sp;              string textData = new StreamReader(e.Result, Encoding.Default).ReadToEnd();  //取返回信息
                             .....
                              String cookie = ((WebClient)sender).ResponseHeaders["Set-Cookie"]; 
                             ((WebClient)sender).Headers.Add("Content-Type", "application/x-www-form-urlencoded");
                            ((WebClient)sender).Headers.Add("Accept-Language", "zh-cn");
                            ((WebClient)sender).Headers.Add("Cookie", cookie);
                            string postData = "......"
                            byte[] byteArray = Encoding.UTF8.GetBytes(postData); // 转化成二进制数组 
                           ((WebClient)sender).UploadDataAsync(new Uri("/tp.asp?Id=129"), "POST", byteArray);
                }
         } 

        private void Pic_UploadDataCompleted2(object sender, UploadDataCompletedEventArgs e)
        {
                 if (e.Error == null)
                { 
                    string returnMessage = Encoding.Default.GetString(e.Result);
                    ......
                }
       } 
       3、HttpWebRequest较为低层，能实现的功能较多，Cookie操作也很简单
             private bool  PostWebRequest()        
             {
                   CookieContainer cc = new CookieContainer();
                    string pos tData = "user=" + strUser + "&pass=" + strPsd;
                    byte[] byteArray = Encoding.UTF8.GetBytes(postData); // 转化 免费资源http://www.it55.com 
                    HttpWebRequest webRequest2 = (HttpWebRequest)WebRequest.Create(new Uri("/chk.asp"));
                    webRequest2.CookieContainer = cc;
                    webRequest2.Method = "POST";
                    webRequest2.ContentType = "application/x-www-form-urlencoded";
                    webRequest2.ContentLength = byteArray.Length;
                    Stream newStream = webRequest2.GetRequestStream(); 
                    // Send the data.
                    newStream.Write(byteArray, 0, byteArray.Length);    //写入参数
                    newStream.Close(); 
                    HttpWebResponse response2 = (HttpWebResponse)webRequest2.GetResponse();
                    StreamReader sr2=new StreamReader(response2.GetResponseStream(), Encoding.Default);
                    string text2 =  sr2.ReadToEnd();
                  ......
        }