问一个关于使用HttpWebRequest发送POST请求的问题
我使用HttpWebRequest发送POST请求去调用一个按钮事件，但由于页面的viewstate老是变来变去，以致服务器端的后台程序总是不会处理，我应该如何办

我把代码贴出来，希望高人帮忙看看 

using System; 
using System.Collections.Generic; 
using System.ComponentModel; 
using System.Data; 
using System.Drawing; 
using System.Text; 
using System.Windows.Forms; 
using System.Net; 
using System.IO; 

namespace WinTest 
{ 
  public partial class AutoPost : Form 
  { 
  public AutoPost() 
  { 
  InitializeComponent(); 
  } 

  private void AutoPost_Load(object sender, EventArgs e) 
  { 
  string viewstate = this.getViewState("http://xihuanet.3322.org:8081/GongDanSystem/list_bill1.aspx"); 
  string postString = "__EVENTTARGET=gvBill$ctl07$lbtnZP" + viewstate; 
  CookieContainer cookieContainer = new CookieContainer(); 
  this.textBox1.Text = this.OpenPageWithPost("http://xihuanet.3322.org:8081/GongDanSystem/list_bill1.aspx", postString, cookieContainer); 
   
  } 

  private string getViewState(string URI) 
  { 

  // 设置打开页面的参数 
  HttpWebRequest request = WebRequest.Create(URI) as HttpWebRequest; 
  request.Method = "GET"; 
  request.KeepAlive = false; 

  // 接收返回的页面 
  HttpWebResponse response = request.GetResponse() as HttpWebResponse; 
  System.IO.Stream responseStream = response.GetResponseStream(); 
  System.IO.StreamReader reader = new System.IO.StreamReader(responseStream, Encoding.UTF8); 
  string srcString = reader.ReadToEnd(); 

  // 获取页面的 VeiwState  
  string viewStateFlag = "id=\"__VIEWSTATE\" value=\""; 
  int i = srcString.IndexOf(viewStateFlag) + viewStateFlag.Length; 
  int j = srcString.IndexOf("\"", i); 
  string viewState = srcString.Substring(i, j - i); 

  // 获取页面的 EventValidation  
  string eventValidationFlag = "id=\"__EVENTVALIDATION\" value=\""; 
  i = srcString.IndexOf(eventValidationFlag) + eventValidationFlag.Length; 
  j = srcString.IndexOf("\"", i); 
  string eventValidation = srcString.Substring(i, j - i); 

  viewState = System.Web.HttpUtility.UrlEncode(viewState); 
  eventValidation = System.Web.HttpUtility.UrlEncode(eventValidation); 

  string formatString = "&__VIEWSTATE={0}&__EVENTVALIDATION={1}"; 
  string postString = string.Format(formatString, viewState, eventValidation); 
  return postString; 

  } 

  private CookieContainer Login(string URI, string postString) 
  { 
  // 将提交的字符串数据转换成字节数组 
  byte[] postData = Encoding.ASCII.GetBytes(postString); 

  // 设置提交的相关参数 
  HttpWebRequest request = WebRequest.C