A Chat Application Using Asynchronous TCP Sockets

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Introduction

In this article, I will discuss a chat application using asynchronous TCP sockets in C#. In the next part of this article, I will present an asynchronous UDP socket based chat application.

TCP Asynchronous Sockets

TCP asynchronous sockets have a Begin and End appended to the standard socket functions, like BeginConnect, BeginAccept, BeginSend, and BeginReceive. Let's take a look at one of them:

IAsyncResult BeginAccept(AsyncCallback callback, object state);

The AsyncCallback function is called when the function completes. Just as events can trigger delegates, .NET also provides a way for methods to trigger delegates. The .NET AsyncCallback class allows methods to start an asynchronous function and supply a delegate method to call when the asynchronous function completes.

The state object is used to pass information between the BeginAccept and the corresponding AsyncCallback function.

The following shows an implementation of it:

Socket sock = new Socket(AddressFamily.InterNetwork, SocketType.Stream,
                         ProtocolType.Tcp);
IPEndPoint iep = new IPEndPoint(IPAddress.Any, 9050);
sock.Bind (iep);
sock.Listen (5);
sock.BeginAccept (new AsyncCallback(CallAccept), sock);

private void CallAccept(IAsyncResult iar)
{
    Socket server = (Socket)iar.AsyncState;
    Socket client = server.EndAccept(iar);
}

The original sock object is retrieved in CallAccept by using the AsyncState property of IAsyncResult. Once we have the original socket (on which the operation completed), we can get the client socket and perform operations on it.

All other asynchronous socket functions work like this, you should look on MSDN for further details. There are a few nice articles explaining the details on the net, so kindly search for them as well.

Getting Started

For exchange of messages between the client and the server, both of them use the following trivial commands:

//The commands for interaction between the server and the client

enum Command
{
    //Log into the server

    Login,      
    //Logout of the server

    Logout,
    //Send a text message to all the chat clients     

    Message,    
    //Get a list of users in the chat room from the server

    List
}

The data structure used to exchange data between the client and the server is shown below. Sockets transmit and receive data as an array of bytes; the overloaded constructor and the ToByte member function performs this conversion.

//The data structure by which the server and the client interact with 

//each other

class Data
{
    //Default constructor

    public Data()
    {
        this.cmdCommand = Command.Null;
        this.strMessage = null;
        this.strName = null;
    }

    //Converts the bytes into an object of type Data

    public Data(byte[] data)
    {
        //The first four bytes are for the Command

        this.cmdCommand = (Command)BitConverter.ToInt32(data, 0);

        //The next four store the length of the name

        int nameLen = BitConverter.ToInt32(data, 4);

        //The next four store the length of the message

        int msgLen = BitConverter.ToInt32(data, 8);

        //Makes sure that strName has been passed in the array of bytes

        if (nameLen > 0)
            this.strName = Encoding.UTF8.GetString(data, 12, nameLen);
        else
            this.strName = null;

        //This checks for a null message field

        if (msgLen > 0)
            this.strMessage = Encoding.UTF8.GetString(data, 
                              12 + nameLen, msgLen);
        else
            this.strMessage = null;
    }

    //Converts the Data structure into an array of bytes

    public byte[] ToByte()
    {
        List<byte> result = new List<byte>();

        //First four are for the Command

        result.AddRange(BitConverter.GetBytes((int)cmdCommand));

        //Add the length of the name

        if (strName != null)
            result.AddRange(BitConverter.GetBytes(strName.Length));
        else
            result.AddRange(BitConverter.GetBytes(0));

        //Length of the message

        if (strMessage != null)
            result.AddRange(BitConverter.GetBytes(strMessage.Length));
        else
            result.AddRange(BitConverter.GetBytes(0));

        //Add the name

        if (strName != null)
            result.AddRange(Encoding.UTF8.GetBytes(strName));

        //And, lastly we add the message text to our array of bytes

        if (strMessage != null)
            result.AddRange(Encoding.UTF8.GetBytes(strMessage));

        return result.ToArray();
    }

    //Name by which the client logs into the room

    public string strName;
    //Message text

    public string strMessage;
    //Command type (login, logout, send message, etc)

    public Command cmdCommand;
}

TCP Server

The server application listens on a particular port and waits for the clients. The clients connect to the server and join the room. The client then sends a message to the server, the server then sends this to all the users in the room.

