当我删除列表框中的项目时，出现问题中的错误，如下面的屏幕截图所示:

我不知道内部异常在哪里，但我尝试了 try, catch 但我在问题中遇到了同样的错误。

这里是所有的代码:

namespace WpfApplication7
 {
/// <summary>
/// Interaction logic for Edit_Rooms.xaml
/// </summary>
public partial class Edit_Rooms : Window
{
    public Edit_Rooms()
    {
        InitializeComponent();
    }

    //initialises entities
    WpfApplication7.AllensCroftEntities1 allensCroftEntities1 = new WpfApplication7.AllensCroftEntities1();
    private Room ObjectIndex;

    private void Window_Loaded(object sender, RoutedEventArgs e)
    {
        // Load data into Rooms. 
        System.Windows.Data.CollectionViewSource roomsViewSource = ((System.Windows.Data.CollectionViewSource)(this.FindResource("roomsViewSource")));
        //runs a query to go the roomsQuery to get the rooms table from the entities
        System.Data.Objects.ObjectQuery<WpfApplication7.Room> roomsQuery = this.GetRoomsQuery(allensCroftEntities1);
        //used when adding new rooms
        roomsViewSource.Source = roomsQuery.Execute(System.Data.Objects.MergeOption.AppendOnly);
    }

    private System.Data.Objects.ObjectQuery<Room> GetRoomsQuery(AllensCroftEntities1 allensCroftEntities1)
    {
        System.Data.Objects.ObjectQuery<WpfApplication7.Room> roomsQuery = allensCroftEntities1.Rooms;
        // Returns an ObjectQuery.
        return roomsQuery;
    }

    private void btnDelete_Click(object sender, RoutedEventArgs e)
    {
        //prevents user trying to delete nothing/unselected row
        if (ObjectIndex == null)
        {
            MessageBox.Show("Cannot delete the blank entry");
        }
        else
        {
                //deletes object from dataset, saves it and outputs a message
                allensCroftEntities1.DeleteObject(ObjectIndex);
                allensCroftEntities1.SaveChanges();
                MessageBox.Show("Room Deleted");
        }
    }

    private void btnSave_Click(object sender, RoutedEventArgs e)
    {
        try
        {
            //attempts to save changes
            allensCroftEntities1.SaveChanges();
            MessageBox.Show("Saved");
        }
        catch (Exception ex)
        {
            //if unsuccessful, outputs an error message
            MessageBox.Show(ex.ToString());
        }
    }

    private void btnFirst_Click(object sender, RoutedEventArgs e)
    {
        listbox.SelectedIndex = 0;
    }

    private void btnPrevious_Click(object sender, RoutedEventArgs e)
    {
        //prevents user going to the previous item before the first item
        if (listbox.SelectedIndex > 0)
        {
            listbox.SelectedIndex -= 1;
        }
    }

    private void btnNext_Click(object sender, RoutedEventArgs e)
    {
        //prevents user going after last item and throwing up an error
        if (listbox.SelectedIndex < listbox.Items.Count)
        {
            listbox.SelectedIndex += 1;
        }
    }

    private void btnLast_Click(object sender, RoutedEventArgs e)
    {
        listbox.SelectedIndex = listbox.Items.Count - 1;
    }

    private void listbox_SelectionChanged(object sender, SelectionChangedEventArgs e)
    {
        //outputs index of the selected room
        ObjectIndex = listbox.SelectedItem as Room;
    }
}
}