屏幕抓取ASP.. NET页面不工作

我试图在以下站点带回页面上的日历事件:http://www.wphospital.org/News-Events/Calendar-of-Events.aspx

请注意，这个站点有一个名为"Month"的链接-我需要能够POST请求特定月份的日历事件的数据。我没法让它工作。下面是代码:

private static void GetData(ref string buf)
{
    try
    {
        //First, request the search form to get the viewstate value 
        HttpWebRequest webRequest = default(HttpWebRequest);
        webRequest = (HttpWebRequest)System.Net.WebRequest.Create("http://www.wphospital.org/News-Events/Calendar-of-Events.aspx");
        StreamReader responseReader = new StreamReader(webRequest.GetResponse().GetResponseStream());
        string responseData = responseReader.ReadToEnd();
        responseReader.Close();
        //Extract the viewstate value and build out POST data 
        string viewState = ExtractViewState(responseData);
        string eventValidation = ExtractEventValidation(responseData);
        string postData = null;
        postData = String.Format("ctl00$manScript={0}&__EVENTTARGET=&__EVENTARGUMENT&__LASTFOCUS=&__VIEWSTATE={1}&lng={2}&__EVENTVALIDATION={3}&ctl00$searchbox1$txtWord={4}&textfield2={5}&ctl00$plcMain$lstbxCategory={6}&ctl00$plcMain$lstbxSubCategory={7}", "ctl00$plcMain$updMonthNav|ctl00$plcMain$btnNextMonth", viewState, "en-US", eventValidation, "Search", "your search here", 0, 0);
        var encoding = new ASCIIEncoding();
        byte[] data = encoding.GetBytes(postData);
        //Now post to the search form 
        webRequest = (HttpWebRequest)System.Net.WebRequest.Create("http://www.wphospital.org/News-Events/Calendar-of-Events.aspx");
        webRequest.UserAgent = "Mozilla/4.0 (compatible; MSIE 7.0; Windows NT 5.1)";
        webRequest.Method = "POST";
        webRequest.ContentType = "application/x-www-form-urlencoded";
        webRequest.ContentLength = data.Length;
        var newStream = webRequest.GetRequestStream();
        newStream.Write(data, 0, data.Length);
        newStream.Close();
        responseReader = new StreamReader(webRequest.GetResponse().GetResponseStream());
        //And read the response 
        responseData = responseReader.ReadToEnd();
        responseReader.Close();
        buf = responseData;
    }
    catch (WebException ex)
    {
        if (ex.Status == WebExceptionStatus.ProtocolError)
        {
            Console.Write("The server returned protocol error ");
            // Get HttpWebResponse so that you can check the HTTP status code.
            HttpWebResponse httpResponse = (HttpWebResponse)ex.Response;
            int sc = (int)httpResponse.StatusCode;
            string strsc = httpResponse.StatusCode.ToString();
        }
    }
}
private static 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));
}