Question

我正在开发Windows Phone 8+应用程序。我有一个包含项目详细信息的页面。每个Item都可以是另一个Item的一个组件，因此我有两个ListBox，其中包含可以组成或组成当前Item的其他Items。

        JItem.Item currItem;
        List<JItem.Item> components;
        List<JItem.Item> isComponentOf;
        protected override void OnNavigatedTo(NavigationEventArgs e)
        {

            base.OnNavigatedTo(e);

            currItemId = NavigationContext.QueryString["item_id"];

            currItem = (JItem.Item)App.itemsDict[currItemId];
            LayoutRoot.DataContext = currItem;

            components = new List<JItem.Item>();
            isComponentOf = new List<JItem.Item>();
            foreach (var x in currItem.components)
            {
                components.Add((JItem.Item)App.itemsDict[x]);
            }
            foreach (var x in currItem.composes)
            {
                isComponentOf.Add((JItem.Item)App.itemsDict[x]);
            }
            componentsLB.ItemsSource = components;
            composesLB.ItemsSource = isComponentOf;
            if (components.Count == 0) 
                componentsLabel.Visibility=Visibility.Collapsed;
            if (isComponentOf.Count == 0) 
                isComponentLabel.Visibility=Visibility.Collapsed;
        }

如果我向前导航选择componentsLB列表框或isComponentOf列表框中的项目，则一切正常。如果我通过“返回”按钮返回页面，即使System.NullReferenceException包含项目并且componentsLB.ItemsSource = components;已分配，我也会在components行上显示componentsLB。< / p>

由于它在NavigationStack上，因此页面不会重新初始化，（不会调用构造函数中的InitializeComponent()），但会分配ListBox。

当通过后退导航访问页面时，我可以使用一些变通方法（不是重新加载数据），但我想了解导致错误的原因。

这是异常的Stacktrace：起泡：

   at DOTA2_Pocket.ItemDetails.componentsLB_SelectionChanged(Object sender, SelectionChangedEventArgs e)
   at System.Windows.Controls.Primitives.Selector.OnSelectionChanged(SelectionChangedEventArgs e)
   at System.Windows.Controls.Primitives.Selector.InvokeSelectionChanged(List`1 unselectedItems, List`1 selectedItems)
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
   at System.Windows.Controls.Primitives.Selector.OnItemsChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ListBox.OnItemsChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemsControl.OnItemCollectionChanged(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Collections.Specialized.NotifyCollectionChangedEventHandler.Invoke(Object sender, NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemCollection.NotifyCollectionChanged(NotifyCollectionChangedEventArgs e)
   at System.Windows.Controls.ItemCollection.UpdateItemsSourceList(IEnumerable newItemsSource)
   at System.Windows.Controls.ItemsControl.ItemsSourceChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
   at System.Windows.DependencyObject.RaisePropertyChangeNotifications(DependencyProperty dp, Object oldValue, Object newValue)
   at System.Windows.DependencyObject.UpdateEffectiveValue(DependencyProperty property, EffectiveValueEntry oldEntry, EffectiveValueEntry& newEntry, ValueOperation operation)
   at System.Windows.DependencyObject.SetValueInternal(DependencyProperty dp, Object value, Boolean allowReadOnlySet)
   at System.Windows.Controls.ItemsControl.set_ItemsSource(IEnumerable value)
   at DOTA2_Pocket.ItemDetails.OnNavigatedTo(NavigationEventArgs e)
   at Microsoft.Phone.Controls.PhoneApplicationPage.InternalOnNavigatedTo(NavigationEventArgs e)
   at Microsoft.Phone.Controls.PhoneApplicationPage.Microsoft.Phone.Controls.IPhoneApplicationPage.InternalOnNavigatedToX(NavigationEventArgs e)
   at System.Windows.Navigation.NavigationService.RaiseNavigated(Object content, Uri uri, NavigationMode mode, Boolean isNavigationInitiator, IPhoneApplicationPage existingContentPage, IPhoneApplicationPage newContentPage)
   at System.Windows.Navigation.NavigationService.CompleteNavigation(DependencyObject content, NavigationMode mode)
   at System.Windows.Navigation.NavigationService.<>c__DisplayClass7.<NavigateCore_StartNavigation>b__4()

Answer 1

根据您提供的信息，您的 App.itemsDict 包含您要添加到组件列表变量的空引用。 ListBox的 SelectedChanged 事件然后抛出 NullReference 异常，因为它试图对该空引用执行操作。

返回导航时出现NullReferenceException

1 个答案: