WebClient post data to .aspx page

 1WebClient client = new WebClient();
 2            client.Headers.Add("Accept", "image/gif,image/x-xbitmap,image/jpeg,image/pjpeg,application/x-shockwave-flash,application/vnd.ms-excel,application/vnd.ms-powerpoint,application/msword,*/*");
 3            client.Headers.Add("Accept-Language", "zh-cn");
 4            client.Headers.Add("Accept-Encoding", "gzip, deflate");
 5            client.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
 6
 7            string postString = "";
 8            postString += HttpUtility.UrlEncode("__VIEWSTATE");
 9            postString += "=";
10            postString += HttpUtility.UrlEncode("/wEPDwUJNzM4ODg2MTUxZGRqlR5kSksCPGmgkbM/P3qjvjwnnQ");
11            postString += "==";
12            postString += "&";
13            postString += HttpUtility.UrlEncode("TextBox1");
14            postString += "=";
15            postString += HttpUtility.UrlEncode("aa"); ;
16            postString += "&";
17            postString += HttpUtility.UrlEncode("Button2");
18            postString += "=";
19            postString += HttpUtility.UrlEncode("Post"); ;
20            postString += "&";
21            postString += HttpUtility.UrlEncode("__EVENTVALIDATION");
22            postString += "=";
23            postString += HttpUtility.UrlEncode("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");
24            postString += "=";
25            byte[] postData = System.Text.Encoding.ASCII.GetBytes(postString);
26            byte[] returnData = client.UploadData(formatUrl, "POST", postData);
27            returnString = Encoding.Default.GetString(returnData);
28
29            //其中"__VIEWSTATE很重要;