RadListBox DragDropBehavior绑定

Question

我正在尝试在RadListBox的DragDropBehavior中使用绑定，就像这样

    <UserControl xmlns:telerik="http://schemas.telerik.com/2008/xaml/presentation"  x:Class="MyProject.Views.MyView"
                 ....
                 xmlns:behaviors="clr-namespace:MyProject.Behaviors"
                 mc:Ignorable="d"
                 d:DesignHeight="300" d:DesignWidth="300">
   <Grid>
        ...
        <telerik:RadListBox ItemsSource="{Binding Items}">
                    <telerik:RadListBox.DragDropBehavior>
                        <behaviors:MyDragDropBehavior AllowReorder="True" DropCommand="{Binding RelativeSource={RelativeSource AncestorType={x:Type UserControl}}, Path=DataContext.DropCommand}"/>
                    </telerik:RadListBox.DragDropBehavior>
        </telerik:RadListBox>
    </Grid>
</UserControl>

View通过注入获取viewmodel

public partial class MyView : UserControl 
    {
        public MyView (ViewModels.MyViewModel viewModel)
        {
            InitializeComponent();
            DataContext = viewModel;
        }
    }

行为代码：

public class MyDragDropBehavior : Telerik.Windows.DragDrop.Behaviors.ListBoxDragDropBehavior
    {
        public override bool CanDrop(Telerik.Windows.DragDrop.Behaviors.DragDropState state)
        {
            return state.IsSameControl;
        }

        public override void Drop(Telerik.Windows.DragDrop.Behaviors.DragDropState state)
        {
            base.Drop(state);
            DropCommand.Execute(null);
        }

        public ICommand DropCommand
        {
            get { return (ICommand)GetValue(DropCommandProperty); }
            set { SetValue(DropCommandProperty, value); }
        }

        public static readonly DependencyProperty DropCommandProperty =
            DependencyProperty.Register("DropCommand", typeof(ICommand), typeof(MyDragDropBehavior), new PropertyMetadata(null));
    }

项目绑定运作良好。行为是有效的，但不是绑定到DropCommand。我获得了绑定错误：

无法找到与引用'RelativeSource FindAncestor绑定的源，AncestorType ='System.Windows.Controls.UserControl'，AncestorLevel ='1''。 BindingExpression：路径= DataContext.DropCommand;的DataItem = NULL; target元素是'MyDragDropBehavior'（HashCode = 25707777）; target属性是'DropCommand'（类型'ICommand'）

ViewModel是

public class MyViewModel
    {
        public MyViewModel()
        {
            DropCommand = new DelegateCommand(OnDrop);
            Items = new ObservableCollection<MyItem>();
        }

        public ObservableCollection<MyItem> Items { get; set; }

        public DelegateCommand DropCommand { get; set; }

        private void OnDrop()
        {

        }
    }

有什么问题？

我找到了一种通过以下方式解决问题的方法

<telerik:RadListBox DragDropBehavior="{Binding DragDropBehavior}">

但我仍然不明白为什么以前的方法不起作用。如果有人知道，我会很感激。