c#代码在silverlight中不显示搜索按钮按下的数据(基础教程)

我是c# silverlight初学者。我正在练习我在这个链接http://weblogs.asp.net/scottgu/pages/silverlight-tutorial-part-3-using-networking-to-retrieve-data-and-populate-a-datagrid.aspx上找到的一些示例

我已经准确地写了这个链接中的代码。这个链接提供了一个GUI从给定的链接搜索"主题"(请参阅代码了解详细信息)。

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 System.Xml;
using System.Xml.Linq;
using System.Windows.Messaging;
namespace shekhar_basic
{
    public partial class MainPage : UserControl
    {
        public MainPage()
        {
            InitializeComponent();
        }
        void SearchBtn_Click(object sender, RoutedEventArgs e)
        {
            string topic = watermark1.Text; //this topic receives the data correctly written on watermark, this watermark1 is the name of that button using xml.
            string diggUrl = String.Format("http://services.digg.com/stories/topic/{0}?count=20&appkey=http%3A%2F%2Fscottgu.com", topic); //I think the problem creating part is here.
            WebClient diggServices = new WebClient();
            diggServices.DownloadStringCompleted += new DownloadStringCompletedEventHandler(DiggService_DownloadStoriesCompleted);
            diggServices.DownloadStringAsync(new Uri(diggUrl));
        }
        void DiggService_DownloadStoriesCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error == null)
            {
                string result = e.Result;                
            }
        }
        void DisplayStories(string xmlContent)
        {
            XDocument xmlStories = XDocument.Parse(xmlContent);
            var stories = from story in xmlStories.Descendants("story")
                          where story.Element("thumbnail") != null && !story.Element("thumbnail").Attribute("src").Value.EndsWith(".gif")
                          select new digStory
                          {
                              Id = (int)story.Attribute("id"),
                              Title = ((string)story.Element("title")).Trim(),
                              Description = ((string)story.Element("description")).Trim(),
                              ThumbNail = (string)story.Element("thumbnail").Attribute("src").Value,
                              HrefLink = new Uri((string)story.Attribute("link")),
                              NumDiggs = (int)story.Attribute("diggs"),
                              UserName = (string)story.Element("user").Attribute("name").Value,
                          };
            dGStoreList1.SelectedIndex = -1;
            dGStoreList1.ItemsSource = stories;
        }
    }
}

我的xml代码是:

<UserControl x:Class="shekhar_basic.MainPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
             xmlns:local="clr-namespace:Microsoft.Windows.Controls;assembly=Microsoft.Windows.Controls.WatermarkedTextBox"
    mc:Ignorable="d" xmlns:sdk="http://schemas.microsoft.com/winfx/2006/xaml/presentation/sdk"> 

    <Grid Background="AntiqueWhite">
        <Grid.RowDefinitions >
            <RowDefinition Height="40" />
            <RowDefinition Height="*" />     
        </Grid.RowDefinitions>
        <Grid Grid.Row="0" Margin="7" ShowGridLines="True" >
            <Grid.ColumnDefinitions>
                <ColumnDefinition Width="*"/>
                <ColumnDefinition Width="120"/>
                <ColumnDefinition Width="50"/>
            </Grid.ColumnDefinitions>
            <Border Grid.Column="0" CornerRadius="10" Background="Turquoise" >
                <TextBlock Text=" Dig search" Foreground="Blue"  />                 
            </Border>
            <local:WatermarkedTextBox Name ="watermark1"  Grid.Column="1" Watermark="Enter the search" Margin="0,0,-7,0" />
            <Button Grid.Column="2" Content="search" Click="SearchBtn_Click" />
        </Grid>
        <TextBlock Grid.Row="1" Margin="10" Foreground="Black">
            Todo : Stories will display here
        </TextBlock>
        <sdk:DataGrid Grid.Row="1"  Margin="5" Name="dGStoreList1">
            </sdk:DataGrid>
    </Grid>
</UserControl>

与此代码对应的GUI是:http://prntscr.com/34k0d3。它应该显示的是这样的东西:http://prntscr.com/34k0lx，你可以看到在"watermark button"里面他们写了"电视"，然后点击"搜索"按钮，它显示了一些结果。但是我的代码不能显示这些结果，我已经调试了我的代码，它显示了我的watermark button在"topic"变量内输入的字符串在我的代码行"string topic = watermark1.Text;"这意味着按钮点击甚至是正确生成的，它正在接收我在"search"点击上的水印按钮输入的内容。

我猜问题可能在这两行:

diggServices.DownloadStringCompleted += new DownloadStringCompletedEventHandler(DiggService_DownloadStoriesCompleted);
            diggServices.DownloadStringAsync(new Uri(diggUrl));

谁能帮我在"search"上显示结果点击按钮在"watermarkbutton"对应的输入字符串?非常感谢你的帮助。编辑:还有一件事我想提一下，我得到这种对话框上运行代码VS. http://prntscr.com/34ka0u，当我点击"是"，然后我得到这种窗口http://prntscr.com/34kabo显示一些错误。