WPF利用WebBrowser获取HTML元素

<Window x:Class="WpfWebBrose.MainWindow"
        xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        Title="MainWindow" Height="768" Width="1024" Loaded="Window_Loaded">
    <Grid>
        <WebBrowser x:Name="wb" />
    </Grid>
</Window>

    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            this.wb.LoadCompleted += (wb_Navigated);
        }

        private void Window_Loaded(object sender, RoutedEventArgs e)
        {
           wb.Navigate(new Uri("http://www.cnblogs.com"));
        }

        private  void wb_Navigated(object sender, NavigationEventArgs e)
        {
            var doc = this.wb.Document as HTMLDocument;
            var item = doc.getElementById("site_nav_top");
            MessageBox.Show(item.innerText);
        }

    }