为什么我的HttpWebRequest重定向到登录页面

我有一个HttpWebRequest来抓取会话ID。然后从响应中获取cookie，将其添加到第二个请求中以获得所需的页面。

如果使用IIS/7.5，会出现什么情况?我正在使用小提琴手，并获得302状态。我正在获取asp.net SessionID。

   CookieContainer myCookies = new CookieContainer();
    HttpWebRequest req = (HttpWebRequest)HttpWebRequest.Create("https://www.secure.com/login.aspx");
    req.Method = "POST";
    req.Credentials = fileretrieve.Credentials;//Network credentials. 
    req.CookieContainer = myCookies;
    req.AllowAutoRedirect = true;
    byte[] bytes = System.Text.Encoding.ASCII.GetBytes(req.Credentials.ToString());
    req.ContentLength = bytes.Length;
    System.IO.Stream os = req.GetRequestStream();
    os.Write(bytes, 0, bytes.Length);
    os.Close();
    HttpWebResponse resp = (HttpWebResponse)req.GetResponse();
    HttpWebRequest xmlreq = (HttpWebRequest)HttpWebRequest.Create("https://www.secure.com/file");
    xmlreq.Method = "GET";
    xmlreq.KeepAlive = true;
    xmlreq.AllowAutoRedirect = true;
    xmlreq.CookieContainer = req.CookieContainer;

    HttpWebResponse xmlresp = (HttpWebResponse)xmlreq.GetResponse();
     string webpage;
    System.IO.StreamReader sr = new System.IO.StreamReader(xmlresp.GetResponseStream());
    webpage = sr.ReadToEnd();