取出ViewState
进行ASP.net的webform开发，关键是了解页面的生命周期，其中一个关键点是ViewState，后台把所需要的信息存放到一个叫做__VIEWSTATE的隐藏input框里，这__VIEWSTATE会往服务器回传服务端控件信息(Server controls)，下面就是一个ViewState的例子。
<div>
<input type="hidden" name="__VIEWSTATE" id="__VIEWSTATE" value="/wEPDwULLTE5NjAwMTc3OTQPZBYCAgMPZBYCAgEPDxYCHgRUZXh0BTRDbGljazogRnVlbD1bRGllc2VsXSwgUG9zdGNvZGVyPVsyMDE0XSwgUHJpY2VzPVsxLjVdZGRkPwaT5ufoa4sSWz2jkkc4+1mjzgA=" />
</div>
<div>
<input type="hidden" name="__EVENTVALIDATION" id="__EVENTVALIDATION" value="/wEWCALvtKiPBwLHjuPuBALEjuPuBALFjuPuBALCjuPuBALDjuPuBAKJr8zPBQLj1NuXC4Mp8caGaDkuTJ50BTH0riQ7xWbK" />
</div>
在CF.NET下进行HttpWebRequest开发，如果服务端是ASP.net的程序，需要处理ViewState。从图3上看，除了__VIEWSTATE还需要处理__EVENTVALIDATION，其实很简单，通过HttpFox看那个需要取那个。
//Get the ViewState and EventValidation
string URI = "http://192.168.1.149/HttpWebRequest/Default.aspx";
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);
取ViewState要先访问提交前的网页，使用HttpWebRequest request = WebRequest.Create(URI) as HttpWebRequest;生成一个HttpWebRequest的对象，参数为Web服务器的地址，提交方式为"GET"，其实这个方式也可以通过HttpFox看到的，如下图：
图4
先启动HttpFox，然后输入网址，看第一行知道这是一个"GET"操作。
HttpWebResponse response = request.GetResponse() as HttpWebResponse;为向Web服务器发送请求，并得到返回信息。返回的信息其实就是图4里的"Content"Tab页，也就是HTML信息，里面包含我们需要的ViewState。
组成Post的数据并发送请求
处理结果得到__VIEWSTATE和__EVENTVALIDATION，然后可以组成提交操作还需要组成Post的数据，如下代码
//Compose the URL
viewState = Uri.EscapeDataString(viewState);
eventValidation = Uri.EscapeDataString(eventValidation);
string formatString = "__VIEWSTATE={0}&__EVENTVALIDATION={1}&DropDownListFuelType={2}&TextBoxPostCode={3}&ButtonSearchPostcode=Search";
string postString = string.Format(formatString, viewState, eventValidation, fuelType, postCode);
//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;
