byte[] response;

WebClient webClient = new WebClient();
response = webClient.DownloadData(LOGIN_URL);

string viewstate = ExtractViewState(
      Encoding.ASCII.GetString(response)
   );

string postData = String.Format(
   "__VIEWSTATE={0}&UsernameTextBox={1}&PasswordTextBox={2}&LoginButton=Login",
   viewstate, USERNAME, PASSWORD);

webClient.Headers.Add("Content-Type", "application/x-www-form-urlencoded");
response = webClient.UploadData(
        LOGIN_URL, "POST", Encoding.ASCII.GetBytes(postData)
    );

private string ExtractViewState(string s)
{
   string viewStateNameDelimiter = "__VIEWSTATE";
   string valueDelimiter = "value=\"";
            
   int viewStateNamePosition = s.IndexOf(viewStateNameDelimiter);     
   int viewStateValuePosition = s.IndexOf(
         valueDelimiter, viewStateNamePosition
      );

   int viewStateStartPosition = viewStateValuePosition + 
                                valueDelimiter.Length;
   int viewStateEndPosition = s.IndexOf("\"", viewStateStartPosition);

   return HttpUtility.UrlEncodeUnicode(
            s.Substring(
               viewStateStartPosition, 
               viewStateEndPosition - viewStateStartPosition
            )
         );  
}

http://odetocode.com/articles/162.aspx

How to use WebClient to post to an ASP.Net page and maintain viewstate for proxy purposes.

Screen Scraping, ViewState, and Authentication using ASP.Net

c, aspnet

C#