WPF 将TreeView节点信息拖拽到TextBox显示，但是TextBox不能作为拖放目标

查找好多资料说用PreviewDrop和PreviewDragOver，PreviewDragOver能进入，但是PreviewDrop不能触发！

 private void _value_PreviewDrop(object sender, DragEventArgs e)
        {
            MessageBox.Show("OK");
            TreeViewItem item = e.OriginalSource as TreeViewItem;
            string s = string.Empty;
            JsonHeaderLogic c = item.DataContext as JsonHeaderLogic;
        }

        private void _value_PreviewDragOver(object sender, DragEventArgs e)
        {
            e.Effects = DragDropEffects.Copy;
            e.Handled = true;
        }

知道了！！！
private void _value_PreviewDragOver(object sender, DragEventArgs e)
{
e.Effects = DragDropEffects.Copy;/****/改为.Move****
e.Handled = true;
}