Logging in with http to Xing

Question

I have this code to log in to xing:

class Programm
{
    private static HttpWebRequest request;
    private static HttpWebResponse response;
    public static void Main()
    {

        //string targetUrl = "/app/forum/seoparser/bpmn/";
        string requestUrl = "http://www.xing.com/app/user";
        //Diese Parameter werden per Post Methode übergeben
        string requestParameter = "login_user_name=" + HttpUtility.UrlEncode("foo")
                                  + "&login_password=" + HttpUtility.UrlEncode("foo")
                                        + "&op=login"; 
                                        //+ "&dest=" + HttpUtility.UrlEncode(targetUrl);


        request = (HttpWebRequest)WebRequest.Create(requestUrl);

        #region Schreib Werte in den RequestStream der an die URL geschickt wird
        byte[] parameter = Encoding.UTF8.GetBytes(requestParameter);
        //request.Method = "GET";
        request.Method = "POST";
        request.ContentType = "application/x-www-form-urlencoded";
        request.ContentLength = parameter.Length;
        //request.Credentials = CredentialCache.DefaultCredentials;

        Stream requestStream = request.GetRequestStream();
        requestStream.Write(parameter, 0, parameter.Length);
        requestStream.Close();

        #endregion

        //Holt sich die Antwort
        response = (HttpWebResponse)request.GetResponse();

        #region Liest die Antwort (ResponseStream) und gibt sie auf der Console aus
        Stream responseStream = response.GetResponseStream();
        StreamReader streamReader = new StreamReader(responseStream);
        //Console.WriteLine(streamReader.ReadToEnd());
        string result = streamReader.ReadToEnd();
        streamReader.Close();
        responseStream.Close();
        #endregion

        #region write to file
        TextWriter tw = new StreamWriter("c:/Upload/result.html");
        tw.WriteLine(result);
        tw.Close();
        #endregion

    }
}

The result I get is the same page I get, when I type

http://www.xing.com/app/user

into the browser, but not logged in.

When I type this into the browser:

https://www.xing.com/app/user?op=login&login_user_name=foo&login_password=foo

I am getting the correct result - i.e. the response page after loggin in. Even when I logged out before.

How can I log in correctly using .Net?