C#的HttpWebRequest编程，支持带ViewState的网页POST请求

 staticprivatestring SearchURL(string id)
 {
 try
 {
 //Get the ViewState and EventValidation
 HttpWebRequest request = WebRequest.Create(URI) as HttpWebRequest;
 request.Method ="GET";
 request.KeepAlive =false;

 //Get the response
 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();

 //Get the ViewState
 string viewStateFlag ="id=\"__VIEWSTATE\" value=\"";
 int i = srcString.IndexOf(viewStateFlag) + viewStateFlag.Length;
 int j = srcString.IndexOf("\"", i);
 string viewState = srcString.Substring(i, j - i);

 //Get the ViewState
 string EventValidationFlag ="id=\"__EVENTVALIDATION\" value=\"";
 i = srcString.IndexOf(EventValidationFlag) + EventValidationFlag.Length;
 j = srcString.IndexOf("\"", i);
 string eventValidation = srcString.Substring(i, j - i);

 //Compose the URL
 viewState = Uri.EscapeDataString(viewState);
 eventValidation = Uri.EscapeDataString(eventValidation);
 string strSearch = Uri.EscapeDataString("查询");

 string formatString ="__VIEWSTATE={0}&__EVENTVALIDATION={1}&TextBox1={2}&Button1={3}";
 string postString =string.Format(formatString, viewState, eventValidation, id,strSearch);

 //Change to byte[]
 byte[] postData = Encoding.ASCII.GetBytes(postString);

 //Compose the new request
 request = WebRequest.Create(URI) as HttpWebRequest;
 request.Method ="POST";
 request.KeepAlive =false;
 request.ContentType ="application/x-www-form-urlencoded";
 request.ContentLength = postData.Length;

 System.IO.Stream outputStream = request.GetRequestStream();
 outputStream.Write(postData, , postData.Length);
 outputStream.Close();

 //Get the new response
 response = request.GetResponse() as HttpWebResponse;
 responseStream = response.GetResponseStream();
 reader =new System.IO.StreamReader(responseStream);
 srcString = reader.ReadToEnd();
 return srcString;
 }
 catch (WebException we)
 {
 Console.WriteLine("Communication error,"+ we.Message +" please check your connectivity and try again.", "Error");
 }
 catch
 {
 Console.WriteLine("Unknow error.", "Error");
 }
 returnnull;
 }