The server application has the following data members:

//The ClientInfo structure holds 

//the required information about every

//client connected to the server

struct ClientInfo
{
    //Socket of the client

    public Socket socket;
    //Name by which the user logged into the chat room

    public string strName;
}

//The collection of all clients logged 

//into the room (an array of type ClientInfo)

ArrayList clientList;

//The main socket on which the server listens to the clients

Socket serverSocket;

byte[] byteData = new byte[1024];

Here is how it starts listening for the clients and accepts the incoming requests:

private void Form1_Load(object sender, EventArgs e)
{
    try
    {
        //We are using TCP sockets

        serverSocket = new Socket(AddressFamily.InterNetwork, 
                                  SocketType.Stream, 
                                  ProtocolType.Tcp);
        
        //Assign the any IP of the machine and listen on port number 1000

        IPEndPoint ipEndPoint = new IPEndPoint(IPAddress.Any, 1000);

        //Bind and listen on the given address

        serverSocket.Bind(ipEndPoint);
        serverSocket.Listen(4);

        //Accept the incoming clients

        serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);
    }
    catch (Exception ex)
    { 
        MessageBox.Show(ex.Message, "SGSserverTCP", 
            MessageBoxButtons.OK, MessageBoxIcon.Error); 
    }
}

With IPAddress.Any, we specify that the server should accept client requests coming on any interface. To use any particular interface, we can use IPAddress.Parse (�192.168.1.1�) instead of IPAddress.Any. The Bind function then bounds the serverSocket to this IP address. The Listen function makes the server wait for the clients, the value passed to Listen specifies how many incoming client requests will be queued by the operating system. If there are pending client requests and another comes in, then it will be rejected.

Below is the corresponding OnAccept function:

private void OnAccept(IAsyncResult ar)
{
    try
    {
        Socket clientSocket = serverSocket.EndAccept(ar);

        //Start listening for more clients

        serverSocket.BeginAccept(new AsyncCallback(OnAccept), null);

        //Once the client connects then start 

        //receiving the commands from her

        clientSocket.BeginReceive(byteData, 0, 
            byteData.Length, SocketFlags.None, 
            new AsyncCallback(OnReceive), clientSocket);
    }
    catch (Exception ex)
    { 
        MessageBox.Show(ex.Message, "SGSserverTCP", 
            MessageBoxButtons.OK, MessageBoxIcon.Error); 
    }
}

EndAccept returns the clientSocket object of the connecting client. We then again start listening for more incoming client requests by making a call to serverSocket.BeginAccept; this is essential as the server won�t be able to process any other incoming client request without it.

With BeginReceive we start receiving the data that will be sent by the client. Note that we pass clientSocket as the last parameter of BeginReceive; the AsyncCallback OnReceive gets this object via the AsyncState property of IAsyncResult, and it then processes the client requests (login, logout, and send messages to the users). Please see the code attached to understand the implementation of OnReceive.

TCP Client

Some of the data members used by the client are:

//The main client socket

public Socket clientSocket;
//Name by which the user logs into the room

public string strName;

private byte[] byteData = new byte[1024];

The client firstly connects to the server:

try
{
    //We are using TCP sockets

    clientSocket = new Socket(AddressFamily.InterNetwork,
                   SocketType.Stream, ProtocolType.Tcp);

    IPAddress ipAddress = IPAddress.Parse(txtServerIP.Text);
    //Server is listening on port 1000

    IPEndPoint ipEndPoint = new IPEndPoint(ipAddress, 1000);

    //Connect to the server

    clientSocket.BeginConnect(ipEndPoint, 
        new AsyncCallback(OnConnect), null);
}
catch (Exception ex)
{ 
    MessageBox.Show(ex.Message, "SGSclient", 
                    MessageBoxButtons.OK, 
                    MessageBoxIcon.Error); 
}

Once connected, a login message is sent to the server. And after that, we send a list message to get the names of clients in the chat room.

//Broadcast the message typed by the user to everyone

private void btnSend_Click(object sender, EventArgs e)
{
    try
    {
        //Fill the info for the message to be send

        Data msgToSend = new Data();
        
        msgToSend.strName = strName;
        msgToSend.strMessage = txtMessage.Text;
        msgToSend.cmdCommand = Command.Message;

        byteData = msgToSend.ToByte();

        //Send it to the server

        clientSocket.BeginSend (byteData, 0, byteData.Length, 
            SocketFlags.None, 
            new AsyncCallback(OnSend), null);

        txtMessage.Text = null;
    }
    catch (Exception)
    {
        MessageBox.Show("Unable to send message to the server.", 
                        "SGSclientTCP: " + strName, 
                        MessageBoxButtons.OK, 
                        MessageBoxIcon.Error);
    }  
}

The message typed by the user is send as a command message to the server which then sends it to all other users in the chat room.

Upon receiving a message, the client processes it accordingly (depending on whether it�s a login, logout, command, or a list message). The code for this is fairly straightforward, kindly see the attached project.