如何在c#的SelectionChanged方法中通过按钮更改Uri

我用c#为windows 8开发了一个游戏指南应用程序。我试图把2个youtube视频在游戏的特定章节，我想通过一个按钮改变Webview的Uri。当按钮未按下时，将显示第一个视频uri，当按钮按下时，将显示第二个视频。我怎么能做到呢?例如:

    private void Chapters_SelectionChanged(object sender, Windows.UI.Xaml.Controls.SelectionChangedEventArgs e)
    {           
        switch (Chapters.SelectedIndex)
        { 
            case 0:
                Chapter2C.Visibility = Visibility.Visible;
                Chapter2T.Visibility = Visibility.Visible;
                next_but.Visibility = Visibility.Visible;
                prev_but.Visibility = Visibility.Visible;
                Mywebview.Visibility = Visibility.Visible;
                if (next_button.Clicked)
                 {
                    Mywebview.Source = new Uri("http://www.youtube.com/embed/F65paVQ0wn8?feature=player_detailpage") 
                 }
                   else 
                 {
                    Mywebview.Source = new Uri ("http://www.youtube.com/embed/rLdrc7K5vdw");
                 }
                 break;
        }
   }

任何人? ?

<ToggleButton x:Name="btnChangeChapter" Content="Change Chapter" Click="btnChangeChapter_Click_1"/>
private void btnChangeChapter_Click_1(object sender, RoutedEventArgs e)
{
    /* TODO: use property btnChangeChapter.IsChecked.Value in SelectionChanged event,
            it's bool value */
}