无边框拖拽

方法
Point mouseOff;//鼠标移动位置变量
        bool leftFlag;//标签是否为左键

        private void MouseDown(object sender, MouseEventArgs e)
        {
            if(e.Button == System.Windows.Forms.MouseButtons.Left)
            {
                mouseOff = new Point(-e.X, -e.Y);
                leftFlag = true;
            }
        }

        private void MouseMove(object sender, MouseEventArgs e)
        {
            if (leftFlag)
            {
                Point mouseSet = Control.MousePosition;
                mouseSet.Offset(mouseOff.X, mouseOff.Y); 
 //设置移动后的位置
                Location = mouseSet;
            }
        }

        private void MouseUp(object sender, MouseEventArgs e)
        {
            if(leftFlag)
            {
                leftFlag = false;
            }