//定义鼠标事件
        private void pictureBox1_MouseDown_1(object sender, MouseEventArgs e)
        {
            this.Cursor = Cursors.Cross;
            this.p1 = new Point(e.X, e.Y);
        }

        private void pictureBox1_MouseUp_1(object sender, MouseEventArgs e)
        {
            this.Cursor = Cursors.Default;
            this.p2 = new Point(e.X, e.Y);
        }

        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            if (this.Cursor == Cursors.Cross)
            {
                this.p2 = new Point(e.X, e.Y);
                this.pictureBox1.Invalidate();
            }
        }
        //定义鼠标事件

        private void pictureBox1_Paint_1(object sender, PaintEventArgs e)//在图片框画图
        {
            Pen p = new Pen(Color.Black, 1);//画笔
            p.DashStyle = System.Drawing.Drawing2D.DashStyle.Dash;
            Rectangle rect = new Rectangle(p1, new Size(p2.X - p1.X, p2.Y - p1.Y));
            e.Graphics.DrawRectangle(p, rect);
        }