i am sending a request to server with json formet and wants to store a result coming from service (json formet)?

public MainPage()
        {
            InitializeComponent();
            StringBuilder postData = new StringBuilder();
            WebClient webClient = new WebClient();
            var tsLong = DateTime.Now.Millisecond / 1000; 
            webClient.Headers[HttpRequestHeader.ContentType] = "application/x-www-form-urlencoded";
            var uri = new Uri("URL");

            postData.AppendFormat("{0}={1}", "className", HttpUtility.UrlEncode("ebuskiMobile"));
            postData.AppendFormat("{0}={1}", "methodName", HttpUtility.UrlEncode("m_getContents"));
            postData.AppendFormat("{0}={1}", "osName", HttpUtility.UrlEncode("Windows"));
            postData.AppendFormat("{0}={1}", "osVersion", HttpUtility.UrlEncode("1.0"));
            postData.AppendFormat("{0}={1}", "appVersion", HttpUtility.UrlEncode("1.0"));
            postData.AppendFormat("{0}={1}", "width", HttpUtility.UrlEncode("1080"));
            postData.AppendFormat("{0}={1}", "height", HttpUtility.UrlEncode("1200"));
          

            webClient.Headers[HttpRequestHeader.ContentLength] = postData.Length.ToString();
            webClient.UploadStringCompleted += new UploadStringCompletedEventHandler(webClient_UploadRegisterStringCompleted);
            //    webClient.UploadProgressChanged += webClient_UploadRegisterProgressChanged;
            webClient.UploadStringAsync(uri, "POST", postData.ToString());
        }

        public class RootObject
        {
            public string message { get; set; }
        }
        private void webClient_UploadRegisterStringCompleted(object sender, UploadStringCompletedEventArgs e)
        {

//exeption here
            string jsonstring = e.Result;
            RootObject rootobj = new RootObject();
            MemoryStream ms = new MemoryStream(Encoding.UTF8.GetBytes(jsonstring));
            DataContractJsonSerializer ser = new DataContractJsonSerializer(rootobj.GetType());
            rootobj = ser.ReadObject(ms) as RootObject;
            string status = rootobj.message;
            MessageBox.Show(status);
          //  NavigationService.Navigate(new Uri("/MISTFMGE;component/Views/Login.xaml", UriKind.RelativeOrAbsolute));
            IsolatedStorageSettings settings = IsolatedStorageSettings.ApplicationSettings;
          
        }



        private void btnSkip_Click(object sender, RoutedEventArgs e)
        {
            NavigationService.Navigate(
            new Uri("/EntryPoll.xaml", UriKind.Relative));
        }



    }