C#

void RetrieveData()
{
    Stream bookStream = new FileStream("books.txt", FileMode.OpenOrCreate);

    using(StreamReader booksReader = new StreamReader(bookStream))
    {
        //Do what you want with booksReader here, the using block will automatically close the streams to avoid memory leaks
    }
}

ListBox.Items.Add()

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;
public class UdpServer
{
    public static void Main()
    {
        try
        {
        int recv;
        byte[] data = new byte[1024];
        IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
        Socket newsock = new Socket(AddressFamily.InterNetwork,SocketType.Dgram, ProtocolType.Udp);
        newsock.Bind(ipep);
        Console.WriteLine("Waiting for a client...");
        IPEndPoint sender = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
        EndPoint tmpRemote = (EndPoint)(sender);
        recv = newsock.ReceiveFrom(data, ref tmpRemote);
        Console.WriteLine("Message received from {0}:", tmpRemote.ToString());
        Console.WriteLine(Encoding.ASCII.GetString(data, 0, recv));
        string wel = "Welcome to my test server";
        data = Encoding.ASCII.GetBytes(wel);        
        newsock.SendTo(data, data.Length, SocketFlags.None, tmpRemote);
         listBox1.Items.Add(data);
        ASCIIEncoding asen = new ASCIIEncoding();
        newsock.Send(asen.GetBytes("The string was recieved by the server."));
        Console.WriteLine("\nSent Acknowledgement");
            listBox1.Items.Add("The string was recieved by the server.");         
        newsock.Close();
       }
            catch(Exception e)
           {
                   Console.WriteLine("Error..... " + e.StackTrace);
            }}}

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text; 
using System.Net;
using System.Net.Sockets;
using System.Windows.Forms;
namespace WindowsApplication13
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
        }

        private void button1_Click(object sender, EventArgs e)
        {
            Callback();
        }
    
        private void Callback()
        {
            try
            {                
                byte[] data = new byte[1024];
                listBox1.Items.Add("Connecting....");
                IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
 Socket server = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
                listBox1.Items.Add("Connected");
                data = Encoding.ASCII.GetBytes("welcome");
                listBox1.Items.Add("Transmitting...");
                server.SendTo(data, data.Length, SocketFlags.None, ipep);
             IPEndPoint sender = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
                EndPoint tmpRemote = (EndPoint)sender;
                // data = new byte[1024];
                int recv = server.ReceiveFrom(data, ref tmpRemote);
      //Console.WriteLine("Message received from {0}:", tmpRemote.ToString());
     listBox1.Items.Add(/*"Message received from {0}:",*/ tmpRemote.ToString());
                //Console.WriteLine(Encoding.ASCII.GetString(data, 0, recv));
                listBox1.Items.Add(Encoding.ASCII.GetString(data, 0, recv));
           listBox1.Items.Add("Stopping client");
                server.Close();
            }
            catch (Exception e)
            {
                Console.WriteLine("Error..... " + e.StackTrace);
            }}}}

listBox1.Items.Add("Connecting....");
listBox1.Items.Add("Connected");
listBox1.Items.Add("Transmitting...");

using System;
using System.Net;
using System.Net.Sockets;
using System.Text;

public class UdpServer
{
    public static void Main()
    {
        try
        {
        int recv;
        byte[] data = new byte[1024];
        IPEndPoint ipep = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);

        Socket newsock = new Socket(AddressFamily.InterNetwork,SocketType.Dgram, ProtocolType.Udp);

        newsock.Bind(ipep);
        Console.WriteLine("Waiting for a client...");

        IPEndPoint sender = new IPEndPoint(IPAddress.Parse("127.0.0.1"), 9050);
        EndPoint tmpRemote = (EndPoint)(sender);

        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
 
        if (Encoding.ASCII.GetString(data)=="hello")
        {
            data = new byte[1024];
            string ss = "Welcome to the Server";
            data = Encoding.ASCII.GetBytes(ss);
            newsock.SendTo(data, data.Length, SocketFlags.None, tmpRemote);
        }

        Console.WriteLine("\nSent Acknowledgement");
        newsock.Close();
    }  
       
            catch(Exception e)
           {
                   Console.WriteLine("Error..... " + e.StackTrace);
            }
    }
}