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

namespace Client
{
    class Program
    {
        //客戶端 Socket對象
        private static Socket clientSocket;
        //服務器端 終點
        private static EndPoint epServer;
        //接收數據的字符數組
        private static byte[] receiveData;

        public static void Main(string[] args)
        {
            //客戶端Socket對象實例化
            clientSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            //設置服務器端IP地址和對應端口
            IPEndPoint server = new IPEndPoint(IPAddress.Parse("192.168.1.165"), 11000);
            //實例化服務器端 終點
            epServer = (EndPoint)server;
            string msg;     //要發送的消息
            byte[] sendData;    //要發送的字符串
            while (true) {
                msg = Console.ReadLine();   //輸入要發送的消息
                if (msg == "exit") break;   //當輸入“exit”時，退出客戶端程序
                //將消息通過ASCII編碼轉換為字符數組，
                //如果要發送漢字或其他特殊符號，可以采用UTF-8
                sendData = Encoding.ASCII.GetBytes(msg);   
                //開始異步發送消息
                //參數：sendData           要發送的數據
                //參數：0：                 要發送數據的起始位置
                //參數：sendData.Length：   要發送數據的字節數
                //參數：SocketFlags.None：  按位組合方式
                //參數：epServer：          接收方設備（包含IP和端口）
                //參數：new AsyncCallback(SendData):   委托
                //參數：null：          請求的狀態信息
                clientSocket.BeginSendTo(sendData, 0, sendData.Length, SocketFlags.None,
                    epServer, new AsyncCallback(SendData), null);
                //實例化接收數據的字符數組
                //若在聲明時已經初始化，此處依然要進行重新初始化
                //當上次接收的數據長度大于本次，則該數組里包含上次接收的殘留數據
                //比如：上次接收“你個小逗逼”。本次接收“開玩笑”。
                //則數組中的數據為：“開玩笑逗逼”。
                receiveData = new byte[1024];
                //開始異步接收消息
                //參數部分與異步發送部分對應，基本一致
                clientSocket.BeginReceiveFrom(receiveData, 0, receiveData.Length, SocketFlags.None,
                    ref epServer, new AsyncCallback(ReceiveData), null);
            }
        }

        //異步發送消息的委托函數
        private static void SendData(IAsyncResult iar)
        {
            //完成異步發送
            clientSocket.EndSend(iar);
        }

        //異步接收消息的委托函數
        private static void ReceiveData(IAsyncResult iar)
        {
            //完成異步接收  recv 表示接收到的字節數
            int recv = clientSocket.EndReceive(iar);
            //將接收到的數據打印出來
            Console.WriteLine("Server: " + Encoding.ASCII.GetString(receiveData, 0, recv));
        }

    }
}

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

namespace Server
{
    class AsyncUdpServer
    {
        //服務器端Socket對象
        private static Socket serverSocket;
        //接收數據的字符數組
        private static byte[] receiveData = new byte[1024];

        public static void Main(string[] args)
        {
            //實例化服務器端Socket對象
            serverSocket = new Socket(AddressFamily.InterNetwork, SocketType.Dgram, ProtocolType.Udp);
            //服務器端的IP和端口，IPAddress.Any實際是：0.0.0.0，表示任意，基本上表示本機IP
            IPEndPoint server = new IPEndPoint(IPAddress.Any, 11000);
            //Socket對象跟服務器端的IP和端口綁定
            serverSocket.Bind(server);
            //客戶端的IP和端口，端口 0 表示任意端口
            IPEndPoint clients = new IPEndPoint(IPAddress.Any, 0);
            //實例化客戶端 終點
            EndPoint epSender = (EndPoint)clients;
            //開始異步接收消息  接收后，epSender存儲的是發送方的IP和端口
            serverSocket.BeginReceiveFrom(receiveData, 0, receiveData.Length, SocketFlags.None,
                ref epSender, new AsyncCallback(ReceiveData), epSender);
            Console.WriteLine("Listening...");
            Console.ReadLine();
        }

        private static void SendData(IAsyncResult iar)
        {
            serverSocket.EndSend(iar);
        }

        private static void ReceiveData(IAsyncResult iar)
        {
            //客戶端的IP和端口，端口 0 表示任意端口
            IPEndPoint client = new IPEndPoint(IPAddress.Any, 0);
            //實例化客戶端 終點
            EndPoint epSender = (EndPoint)client;
            //結束異步接收消息  recv 表示接收到的字符數
            int recv = serverSocket.EndReceiveFrom(iar, ref epSender);           
            //將接收到的數據打印出來，發送方采用什么編碼方式，此處就采用什么編碼方式 轉換成字符串
            Console.WriteLine("Client:" + Encoding.ASCII.GetString(receiveData, 0, recv));
            //定義要發送回客戶端的消息，采用ASCII編碼，
            //如果要發送漢字或其他特殊符號，可以采用UTF-8           
            byte[] sendData = Encoding.ASCII.GetBytes("hello");
            //開始異步發送消息  epSender是上次接收消息時的客戶端IP和端口信息
            serverSocket.BeginSendTo(sendData, 0, sendData.Length, SocketFlags.None,
                epSender, new AsyncCallback(SendData), epSender);
            //重新實例化接收數據字節數組
            receiveData = new byte[1024];
            //開始異步接收消息，此處的委托函數是這個函數本身，遞歸
            serverSocket.BeginReceiveFrom(receiveData, 0, receiveData.Length, SocketFlags.None,
                ref epSender, new AsyncCallback(ReceiveData), epSender);
        }

    }
}