问https抛出“找不到远程服务器”windows phone 7

EN

private void LoginRequest()
        {
            try
            {

         var httpLoginRequest = (HttpWebRequest)WebRequest.Create(new Uri("https:xxxxx"                             + "?" +"username=" + UserName_textBox.Text + "&" + "password="+ Password_textBox.Password));
                httpLoginRequest.Method = DisplayMessage.Get_Method;
                Parameters = new Dictionary<string, string>();
                httpLoginRequest.BeginGetResponse(new AsyncCallback(GetLoginCallback), httpLoginRequest);
            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        private void GetLoginCallback(IAsyncResult asynchronousResult)
        {
            try
            {
               HttpWebRequest httpRequest = HttpWebRequest)asynchronousResult.AsyncState;
               HttpWebResponse httpresponse = (HttpWebResponse)httpRequest.EndGetResponse(asynchronousResult);
                Stream streamResponse = httpresponse.GetResponseStream();
                using(StreamReader streamRead = new StreamReader(streamResponse))
                {
                    var response = streamRead.ReadToEnd(); 
                }
         }
catch(WebException ex){}}