为什么网络客户端不显示网页

下面的代码可以编译，但不显示网站的文字或照片。我只想运行一个涉及与 Silverlight 进行 HTTP 通信的程序。知道为什么我没有显示网站吗？我只是在文本块中显示"发生错误"

namespace SilverlightApplication4
{
    public partial class MainPage : UserControl
    {
        string baseUri = "http://yahoo.com";
        WebClient client = new WebClient();            
        public MainPage()
        {
            InitializeComponent();
            client.DownloadStringCompleted += new DownloadStringCompletedEventHandler(client_DownloadStringCompleted);        
        }
        private void Button1_Click(object sender, RoutedEventArgs e)
        {
            client.DownloadStringAsync(new Uri(baseUri));
        }
        void client_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
        {
            if (e.Error == null)
                textBox.Text = "Using WebClient: "+  e.Result;
            else
            {
                textBox.Text = e.Error.Message;
                textBlock.Text = "error occurred";
            }
        }
    }
}