异步下载类 循环调用 的问题

Question

using System.Windows.Forms;
using System.Net;
using System.IO;namespace DataGather
{
//异步下载类
    public class MyHttpWebRequest
    {
        public static ManualResetEvent allDone = new ManualResetEvent(false);
        const int BUFFER_SIZE = 1024;
        const int DefaultTimeout = 2 * 60 * 1000; // 2 minutes timeout        public static string ReadStr;
        public static bool Suc;
             
        public void MyDataGather(string ServerUrl,int i)
        {
                try
                {
                    HttpWebRequest MyHttpWebRequest = (HttpWebRequest)WebRequest.Create(ServerUrl + i);                    // Create an instance of the RequestState and assign the previous myHttpWebRequest
                    // object to its request field.  
                    RequestState MyRequestState = new RequestState();
                    MyRequestState.Request = MyHttpWebRequest;                    // Start the asynchronous request.
                    IAsyncResult Result =
                      (IAsyncResult)MyHttpWebRequest.BeginGetResponse(new AsyncCallback(RespCallback), MyRequestState);                    // this line implements the timeout, if there is a timeout, the callback fires and the request becomes aborted
                    ThreadPool.RegisterWaitForSingleObject(Result.AsyncWaitHandle, new WaitOrTimerCallback(TimeoutCallback), MyHttpWebRequest, DefaultTimeout, true);                    // The response came in the allowed time. The work processing will happen in the 
                    // callback function.
                    allDone.WaitOne();                    // Release the HttpWebResponse resource.
                    //此处在循环第二次出现 未将对象引用设置到对象实例  的异常
                    MyRequestState.Response.Close();
                }
                catch (WebException)
                {
                    ReturnString("失败(一)!", false);
                }
                catch (Exception)
                {
                    ReturnString("失败(二)!", false);
                }
        }        private static void RespCallback(IAsyncResult AsynchronousResult)
        {
            try
            {
                // State of request is asynchronous.
                RequestState MyRequestState = (RequestState)AsynchronousResult.AsyncState;
                HttpWebRequest MyHttpWebRequest = MyRequestState.Request;
                MyRequestState.Response = (HttpWebResponse)MyHttpWebRequest.EndGetResponse(AsynchronousResult);                // Read the response into a Stream object.
                Stream ResponseStream = MyRequestState.Response.GetResponseStream();
                MyRequestState.StreamResponse = ResponseStream;                // Begin the Reading of the contents of the HTML page and print it to the console.
                IAsyncResult AsynchronousInputRead = ResponseStream.BeginRead(MyRequestState.BufferRead, 0, BUFFER_SIZE, new AsyncCallback(ReadCallBack), MyRequestState);
                return;
            }
            catch (WebException)
            {
                ReturnString("失败(三)!", false);
            }
            allDone.Set();
        }        private static void ReadCallBack(IAsyncResult AsyncResult)
        {
            try
            {                RequestState MyRequestState = (RequestState)AsyncResult.AsyncState;
                Stream ResponseStream = MyRequestState.StreamResponse;
                int Read = ResponseStream.EndRead(AsyncResult);
                // Read the HTML page and then print it to the console.
                if (Read > 0)
                {
                    //MyRequestState.RequestData.Append(Encoding.ASCII.GetString(MyRequestState.BufferRead, 0, Read));
                    MyRequestState.RequestData.Append(Encoding.GetEncoding("GB2312").GetString(MyRequestState.BufferRead, 0, Read));
                    IAsyncResult AsynchronousResult = ResponseStream.BeginRead(MyRequestState.BufferRead, 0, BUFFER_SIZE, new AsyncCallback(ReadCallBack), MyRequestState);
                    return;
                }
                else
                {
                    if (MyRequestState.RequestData.Length > 1)
                    {
                        string StringContent;
                        StringContent = MyRequestState.RequestData.ToString();
                        ReturnString(StringContent, true);
                    }
                    ResponseStream.Close();
                    allDone.Set();
                }
            }
            catch (WebException)
            {
                ReturnString("失败(四)!", false);
            }
        }        private static void TimeoutCallback(object State, bool TimedOut)
        {
            if (TimedOut)
            {
                HttpWebRequest Request = State as HttpWebRequest;
                if (Request != null)
                {
                    Request.Abort();
                }
            }
        }        public static void ReturnString(string TempStr,bool TempBool)
        {
            ReadStr = TempStr;
            Suc = TempBool;
        }    }
    public class RequestState
    {
        // This class stores the State of the request.
        const int BUFFER_SIZE = 1024;
        public StringBuilder RequestData;
        public byte[] BufferRead;
        public HttpWebRequest Request;
        public HttpWebResponse Response;
        public Stream StreamResponse;
        public RequestState()
        {
            BufferRead = new byte[BUFFER_SIZE];
            RequestData = new StringBuilder("");
            Request = null;
            StreamResponse = null;
        }
    }    public class WebDataGather
   {
       private string ServerUrl = "http://web.com?id=" ;       
       public void StartGather()
       {
            for(int i=1;i<100;i++)
           {     
                MyHttpWebRequest MyHttpWebRequest = new MyHttpWebRequest();
                MyHttpWebRequest.MyDataGather(ServerUrl,i);                string ReadStr = MyHttpWebRequest.ReadStr;
                bool Suc = MyHttpWebRequest.Suc;                 if(Suc)
                 {
                    //输出ReadStr
                 } 
            }       }   }
}===================================================================大概就是这个样子的
但是 循环调用 异步下载的类  总会有问题//此处在循环第二次出现 未将对象引用设置到对象实例  的异常
 MyRequestState.Response.Close();当把这里注释掉
执行没有问题  但是  后面字符串的输出会有问题  连续输出相同字符串   也就是循环调用的类执行时间长  而循环本身执行过快=========================================================
请朋友指明问题所在   并提出解决办法   谢谢  

Answer 1

在MyRequestState.Response.Close();之前加判断吧
if(MyRequestState.Response !=null)