C# 的异步

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Net;
using System.IO;
using System.Threading;

namespace AsncyHandle
{
    public class RequestState
    {
        const int BUFFER_SIZE = 1024;
        public StringBuilder RequestData;
        public byte[] BufferRead;
        public HttpWebRequest Request;
        public Stream ResponseStream;
        //创建适当编码类型的解码器
        public Decoder StreamDecode = Encoding.UTF8.GetDecoder();

        public RequestState()
        {
            BufferRead = new byte[BUFFER_SIZE];
            RequestData = new StringBuilder("");
            Request = null;
            ResponseStream = null;

        }
    }

    class Program
    {
        public static ManualResetEvent allDone = new ManualResetEvent(false);
        const int BUFFER_SIZE = 1024;

        static void Main(string[] args)
        {
            Uri HttpSite = new Uri("http://www.baidu.com");

            //创建请求对象
            HttpWebRequest wreq = (HttpWebRequest)WebRequest.Create(HttpSite);
            //创建请求对象
            RequestState rs = new RequestState();

            //将请求添加到状态，以便可以被来回传涕
            rs.Request = wreq;

            //发出异步请求
            IAsyncResult r = (IAsyncResult)wreq.BeginGetResponse(new AsyncCallback(RespCallback), rs);

            // 将 ManualResetEvent 设置为 Wait，
            // 以便在调用回调前，应用程序不退出
            allDone.WaitOne();
        }

        public static void showusage()
        {
            Console.WriteLine("尝试获取 (GET) 一个 URL");
            Console.WriteLine("\r\n用法：:");
            Console.WriteLine("ClientGetAsync URL");
            Console.WriteLine("示例：:");
            Console.WriteLine("ClientGetAsync http://www.microsoft.com/net/");
        }

        public static void RespCallback(IAsyncResult ar)
        {
            //从异步结果获取RequestState 对象
            RequestState rs = (RequestState)ar.AsyncState;

            //从RequestState 获取HttpWebRequest
            HttpWebRequest req = rs.Request;

            //调用 EndGetResponse  生成 HttpWebResponse 对象

            HttpWebResponse resp = (HttpWebResponse)req.EndGetResponse(ar);

            // // 既然我们拥有了响应，就该从
            // 响应流开始读取数据了
            Stream responseStream = resp.GetResponseStream();

            //该读取操作也使用异步完成，所以我们
            //将要以 RequestState 存储流
            rs.ResponseStream = responseStream;

            // 请注意，rs.BufferRead 被传入到 BeginRead。
            // 这是数据将被读入的位置。
            IAsyncResult iarRead = responseStream.BeginRead(rs.BufferRead, 0, BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs);
        }

        private static void ReadCallBack(IAsyncResult asyncResult)
        {
            //从asyncresult 获取 RequestState 对象
            RequestState rs = (RequestState)asyncResult.AsyncState;

            //取出在respCallBack 中设置的ResponseStream
            Stream responseStream = rs.ResponseStream;

            int read = responseStream.EndRead(asyncResult);

            if (read > 0)
            {
                //准备Char数组缓冲区，用于向Unicode 转换

                Char[] charBuffer = new Char[BUFFER_SIZE];

                //将字节流转换为Char 数组，然后转换为字符串
                //len 显示多少字符被转换为 unicode

                int len = rs.StreamDecode.GetChars(rs.BufferRead, 0, read, charBuffer, 0);

                String str = new String(charBuffer, 0, len);

                //将最近读取的数据追加到RequestData stirngBuilder对象中
                //该对想包含在RequestState中
                rs.RequestData.Append(str);
                IAsyncResult ar = responseStream.BeginRead(rs.BufferRead, 0, BUFFER_SIZE, new AsyncCallback(ReadCallBack), rs);

            }
            else
            {
                if(rs.RequestData.Length>1)
                {
                    //所有数据都已被读取，因此将其显示 到控制台
                    string strContent;
                    strContent = rs.RequestData.ToString();
                    Console.WriteLine(strContent);
                }
                //关闭响应流
                responseStream.Close();

                //设置ManualResetEvent ,以便主线程可以退出
                allDone.Set();
            }
            return;

        }

    }
}