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, 0, 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;
}