I want to send data from server to client using udp

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;

namespace Server
{
    //The commands for interaction between the server and the client
    enum Command
    {
        Login,      //Log into the server
        Logout,     //Logout of the server
        Message,    //Send a text message to all the chat clients
        List,       //Get a list of users in the chat room from the server
        Null        //No command
    }
    public partial class SGSserverForm : Form
    {
        //The main socket on which the server listens to the clients
        Socket serverSocket;
        public EndPoint epSender;
        public string strName = "Raj";

        byte[] byteData = new byte[1024];
        public SGSserverForm()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            try
            {
                CheckForIllegalCrossThreadCalls = false;

                //We are using UDP sockets
                serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
                IPEndPoint ipeSender = new IPEndPoint(IPAddress.Any, 1000);
                //Bind this address to the server
                serverSocket.Bind(ipeSender);
                //The epSender identifies the incoming clients
                EndPoint epSender = (EndPoint)ipeSender;

                //Start receiving data
                serverSocket.BeginReceiveFrom(byteData, 0, byteData.Length, SocketFlags.None, ref epSender, new AsyncCallback(OnReceive), epSender);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "SGSServerUDP", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void OnReceive(IAsyncResult ar)
        {
            try
            {
                IPEndPoint ipeSender = new IPEndPoint(IPAddress.Any, 1000);
                EndPoint epSender = (EndPoint)ipeSender;
                serverSocket.EndReceiveFrom(ar, ref epSender);

                Data msgReceived = new Data(byteData);

                //We will send this object in response the users request
                Data msgToSend = new Data();
                byte[] message;

                msgToSend.cmdCommand = msgReceived.cmdCommand;
                msgToSend.strName = msgReceived.strName;
                msgToSend.strMessage = msgReceived.strName + ":" + msgReceived.strMessage;

                message = msgToSend.ToByte();
                serverSocket.BeginSendTo(message, 0, message.Length, SocketFlags.None, epSender, new AsyncCallback(OnSend), epSender);
                txtLog.Text += msgToSend.strMessage + "\r\n";
                serverSocket.BeginReceiveFrom(byteData, 0, byteData.Length, SocketFlags.None, ref epSender, new AsyncCallback(OnReceive), epSender);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "SGSServerUDP", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        public void OnSend(IAsyncResult ar)
        {
            try
            {
                serverSocket.EndSend(ar);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "SGSServerUDP", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void textBox1_TextChanged(object sender, EventArgs e)
        {
            try
            {
                //Fill the info for the message to be send
                Data msgToSend = new Data();

                // msgToSend.strName = strName;
                msgToSend.strMessage = textBox1.Text;
                msgToSend.cmdCommand = Command.Message;

                byte[] byteData = msgToSend.ToByte();

                //Send it to the server
                serverSocket.BeginSendTo(byteData, 0, byteData.Length, SocketFlags.None, epSender, new AsyncCallback(OnSend), null);

                //txtMessage.Text = null;
            }
            catch (Exception)
            {
                MessageBox.Show("Unable to send message to the server.", "SGSclientUDP: ", MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void textBox1_Enter(object sender, EventArgs e)
        { }
    }
    //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);

            //This check 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 result = new List();

            //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();
        }
        public string strName;      //Name by which the client logs into the room
        public string strMessage;   //Message text
        public Command cmdCommand;  //Command type (login, logout, send message, etcetera)
    }
}

And this is my client code :

using System;
using System.Collections.Generic;
using System.Text;
using System.Windows.Forms;
using System.Net;
using System.Net.Sockets;

namespace SGSclient
{
    //The commands for interaction between the server and th
    enum Command
    {
        Login,      //Log into the server
        Logout,     //Logout of the server
        Message,    //Send a text message to all the chat clients
        List,       //Get a list of users in the chat room from the server
        Null        //No command
    }
    public partial class SGSClient : Form
    {
        public Socket clientSocket; //The main client socket
        public string strName = "Naveen";      //Name by which the user logs into the room
        public EndPoint epServer;   //The EndPoint of the server

        byte[] byteData = new byte[1024];

        public SGSClient()
        {
            InitializeComponent();
        }
        private void Form1_Load(object sender, EventArgs e)
        {
            CheckForIllegalCrossThreadCalls = false;

            clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);       //IP address of the server machine
            IPAddress ipAddress = IPAddress.Parse("127.0.0.1");            //Server is listening on port 1000
            IPEndPoint ipEndPoint = new IPEndPoint(ipAddress, 1000);
            epServer = (EndPoint)ipEndPoint;

            this.Text = "SGSclient: " + strName;

            //The user has logged into the system so we now request the server to send
            Data msgToSend = new Data();
            msgToSend.cmdCommand = Command.List;
            msgToSend.strName = strName;
            msgToSend.strMessage = null;

            byteData = msgToSend.ToByte();

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

            byteData = new byte[1024];
            //Start listening to the data asynchronously
            clientSocket.BeginReceiveFrom(byteData, 0, byteData.Length, SocketFlags.None, ref epServer, new AsyncCallback(OnReceive), null);
        }
        private void OnSend(IAsyncResult ar)
        {
            try
            {
                clientSocket.EndSend(ar);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "SGSclient: " + strName, MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void OnReceive(IAsyncResult ar)
        {
            try
            {
                clientSocket.EndReceive(ar);

                //Convert the bytes received into an object of type Data
                Data msgReceived = new Data(byteData);

                string data = Encoding.UTF8.GetString(byteData);

                txtChatBox.Text += data.ToString() + "\r\n";

                //Accordingly process the message received

                switch (msgReceived.cmdCommand)
                {
                    case Command.Message:
                        break;

                    case Command.List:
                        txtChatBox.Text += "<<<" + strName + " has joined the room>>>\r\n";
                        break;
                }
                if (msgReceived.strMessage != null)
                    txtChatBox.Text += msgReceived.strMessage + "\r\n";

                byteData = new byte[1024];

                //Start listening to receive more data from the user
                clientSocket.BeginReceiveFrom(byteData, 0, byteData.Length, SocketFlags.None, ref epServer, new AsyncCallback(OnReceive), null);
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message, "SGSclient: " + strName, MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
        private void txtMessage_TextChanged(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;

                byte[] byteData = msgToSend.ToByte();

                byte[] bytes = Encoding.ASCII.GetBytes(txtMessage.Text);
                //Send it to the server
                clientSocket.BeginSendTo(byteData, 0, byteData.Length, SocketFlags.None, epServer, new AsyncCallback(OnSend), null);
            }
            catch (Exception)
            {
                MessageBox.Show("Unable to send message to the server.", "SGSclientUDP: " + strName, MessageBoxButtons.OK, MessageBoxIcon.Error);
            }
        }
    }
    //The data structure by which the server and the client interact.
    class Data
    {
        //Default constructor
        public Data()
        {
            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);

            //This check 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 result = new List();

            //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();
        }
        public string strName;      //Name by which the client logs into the room
        public string strMessage;   //Message text
        public Command cmdCommand;  //Command type (login, logout, send message, etcetera)
    }
}

Now i am able to send data from client to server but when i want to send data from server to client it is catching exception.why??

Thanks!