เขียนแอพ WP 8 Connect to Web Server ไม่สำเร็จ ควรแก้ไขอย่างไร ต้อง Add Reference เพิ่มไหม

using System;
using System.Windows;
using Microsoft.Phone.Controls;
using Microsoft.Phone.Shell;
using System.Net;

namespace PhoneApp
{
    public partial class MainPage : PhoneApplicationPage
    {

        // Constructor
        public MainPage()
        {
            InitializeComponent();

            Loaded += MainPage_Loaded;
        }

        WebClient client;

        private void MainPage_Loaded(object sender, System.Windows.RoutedEventArgs e)
        {
            string url = "http://localhost/myphp/hello.php";
            Uri uri = new Uri(url);
            client = new WebClient();
            client.AllowReadStreamBuffering = true;

            client.DownloadStringCompleted += client_DownloadStringCompleted;
            client.DownloadProgressChanged += client_DownloadProgressChanged;

            client.DownloadStringAsync(uri);
        }

        private void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {

            if (e.Cancelled == false && e.Error == null)
            {
                System.Threading.Thread.Sleep(2000);
                string textString = Convert.ToString(e.Result);
                this.txtResult.Text = textString;
            }

        }

        private void client_DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
        {
            this.txtResult.Text = "Loading...";
        }

    }
}