HttpwebRequest - 带ViewState的网页POST请求

这是我今天下午碰到的案例，一个退订页面的post请求，请求头信息都很明确，but看看下面这个请求体，除了最后一个key是我的页面控件名称，其他的几个ViewState相关都是what呢？（ViewState原理参考http://www.cnblogs.com/wwan/archive/2010/11/18/1880357.html），下面是怎么处理这种情况的Post请求的办法。

1. 拿到接口地址，参数，拼串发送Get请求，拿到响应体(如下图，看高亮)

2. 从响应体里提取我们想要的ViewState相关，并保持在相应的字符串里面

3. 拼出来我们想要的请求体，对刚才的地址重新发送Post请求

亲测，可以解决问题。

请求头 Content-Type：Content-Type: application/x-www-form-urlencoded

  public static string unsubResponse(string UnsubURL)
        {
            try
            {
                // Get the ViewState and EventValidation
                HttpWebRequest request = WebRequest.Create(UnsubURL) as HttpWebRequest;
                request.Method = "GET";
                request.KeepAlive = false;

                // Get the response
                HttpWebResponse response = request.GetResponse() as HttpWebResponse;
                Stream responseStream = response.GetResponseStream();
                StreamReader reader = new 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 __VIEWSTATEGENERATOR
                string viewStateGeneratorFlag = "id=\"__VIEWSTATEGENERATOR\" value=\"";
                i = srcString.IndexOf(viewStateGeneratorFlag) + viewStateGeneratorFlag.Length;
                j = srcString.IndexOf("\"", i);
                string viewStateGenerator = srcString.Substring(i, j - i);

                // Get the __EVENTVALIDATION
                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);
                viewStateGenerator = Uri.EscapeDataString(viewStateGenerator);
                eventValidation = Uri.EscapeDataString(eventValidation);
                string unsubButton = Uri.EscapeDataString("Unsubscribe");

                string formatString = "__VIEWSTATE={0}&__VIEWSTATEGENERATOR={1}&__EVENTVALIDATION={2}&ButtonUnsubscribe={3}";
                string postString = string.Format(formatString, viewState, viewStateGenerator, eventValidation, unsubButton);

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

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

                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 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");
            }
            return (null);
        }