WPF DatePicker 的textbox的焦点

要得到DatePicker的textchange属性， 必须通过TextBoxBase.TextChanged 事件来处理。 

想要判断是否当前DatePicker的textbox获取到焦点， 可以通过以下代码：

  private void dtpBirth_TextChanged(object sender, TextChangedEventArgs e)

        {

            //

            IInputElement child = FocusManager.GetFocusedElement(window);

            DatePicker dtp = sender as DatePicker;

            if (null != child)

            {

                IInputElement element = dtp.InputHitTest(Mouse.GetPosition(dtp));

                if (null != element)

                {

                    dtpBirth.Tag = true;

                }               

            }

        }