C#

   private void Send_Click(object sender, EventArgs e)
        {
           
            Callback();
        }

        private void Callback()
        {
            try
            {
                byte[] data = new byte[1024];
                listBox3.Items.Add("Connecting....");
                IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8000);
                Socket newsock= new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
                newsock.Bind(ipep);

                listBox3.Items.Add("Connected");                
               
                data = Encoding.ASCII.GetBytes("hello");
                listBox3.Items.Add("Transmitting...");
                newsock.SendTo(data, data.Length,SocketFlags.None,ipep);//sent hello              
           
                listBox3.Items.Add("Sent...");

                IPEndPoint sender = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8000);
                EndPoint tmpRemote = (EndPoint)sender;

                listBox3.Items.Add("Message received from {0}:");
                listBox3.Items.Add(sender.ToString());

               // newsock.Bind(tmpRemote);
                byte[] data1 = new byte[1024];
                int recv = newsock.ReceiveFrom(data1,ref tmpRemote);
                listBox3.Items.Add(Encoding.ASCII.GetString(data1, 0, recv));
                               
                //listBox3.Items.Add("Stopping client");
                //server.Close();            
               
            }
            catch (Exception e)
            {
                //Console.WriteLine("Error..... " + e.StackTrace);
                MessageBox.Show(e.StackTrace);
               
            }
         
        }
   
    }
       
}

namespace TestServer
{
    public class UdpServer
    {
       
        //int recv;
        public static void Main(string[] args)
        {

            UdpServer us = new UdpServer();
        }
        public UdpServer()
        {
            Thread startServer = new Thread(new ThreadStart(start_server));
            //Start the Thread
            startServer.Start();
            Console.WriteLine("Waiting for a client...");
        }
        public static void start_server()
        {
            byte[] data = new byte[1024];
            IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8000);
            Socket newsock = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            newsock.Bind(ipep);
                       
            while (true)
            {
                IPEndPoint sender = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 8000);
                EndPoint tmpRemote = (EndPoint)(sender);
               
                int recv = newsock.ReceiveFrom(data, ref tmpRemote);
                Console.WriteLine("Message received from {0}:", tmpRemote.ToString());
                Console.WriteLine(Encoding.ASCII.GetString(data, 0, recv));//hello is received                    

                byte[] data1 = new byte[1024];
                string ss = "Welcome to the Server";
                data1 = Encoding.ASCII.GetBytes(ss);
                newsock.SendTo(data1, data1.Length, SocketFlags.None, tmpRemote);

                Console.WriteLine("\nSent Acknowledgement");
                // startServer.Start();
            }

        }
    }
}