小白求教，DispatcherTimer定时刷新有关问题。

小白求教，DispatcherTimer定时刷新问题。。。
小弟现在想让 手机 从某个网站定时 提取数据，并在一个textbox中显示出来。。然后代码如下

using System;

using System.Collections.Generic;

using System.Linq;

using System.Net;

using System.Windows;

using System.Windows.Controls;

using System.Windows.Documents;

using System.Windows.Input;

using System.Windows.Media;

using System.Windows.Media.Animation;

using System.Windows.Shapes;

using Microsoft.Phone.Controls;

using System.IO;

using System.Windows.Threading;

namespace WebClientHttpWebRequest

{

    public partial class MainPage : PhoneApplicationPage

    {

        public MainPage()

        {

            InitializeComponent();

            DispatcherTimer timer = new DispatcherTimer();

            timer.Interval = TimeSpan.FromSeconds(10);

            timer.Tick += timer_Tick;

            timer.Start();

        }


        

        private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)

        {

            DoWebClient();

        }


        private void DoWebClient()

        {

        }

        void timer_Tick(object sender, EventArgs e)

        {


            WebClient webClient = new WebClient();

            webClient.OpenReadAsync(new Uri("http://quote.zhijinwang.com/xml/ag.txt"));

            webClient.OpenReadCompleted += new OpenReadCompletedEventHandler(webClient_OpenReadCompleted);

        }

        

       

        void webClient_OpenReadCompleted(object sender, OpenReadCompletedEventArgs e)

        {

            using (StreamReader reader = new StreamReader(e.Result))

            {

                string contents = reader.ReadToEnd();

                int begin = contents.ToString().IndexOf("t");

                int end = contents.ToString().IndexOf("&");

                webClientTextBlock.Text = contents.ToString().Substring(begin + 5, end-begin-5 );