如何安全地结束NetworkStream.Read()？(在C#或VB.net中)

Question

我写了一个类,试图读取服务器发送的字节数据,当我的应用程序结束时,我也希望循环结束,但是NetworkStream.Read()如果没有数据可用,它似乎等待.

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net.Sockets;
using System.Threading;
using System.Net;
using System.Threading.Tasks;
namespace Stream
{
    class Program
    {
        private static TcpClient client = new TcpClient("127.0.0.1", 80);
        private static NetworkStream stream = client.GetStream();

        static void Main(string[] args)
        {
            var p = new Program();
            p.Run();
        }

        void Run()
        {
            ThreadPool.QueueUserWorkItem(x =>
            {
                while (true)
                {
                    stream.Read(new byte[64], 0, 64);
                }
            });

            client.Close();
            // insert Console.ReadLine(); if you want to see an exception occur.
        }
    }
}

有了这段代码,我们就可以了

1. System.IO.IOException 说"无法从传输连接读取数据:现有连接被远程主机强行关闭",
2. ObjectDisposedException 说"无法访问已处置的对象",或
3. System.IO.IOException 说"通过调用WSACancelBlockingCall来中断阻塞操作".

那我怎么能安全地结束这个方法呢？

Answer 1

1. 为避免阻塞,只在DataAvailable为true时调用Read().
2. ThreadPool不适合长时间运行的任务,因此重构你的while循环
3. 在读取连接之前关闭连接(因为工作项执行异步).

这可能有所帮助

     void Run() {
        ThreadPool.QueueUserWorkItem(ReadLoop);
     }

     void ReadLoop(object x) {
         if (stream.DataAvailable) 
           stream.Read(new byte[64], 0, 64);
         else 
             Thread.Sleep(TimeSpan.FromMilliseconds(200));

         if (Finished)
               client.Close();
         else if (!Disposed && !Finished) 
               ThreadPool.QueueUserWorkItem(ReadLoop);
     }

您需要在真实班级中自己管理完成和处置.

Answer 2

从我在运行程序时可以看到,第一个异常"无法从传输连接中读取数据......"不是由Stream.Read引发的,而是由TcpClient的构造函数引发的,很可能是由于存在这样的事实.没有服务器接受127.0.0.1:80上的连接.

现在,如果你想在Stream.Read上很好地结束,我会异步进行.通过这种方式,您可以捕获在Read和清理程序期间抛出的任何异常.这是您的程序的修改版本:

using System;
using System.Net.Sockets;

namespace Stream
{
    class Program
    {
        private static TcpClient client;
        private static NetworkStream stream;

        static void Main(string[] args)
        {
            var p = new Program();
            p.Run();
        }

        void Run()
        {
            try
            {
                client = new TcpClient("127.0.0.1", 80);
                stream = client.GetStream();
                byte[] buffer = new byte[64];

                stream.BeginRead(buffer, 0, buffer.Length, new AsyncCallback(OnRead), buffer);

                client.Close();
                Console.ReadKey();
            }
            catch (Exception)
            {
                //...
            }
        }

        void OnRead(IAsyncResult result)
        {
            try
            {
                stream.EndRead(result);
                byte[] buffer = result.AsyncState as byte[];

                if (buffer != null)
                {
                    //...
                }

                // continue to read the next 64 bytes
                buffer = new byte[64];
                stream.BeginRead(buffer, 0, buffer.Length, new AsyncCallback(OnRead), buffer);
            }
            catch (Exception)
            {
                // From here you can get exceptions thrown during the asynchronous read
            }
        }
    }
}