服务器代码：

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.IO;
using System.Linq;
using System.Net;
using System.Net.Sockets;
using System.Text;
using System.Threading;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace Missuin
{
public partial class Form2 : Form
{
public Form2()
{
InitializeComponent();
}

private void button1_Click(object sender, EventArgs e)
{
//当点击开始监听的时候 在服务器端创建一个负责监IP地址和端口号的Socket
Socket socket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.IP);
IPAddress ip = IPAddress.Any;//IPAddress.Parse(txtServer.Text);
//创建端口号对象
IPEndPoint point = new IPEndPoint(ip, Convert.ToInt32(txtPort.Text));
//监听
socket.Bind(point);
ShowMsg("监听成功");
socket.Listen(10);

Thread th = new Thread(Listen);
th.IsBackground = true;
th.Start(socket);
}

Socket socketSend;
/// <summary>
/// 被线程所执行的函数，只能传object参数
/// </summary>
/// <param name="o"></param>
void Listen(Object o)
{
Socket socketWatch = o as Socket;
//等待客户端连接 并创建一个负责通信的Sokcet
while (true)
{
//负责根客户通信的Socket
socketSend = socketWatch.Accept();
dictSocket.Add(socketSend.RemoteEndPoint.ToString(), socketSend);

cbuUsers.Items.Add(socketSend.RemoteEndPoint.ToString());
ShowMsg(socketSend.RemoteEndPoint.ToString() + ":" + "连接成功");
////客户端连接成功后，服务器应该接受客户端发来的消息
//byte[] buffer = new byte[1024 * 1024 * 2];
////实际接受到的有效字节数
//int r = socketSend.Receive(buffer);
//string str = Encoding.UTF8.GetString(buffer, 0, r);
//Console.WriteLine(socketSend.RemoteEndPoint+":"+str);
Thread th = new Thread(Recive);
th.IsBackground = true;
th.Start(socketSend);
}
}

Dictionary<String, Socket> dictSocket = new Dictionary<string, Socket>();
/// <summary>
/// 服务器端不修改的接收客户发来的消息
/// </summary>
/// <param name="o"></param>
void Recive(object o)
{
Socket socketSend = o as Socket;
while (true)
{
try
{
//客户端连接成功后，服务器应该接受客户端发来的消息
byte[] buffer = new byte[1024 * 1024 * 2];
//实际接受到的有效字节数
int r = socketSend.Receive(buffer);
if (r == 0)
break;
string str = Encoding.UTF8.GetString(buffer, 0, r);
ShowMsg(socketSend.RemoteEndPoint + ":" + str);
}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}
}

void ShowMsg(string msg)
{
txtLog.AppendText(msg + "\r\n");
}

private void Form2_Load(object sender, EventArgs e)
{
Control.CheckForIllegalCrossThreadCalls = false;
}
/// <summary>
/// 服务器给客户端发消息
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void button5_Click(object sender, EventArgs e)
{
byte[] msgs = Encoding.UTF8.GetBytes(this.txtmsg.Text);

List<byte> list = new List<byte>();
list.Add(0);
list.AddRange(msgs);
byte[] newmsgs = list.ToArray();
//获得用户在下拉框中选中的IP地址
string ip = cbuUsers.SelectedItem.ToString();
Socket sendSocket = dictSocket[ip];
sendSocket.Send(newmsgs);
ShowMsg(sendSocket.RemoteEndPoint + ":" + this.txtmsg.Text);

}
/// <summary>
/// 选择要发送的文件
/// </summary>
/// <param name="sender"></param>
/// <param name="e"></param>
private void btnSelect_Click(object sender, EventArgs e)
{
OpenFileDialo