为什么 ObservableCollection 被修改时会抛出异常？

Question

我的应用程序使用 WPF DataGrid。其中一列是模板列，其中包含绑定到提供该行的实体的 ObservableCollection 的 ComboBox。当我向 ObservableCollection 添加值时，会引发 NullReferenceException。

有人知道为什么会发生这种情况吗？这是异常的堆栈跟踪：

   at MS.Internal.Data.PropertyPathWorker.DetermineWhetherDBNullIsValid()
   at MS.Internal.Data.PropertyPathWorker.get_IsDBNullValidForUpdate()
   at MS.Internal.Data.ClrBindingWorker.get_IsDBNullValidForUpdate()
   at System.Windows.Data.BindingExpression.ConvertProposedValue(Object value)
   at System.Windows.Data.BindingExpressionBase.UpdateValue()
   at System.Windows.Data.BindingExpression.Update(Boolean synchronous)
   at System.Windows.Data.BindingExpressionBase.Dirty()
   at System.Windows.Data.BindingExpression.SetValue(DependencyObject d, DependencyProperty dp, Object value)
   at System.Windows.DependencyObject.SetValueCommon(DependencyProperty dp, Object value, PropertyMetadata metadata, Boolean coerceWithDeferredReference, OperationType operationType, Boolean isInternal)
   at System.Windows.DependencyObject.SetValue(DependencyProperty dp, Object value)
   at System.Windows.Controls.Primitives.Selector.UpdatePublicSelectionProperties()
   at System.Windows.Controls.Primitives.Selector.SelectionChanger.End()
   at System.Windows.Controls.Primitives.Selector.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.Data.CollectionView.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.Windows.Controls.ItemCollection.System.Windows.IWeakEventListener.ReceiveWeakEvent(Type managerType, Object sender, EventArgs e)
   at System.Windows.WeakEventManager.DeliverEventToList(Object sender, EventArgs args, ListenerList list)
   at System.Windows.WeakEventManager.DeliverEvent(Object sender, EventArgs args)
   at System.Collections.Specialized.CollectionChangedEventManager.OnCollectionChanged(Object sender, NotifyCollectionChangedEventArgs args)
   at System.Windows.Data.CollectionView.OnCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.Windows.Data.ListCollectionView.ProcessCollectionChangedWithAdjustedIndex(NotifyCollectionChangedEventArgs args, Int32 adjustedOldIndex, Int32 adjustedNewIndex)
   at System.Windows.Data.ListCollectionView.ProcessCollectionChanged(NotifyCollectionChangedEventArgs args)
   at System.Windows.Data.CollectionView.OnCollectionChanged(Object sender, NotifyCollectionChangedEventArgs args)
   at System.Collections.ObjectModel.ObservableCollection`1.OnCollectionChanged(NotifyCollectionChangedEventArgs e)
   at System.Collections.ObjectModel.ObservableCollection`1.InsertItem(Int32 index, T item)
   at System.Collections.ObjectModel.Collection`1.Add(T item)
   at ORF.PersonBook.IdentityModule.Model.SubsidiaryModel.AddRoom(RoomModel room) in C:\Project\Phoenix\Development\src\ORF.PersonBook.IdentityModule\Model\SubsidiaryModel.cs:line 127

Answer 1

最后我找到了异常的原因。当您从列表中删除所选项目时会出现此问题。下面我发布了一段不完整的代码。

<!-- XAML -->
<ListBox ItemsSource="{Binding Rooms}" SelectedItem="{Binding SelectedRoom}">
    <ListBox.ItemTemplate>
        <DataTemplate>
            <DockPanel>
                <Button Content="remove" DockPanel.Dock="Right" Command="{Binding Some Remove Command}" />
                <TextBlock Text="{Binding Name}" />
            </DockPanel>
        </DataTemplate>
    </ListBox.ItemTemplate>
</ListBox>

和视图模型：

// View Model
public ObservableCollection<Room> Rooms { get; private set; }
public Room SelectedRoom { get; set; }

public RemoveRoom()
{
    var room = SelectedRoom;
    //SelectedRoom = null; // Uncomment this line to avoid the exception

    Rooms.Remove(room);
}

解决方案是首先将所选项目设置为 null（或任何其他项目），然后再实际从列表中删除该项目。

奥利弗·哈纳皮

Answer 2

您的 Room 类对象实际上已实例化吗？
看起来好像您添加了一个未初始化的值。