我在尝试使用我的 RSS 阅读器中的以下代码导航回之前的 MainPage.xaml 时遇到空引用异常。有人可以通过建议我需要做些什么来避免这个错误来帮助我吗？
public partial class FeedDetailsPage : PhoneApplicationPage
{
object _selectedFeedItem;
object _selectedFeed;
public FeedDetailsPage()
{
InitializeComponent();
Loaded += new RoutedEventHandler(PhoneApplicationPage_Loaded);
LoadFeed();
}
private void LoadFeed()
{
FrameworkElement root = Application.Current.RootVisual as FrameworkElement;
var currentFeed = root.DataContext as ATP_Tennis_App.ViewModels.FeedViewModel;
WebClient wc = new WebClient();
wc.DownloadStringCompleted += new DownloadStringCompletedEventHandler(wc_DownloadStringCompleted);
wc.DownloadStringAsync(new Uri(currentFeed.FeedUrl));
_selectedFeed = currentFeed;
}
void wc_DownloadStringCompleted(object sender, DownloadStringCompletedEventArgs e)
{
// if (e.Error != null) { return; }
var rssFeed = XElement.Parse(e.Result);
var currentFeed = this.DataContext as ATP_Tennis_App.ViewModels.FeedViewModel;
var items = from item in rssFeed.Descendants("item")
select new ATP_Tennis_App.ViewModels.FeedItemViewModel()
{
Title = item.Element("title").Value,
DatePublished = DateTime.Parse(item.Element("pubDate").Value),
Url = item.Element("link").Value,
Description = item.Element("description").Value
};
foreach (var item in items)
currentFeed.Items.Add(item);
}
private void PhoneApplicationPage_BackKeyPress(object sender, System.ComponentModel.CancelEventArgs e)
{
// Cancel default navigation
e.Cancel = true;
PhoneApplicationFrame root = (PhoneApplicationFrame)Application.Current.RootVisual;
root.GoBack();
}
private void PhoneApplicationPage_Loaded(object sender, RoutedEventArgs e)
{
this.DataContext = _selectedFeed;
}
private void lstFeeds_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
_selectedFeedItem = (sender as ListBox).SelectedItem;
NavigationService.Navigate(new Uri("/FeedItemDetailsPage.xaml", UriKind.Relative));
FrameworkElement root = Application.Current.RootVisual as FrameworkElement;
root.DataContext = _selectedFeedItem;
}
}
在线抛出异常
wc.DownloadStringAsync(new Uri(currentFeed.FeedUrl));