HiddenValues.cs

public static string ExtractHiddenValuesFromHtmlSource(string htmlSource, string hiddenControlName)
{
    string requestControlDelimiter = hiddenControlName;
    string valueDelimiter = "value=\"";

    int requestControlPosition = htmlSource.IndexOf(requestControlDelimiter);
    int requestControlValuePosition = htmlSource.IndexOf(
          valueDelimiter, requestControlPosition
       );

    int requestControlStartPosition = requestControlValuePosition +
                                 valueDelimiter.Length;
    int requestControlEndPosition = htmlSource.IndexOf("\"", requestControlStartPosition);


    string requestControlValue = htmlSource.Substring(
       requestControlStartPosition,
       requestControlEndPosition - requestControlStartPosition);

    return HttpUtility.UrlEncode(requestControlValue);
}