独立存在是什么意思？

先来看一个例子.我们知道一个Socket对象（我们叫他ListenScoket）可以调用AcceptAsync并接受一个SocketAsyncEventArgs对象，如果操作成功则ListenSocket会为SocketAsyncEventArgs对象创建一个新的Socket对象（我们叫它AcceptSccket）。也就是这样

listenSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);

listenSocket.Bind(new IPEndPoint(IPAddress.Parse("10.175.5.172"), 60000));

listenSocket.Listen(10);

SocketAsyncEventArgs asyncEventArgs =new SocketAsyncEventArgs();

asyncEventArgs.Completed += asyncEventArgs_Completed;

listenSocket.AcceptAsync(asyncEventArgs);

如果ListenSocket关闭之后AcceptSocket还可以正常工作,那么我们就说AcceptSocket是独立存在的.

那到底AcceptSocket能否独存在呢?

首先我们来看AcceptSocket是什么时候创建的.其实当listenSocket.AcceptAsync(asyncEventArgs)执行的时候SocketAsyncEventArgs内部的AcceptSocket就已经被创建了,但是它不能被用来发送和接收数据.假如我们在AcceptAsync之后紧跟着调用AcceptSocket来发送一个”OK”字符串,那么会得到这样一个错误:

'asyncEventArgs.AcceptSocket.Send(ASCIIEncoding.ASCII.GetBytes("OK"))' threw an exception of type 'System.Net.Sockets.SocketException'

    base: {"A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket using a sendto call) no address was supplied"}

    ErrorCode: 10057

    Message: "A request to send or receive data was disallowed because the socket is not connected and (when sending on a datagram socket using a sendto call) no address was supplied"

    SocketErrorCode: NotConnected

注意错误消息中被标为红色的部分,这说明AcceptSocket对象已经创建了,不过没有可用的地址,也就是没有客户端IP和端口.这个很好理解,因为客户端还没有连接上呢.当客户端连接成功的时候,ListenSocket会将客户端的地址(IP和端口)赋予AcceptSocket.同时,SocketAsyncEventArgs的Completed事件会被触发,我们就知道AcceptSocket可以正常使用了.

为什么会有这个疑问呢? 在我们的应用中ListenSocket会创建很多的AcceptSocket, 某一个时刻我们想停止接收更多新的Socket连接,同时又想保持现有的连接,那么我们可以直接把ListenSocket关闭.经过测试是不需要担心AcceptSocket会受到影响的.也就是说AcceptSocket可以独立存在的. 下面是测试代码. 

using System;using System.Net;using System.Net.Sockets;using System.Text;using System.Threading.Tasks;namespace SocketAsyncEventArgsTest{    class Program    {        static int socketCount = 0;        static Socket listenSocket;        static void Main(string[] args)        {            listenSocket = new Socket(AddressFamily.InterNetwork, SocketType.Stream, ProtocolType.Tcp);            listenSocket.Bind(new IPEndPoint(IPAddress.Parse("10.175.5.172"), 60000));            listenSocket.Listen(10);            SocketAsyncEventArgs asyncEventArgs =new SocketAsyncEventArgs();            asyncEventArgs.Completed += asyncEventArgs_Completed;            listenSocket.AcceptAsync(asyncEventArgs);            SocketAsyncEventArgs asyncEventArgs2 = new SocketAsyncEventArgs();            asyncEventArgs2.Completed += asyncEventArgs_Completed;            listenSocket.AcceptAsync(asyncEventArgs2);            //listenSocket.Close();            Console.Read();        }        static void asyncEventArgs_Completed(object sender, SocketAsyncEventArgs e)        {            if (SocketError.Success == e.SocketError)            {                socketCount++;                if (2 == socketCount)                    // Close the listen socket will not close the accept sockets opened by it.                    // so the accept socket can continue receive/send data normally.                    listenSocket.Close();                Socket acceptSocket = e.AcceptSocket;                byte[] buffer=new byte[1024];                Task.Factory.StartNew(() =>                {                    while(true)                    {                        acceptSocket.Receive(buffer);                        Console.WriteLine(ASCIIEncoding.ASCII.GetString(buffer));                        acceptSocket.Send(ASCIIEncoding.ASCII.GetBytes("OK"));                    }                });            }        }    }}