private void puzzles_PreviewMouseLeftButtonDown_1(object sender, MouseButtonEventArgs e)
        {
            ListBox parent = (ListBox)sender;
            dragSource = parent;
            //MessageBox.Show(e.GetPosition(parent).X.ToString());
            //MessageBox.Show(e.GetPosition(parent).Y.ToString());
            object data = GetDataFromListBox(dragSource, e.GetPosition(parent));

            if (data != null)
            {
                DragDrop.DoDragDrop(parent, parent.Items.GetItemAt(2), DragDropEffects.Move);
            }
          

        }

private static object GetDataFromListBox(ListBox source, Point point)
        {
            UIElement element = source.InputHitTest(point) as UIElement;
            if (element == null)
            {
                MessageBox.Show("空的！");
                return null;
            }
            else
            {
                object data = DependencyProperty.UnsetValue;
                while (data == DependencyProperty.UnsetValue)
                {
                    //data = source.ItemContainerGenerator.ItemFromContainer(element);
                    data = source.ItemContainerGenerator.ItemFromContainer(element);
                 
                    if (data == DependencyProperty.UnsetValue)
                    {
                        element = VisualTreeHelper.GetParent(element) as UIElement;
                    }

                    if (element == source)
                    {
                        return null;
                    }
                }

                if (data != DependencyProperty.UnsetValue)
                {
                    return data;
                }
              
            }          

           return null;
        }

 puzzles是个ListBox，它的项是一个img对象

data = source.ItemContainerGenerator.ItemFromContainer(element);老是报不能为空，

请高手指教一下！