private Bitmap bt;
        private Bitmap bt1;
        Point p1;
        Point p2;
        bool b = false;
        Image img;

/// <summary>
        /// 剪切图片的按钮
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void btnShear_Click(object sender, EventArgs e)
        {
            b = true;
        }

        /// <summary>
        /// 获取按下鼠标指针的坐标
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void pictureBox_MouseDown(object sender, MouseEventArgs e)
        {
            if (b)
            {
                this.Cursor = Cursors.Cross;
                p1 = new Point(e.X, e.Y);
            }
        }

        /// <summary>
        /// 记录鼠标的移动
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void pictureBox_MouseMove(object sender, MouseEventArgs e)
        {
            if (b)
            {
                if (this.Cursor == Cursors.Cross)
                {
                    p2 = new Point(e.X, e.Y);
                    pictureBox.Invalidate();
                }
            }
        }

        /// <summary>
        /// 获取松开鼠标时指针的坐标
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void pictureBox_MouseUp(object sender, MouseEventArgs e)
        {
            if (b)
            {
                this.Cursor = Cursors.Default;
                p2 = new Point(e.X, e.Y);
                CutImage();
                b = false;
            }
        }

        /// <summary>
        /// 记录鼠标在PictureBox上面画的矩形
        /// </summary>
        /// <param name="sender"></param>
        /// <param name="e"></param>
        private void pictureBox_Paint(object sender, PaintEventArgs e)
        {
            if (b)
            {
                Pen p = new Pen(Color.White, 1);
                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);
                p.Dispose();
            }
        }

        /// <summary>
        /// 剪切图片
        /// </summary>
        private void CutImage()
        {
            try
            {
                img = this.pictureBox.Image;
                bt1 = new Bitmap(p2.X - p1.X, p2.Y - p1.Y);
                Rectangle tgtRect = new Rectangle(0, 0, bt1.Width, bt1.Height);
                Rectangle srcRect = new Rectangle(p1.X, p1.Y, bt1.Width, bt1.Height);
                Graphics g = Graphics.FromImage(bt1);
                g.DrawImage(img, tgtRect, srcRect, GraphicsUnit.Pixel);
                this.pictureBox.Image = bt1;
                g.Dispose();
                pictureBox.Refresh();
            }
            catch(Exception)
            {

            }
        }

当剪切后为什么用pen画的矩形还在，怎么去掉？

在pictureBox_MouseMove里面， e.Graphics.DrawRectangle(p, rect);        e为什么点不出来啊，是不是参数不一样

非常非常非常感谢baikil大哥的帮助

刷新的太频繁了，还是不行