. As you can see, we spawn off a new thread where we listen for connections. This is so that we can return control to the calling class. We use the AcceptTcpClient method of the TcpListener class which returns a TcpClient object that is a reference to the connected client. Immediately we instantiate a new SpookClient object passing the returned TcpClient object to it's constructor. And we go back to AcceptTcpClient, thus allowing a seemingly innumerable number of clients to be able to connect at any one time.

The SpookClient class

__gc class SpookClient
{
private:
    TcpClient* m_c;
    Thread* t;
public:
    SpookClient(TcpClient* c)
    {
        m_c = c;
        t = new Thread(new ThreadStart(this,&SpookClient::StartChat));
        t->IsBackground = true;
        t->Start();
    }
private:
    void StartChat()
    {
        NetworkStream* ns = m_c->GetStream();
        unsigned char buffer __gc[] = new unsigned char __gc[128];
        String* str = "";
        while(true)
        {
            int d = ns->Read(buffer,0,127);
            str = String::Concat(str,Encoding::ASCII->GetString(buffer,0,d));
            if(str->LastIndexOf('\n') != -1)
                break;
        }

        str = str->Trim();
        str = String::Concat("\n",str);     
        buffer = Encoding::ASCII->GetBytes(str);
        Array::Reverse(buffer);
        ns->Write(buffer,0,buffer->Length);     
        ns->Flush();
        ns->Close();
        m_c->Close();
    }
};

The SpookClient class has a TcpClient member and we assign the TcpClient object passed to the constructor to this member object. And we start off a new thread. Inside the thread, we first call TcpClient.GetStream to get the underlying NetworkStream object. Once we have the NetworkStream object we can use Write and Read on it to send and receive data, respectively. We simply accept a line of text, by looping till a \n is encountered and then we reverse it and send it back. Remember to Flush and Close the NetworkStream object and then call Close on the TcpClient object as well.

TCP client - MFC

void CTcpClientMFCDlg::OnBnClickedButton1()
{
    UpdateData(TRUE);
    m_text += "\n";
    CSocket s;
    s.Create();
    if(s.Connect("127.0.0.1",20248))
    {
        s.Send(m_text,m_text.GetLength());
                
        m_text = "";
        char buff[17];
        
        int i;
        
        while(m_text.Find('\n') == -1)
        {
            i=s.Receive(buff,16);
            buff[i]=0;
            m_text += buff;         
        }
        
        m_text.Trim();      
        
        UpdateData(false);      
    }
    s.Close();
}

Well, we don't do anything spectacular here. We simply send the text using CSocket::Send() and receive back the reversed line of text using CSocket::Receive. You'd think that with using network streams and all, the data would have been slightly personalized for .NET purposes, but apparently the raw data is preserved. This indicates that you can use the TcpListener and TcpClient classes for any kind of Tcp programming including popular protocols like HTTP, SMTP, POP3 etc.

TCP client - MC++

__gc class SpookClient
{
public:
    String* Reverse(String* s)
    {
        TcpClient* tl = new TcpClient("127.0.0.1",20248);
        NetworkStream* nw = tl->GetStream();
        unsigned char buffer __gc[] = new unsigned char __gc[128];
        buffer = Encoding::ASCII->GetBytes(s);
        nw->Write(buffer,0,buffer->Length);
        unsigned char retbuffer __gc[] = new unsigned char __gc[128];
        String* str = "";
        while(true)
        {
            int d = nw->Read(retbuffer,0,127);
            str = String::Concat(str,Encoding::ASCII->GetString(retbuffer,0,d));
            if(str->LastIndexOf('\n') != -1)
                break;
        }
        nw->Close();
        tl->Close();
        return str;
    }
};

I have written another class here called SpookClient, not to be confused with the other class in the server program that has the same name. This uses a TcpClient object to connect to the server, gets the underlying NetworkStream using GetStream and uses Write and Read for sending and receiving data, to and from the server. This code snippet would look very similar to our client handler class in the server program because they both demonstrate the use of the NetworkStream class.