/// <summary>
/// 返回最顶层的元素的在dragpoint型坐标。
/// Returns the top-most element of the given type
/// at the DragPoint coordinates.
/// </summary>
/// <typeparam name="T">The type of the element to be returned.返回元素的类型。</typeparam>
public T GetElement <T>(System.Windows.Point dragPoint) where T : System.Windows.FrameworkElement
{
// 注意：这不是要工作在WinForms！你可以使用扩展方法，将发现根视觉相对第一个参数（这个）。
// NOTE: This is not going to work in WinForms! You could use the extension method that will find to root visual
// relatively to the first argument (this).
return(VisualTreeHelperExtensions.GetElementsInScreenCoordinates <T>(dragPoint).FirstOrDefault());
}