Socket实现简单的指定客户端发送信息（效果如下图） 

不多说了，直接上代码：

server端：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;
using System.Net.Sockets;
using System.Net;
 
class server
{
    //创建一个和客户端通信的套接字
    static Socket SocketWatch = null;
    //定义一个集合，存储客户端信息
    static Dictionary<string, Socket> ClientConnectionItems = new Dictionary<string, Socket> { };
    public static string cID;
    static void Main(string[] args)
    {
        //端口号（用来监听的）
        int port = 6000;
        IPAddress ip = IPAddress.Any;
        //将IP地址和端口号绑定到网络节点point上  
        IPEndPoint ipe = new IPEndPoint(ip, port);
        //定义一个套接字用于监听客户端发来的消息，包含三个参数（IP4寻址协议，流式连接，Tcp协议）  
        SocketWatch = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
        //监听绑定的网络节点  
        SocketWatch.Bind(ipe);
        //将套接字的监听队列长度限制为20  
        SocketWatch.Listen(20);
        //负责监听客户端的线程:创建一个监听线程  
        Thread threadwatch = new Thread(WatchConnecting);
        //将窗体线程设置为与后台同步，随着主线程结束而结束  
        threadwatch.IsBackground = true;
        //启动线程     
        threadwatch.Start();
        Console.WriteLine("开启监听......");
        Console.WriteLine("点击输入任意数据回车退出程序......");
        Console.ReadKey();
        SocketWatch.Close();  
    }
    //监听客户端发来的请求  
    static void WatchConnecting()
    {
        Socket connection = null;
        //持续不断监听客户端发来的请求     
        while (true)
        {
           // string cID;
            try
            {
                connection = SocketWatch.Accept();
            }
            catch (Exception ex)
            {
                //提示套接字监听异常     
                Console.WriteLine(ex.Message);
                break;
            }
            byte[] idrec = new byte[1024 * 1024];
            int length = connection.Receive(idrec);
            cID = Encoding.UTF8.GetString(idrec, 0, length);
            ClientConnectionItems.Add(cID, connection);
            //显示与客户端连接情况
            Console.WriteLine("\r\n[客户端\"" + cID + "\"建立连接成功！ 客户端数量：" + ClientConnectionItems.Count + "]");
            //获取客户端的IP和端口号  
            IPAddress clientIP = (connection.RemoteEndPoint as IPEndPoint).Address;
            int clientPort = (connection.RemoteEndPoint as IPEndPoint).Port;
            string sendmsg = "[" + "本端：" + cID + " 连接服务端成功！]";
            byte[] arrSendMsg = Encoding.UTF8.GetBytes(sendmsg);
            connection.Send(arrSendMsg);
            //创建一个通信线程      
            Thread thread = new Thread(recv);
            //设置为后台线程，随着主线程退出而退出 
            thread.IsBackground = true;
            //启动线程     
            thread.Start(connection);
        }
    }
   
    static void recv(object socketclientpara)
    {
        Socket socketServer = socketclientpara as Socket;
 
        while (true)
        {
            try
            {
                byte[] arrServerRecMsg = new byte[1024 * 1024];
                int length = socketServer.Receive(arrServerRecMsg);
                //将机器接受到的字节数组转换为人可以读懂的字符串     
                string strSRecMsg = Encoding.UTF8.GetString(arrServerRecMsg, 0, length);
                string ccID = strSRecMsg.Substring(0, 4);
                int len = strSRecMsg.Length;
                string hc=cID+":"+strSRecMsg.Substring(4,len-4);
                Console.WriteLine( DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss:fff") + "]\r\n" + strSRecMsg);
                //判断是否包含这个客户端
                bool contains = ClientConnectionItems.ContainsKey(ccID);
                if (contains)
                {
                    ClientConnectionItems[ccID].Send(Encoding.UTF8.GetBytes( hc));
                }
                else
                {
                    Console.WriteLine("输入有误，不予转发\r\n");
                }
                arrServerRecMsg.DefaultIfEmpty();
            }
            catch (Exception)
            {
                string temp = ClientConnectionItems.First().Key;
                //提示套接字监听异常  
                Console.WriteLine("\r\n[客户端\"" + socketServer.RemoteEndPoint + "\"已经中断连接！ 客户端数量：" + ClientConnectionItems.Count + "]");
                ClientConnectionItems.Remove(ClientConnectionItems.First().Key);
                Console.WriteLine("\r\n[客户端\"" + temp + "\"已经中断连接！ 客户端数量：" + ClientConnectionItems.Count + "]");
                break;
            }
        }
    }
}

 client端（这里只上传了num3端，想创建更多客户端改改名称就行）：

using System;
using System.Text;
using System.Threading;
using System.Net.Sockets;
using System.Net;
 
 
class client1
{
    //创建1个客户端套接字和1个负责监听服务端请求的线程  
    static Thread ThreadClient = null;
    static Socket SocketClient = null;
    static void Main(string[] args)
    {
        try
        {
            int port = 6000;
            string host = "127.0.0.1";//服务器端ip地址
            IPAddress ip = IPAddress.Parse(host);
            IPEndPoint ipe = new IPEndPoint(ip, port);
            //定义一个套接字监听  
            SocketClient = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);
            try
            {
                //客户端套接字连接到网络节点上，用的是Connect  
                SocketClient.Connect(ipe);
                //本客户端的ID
                string sendsStr = "num3";
                ClientSendMsg(sendsStr);
            }
            catch (Exception)
            {
                Console.WriteLine("连接失败！\r\n");
                Console.ReadLine();
                return;
            }
            ThreadClient = new Thread(Recv);
            ThreadClient.IsBackground = true;
            ThreadClient.Start();
            Thread.Sleep(1000);
            Console.WriteLine("请输入内容<按Enter键发送>：\r\n");
            while(true)
            {
                string sendStr = Console.ReadLine();
                
                if (sendStr.Length <= 4)
                {
                    Console.WriteLine("输入信息长度小于5个字符，请重新输入");
                }
                else if (sendStr.Substring(0,4)=="num3")
                {
                    
                    Console.WriteLine("请不要给自己发送信息");
                }
                else
                {
                    ClientSendMsg(sendStr);
                }
            }
        }
        catch (Exception ex)
        {
            Console.WriteLine(ex.Message);
            Console.ReadLine();
        }
    }
    //接收服务端发来信息的方法    
    public static void Recv()
    {
        //持续监听服务端发来的消息 
        while (true)
        {
            try
            {
                //定义一个1M的内存缓冲区，用于临时性存储接收到的消息  
                byte[] arrRecvmsg = new byte[1024 * 1024];
                //将客户端套接字接收到的数据存入内存缓冲区，并获取长度  
                int length = SocketClient.Receive(arrRecvmsg);
                //将套接字获取到的字符数组转换为人可以看懂的字符串  
                string strRevMsg = Encoding.UTF8.GetString(arrRecvmsg, 0, length);
                Console.WriteLine(DateTime.Now.ToString("yyyy-MM-dd HH:mm:ss:fff") + "\r\n" + strRevMsg + "\r\n");
            }
            catch (Exception ex)
            {
                Console.WriteLine("远程服务器已经中断连接！" + ex.Message + "\r\n");
                break;
            }
        }
    }
    //发送字符信息到服务端的方法  
    public static void ClientSendMsg(string sendMsg)
    {
        //将输入的内容字符串转换为机器可以识别的字节数组     
        byte[] arrClientSendMsg = Encoding.UTF8.GetBytes(sendMsg);
        //调用客户端套接字发送字节数组     
        SocketClient.Send(arrClientSendMsg);
    }
}