/// <summary>
        /// 鼠标按下
        /// </summary>
        /// <param name="e"></param>
        protected override void OnMouseDown(MouseEventArgs e)
        {
            //继承过来就自带这个方法
            base.OnMouseDown(e);
            //如果是左键点击，且点击次数是1的情况
            if (e.Button != MouseButtons.Left || e.Clicks != 1)
                return;

        }