public partial class PainterForm : Form
    {
        private Color color = Color .Black ;
        int size = 4;
        Point startpoint;
        Image orginalImg;//原始图像
        Image finishingImg;//中间图像
        public Bitmap bitmap;
        private string sType = "Pencil";
        bool shouldPaint = false;
        public PainterForm()
        {
            InitializeComponent();
            bitmap = new Bitmap(pictureBox1.Width, pictureBox1.Height);
            Graphics g = Graphics.FromImage(bitmap);
            g.FillRectangle(new SolidBrush(pictureBox1.BackColor), new Rectangle(0, 0, pictureBox1.Width, pictureBox1.Height));
            g.Dispose();
            orginalImg  = (Image)bitmap.Clone();
            finishingImg  = (Image)bitmap.Clone();
        }
        private void pictureBox1_MouseDown(object sender, MouseEventArgs e)
        {
            shouldPaint = true;
            startpoint = new Point(e.X, e.Y);
        }
        private void pictureBox1_MouseUp(object sender, MouseEventArgs e)
        {
            shouldPaint = false;
            orginalImg = finishingImg;
        }
        private void pictureBox1_MouseMove(object sender, MouseEventArgs e)
        {
            Graphics graphics = pictureBox1.CreateGraphics();
            Image img = (Image)orginalImg.Clone();

            if (shouldPaint && sType !="Pencil" && sType !="Eraser" )            {
                Graphics g = Graphics.FromImage(img);
                switch (sType)
                {
                    case "Line":
                        {
                            g.DrawLine(new Pen(color , size), startpoint , new Point(e.X, e.Y));
                            break;
                        }
                    case "Rect":
                        {
                            float width = Math.Abs(e.X - startpoint.X);//确定矩形的宽
                            float heigth = Math.Abs(e.Y - startpoint.Y);//确定矩形的高
                            PointF rectStartPoint = startpoint;
                            if (e.X < startpoint.X)
                                rectStartPoint.X = e.X;
                            if (e.Y < startpoint.Y)
                                rectStartPoint.Y = e.Y;
                            g.DrawRectangle(new Pen(color ,size), rectStartPoint.X, rectStartPoint.Y, width, heigth);
                            break;
                        }
                    case "Circle":
                        {
                            Point Cir = startpoint ;
                            if (e.X < startpoint.X)
                                Cir.X = e.X;
                            if (e.Y < startpoint.Y)
                                Cir.Y = e.Y;
                            g.DrawEllipse(new Pen(color, size), Cir.X, Cir.Y, Math.Abs(e.X - startpoint.X), Math.Abs(e.Y - startpoint.Y));
                            break;
                        }
                        g.Dispose();
                        finishingImg  = (Image)img.Clone();
                        //this.pictureBox.CreateGraphics().DrawImage(img, 0, 0);
                        graphics.DrawImage(finishingImg , 0, 0);
                        img.Dispose();
                }
            }
                g.Dispose();
                graphics.DrawImage(finishingImg , 0, 0);

            } 
        }
 private void pictureBox1_Paint(object sender, PaintEventArgs e)
        {
            Graphics gra = pictureBox1.CreateGraphics();
            gra .DrawImage(bitmap  , 0, 0);
        }