asp.net 生成验证码

        Random ran = new Random();
        protected void Page_Load(object sender, EventArgs e)
        {

            string str = getRandomValidate(4);

            //这一步是为了将验证码写入Session，进行验证，不能缺省，也可一使用cookie

            Session["check"] = str;
            getImageValidate(str);
        }

        //得到随机字符串
        private string getRandomValidate(int len)
        {
            int num;
            int tem;
            string rtuStr = "";
            for (int i = 0; i < len; i++)
            {
                num = ran.Next();
                /*
                 * 这里可以选择生成字符和数字组合的验证码
                 */
                tem = num % 10 + '0';//生成数字
                //tem = num % 26 + 'A';//生成字符
                rtuStr += Convert.ToChar(tem).ToString();
            }
            return rtuStr;

        }

        //将生成的随机数字转化成图像
        private void getImageValidate(string strValue)
        {
            //string str = "OO00"; //前两个为字母O，后两个为数字0
            int width = Convert.ToInt32(strValue.Length * 12);    //计算图像宽度
            Bitmap img = new Bitmap(width, 23);
            Graphics gfc = Graphics.FromImage(img);           //产生Graphics对象，进行画图

            gfc.Clear(Color.White);

            drawLine(gfc, img);

            //写验证码，需要定义Font
            Font font = new Font("arial", 12, FontStyle.Bold);
            System.Drawing.Drawing2D.LinearGradientBrush brush =
                new System.Drawing.Drawing2D.LinearGradientBrush(new Rectangle(0, 0, img.Width, img.Height), Color.DarkOrchid, Color.Blue, 1.5f, true);

            gfc.DrawString(strValue, font, brush, 3, 2);

            drawPoint(img);

            gfc.DrawRectangle(new Pen(Color.DarkBlue), 0, 0, img.Width - 1, img.Height - 1);
            //将图像添加到页面
            MemoryStream ms = new MemoryStream();
            img.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
            //更改Http头
            Response.ClearContent();
            Response.ContentType = "image/gif";
            Response.BinaryWrite(ms.ToArray());
            //Dispose
            gfc.Dispose();
            img.Dispose();
            Response.End();

        }

        //画背景线

        private void drawLine(Graphics gfc, Bitmap img)
        {
            //选择画10条线,也可以增加，也可以不要线，只要随机杂点即可
            for (int i = 0; i < 10; i++)
            {
                int x1 = ran.Next(img.Width);
                int y1 = ran.Next(img.Height);
                int x2 = ran.Next(img.Width);
                int y2 = ran.Next(img.Height);
                gfc.DrawLine(new Pen(Color.Silver), x1, y1, x2, y2);      //注意画笔一定要浅颜色，否则验证码看不清楚
            }

        }

       //画背景点

        private void drawPoint(Bitmap img)
        {
            /*
            //选择画100个点,可以根据实际情况改变
            for (int i = 0; i < 100; i++)
            {
                int x = ran.Next(img.Width);
                int y = ran.Next(img.Height);
                img.SetPixel(x,y,Color.FromArgb(ran.Next()));//杂点颜色随机
            }
             */
            int col = ran.Next();//在一次的图片中杂店颜色相同
            for (int i = 0; i < 100; i++)
            {
                int x = ran.Next(img.Width);
                int y = ran.Next(img.Height);
                img.SetPixel(x, y, Color.FromArgb(col));
            }

        }

前台image控件：

<asp:Image ID="Image1" runat="server" ImageUrl="CreateImg.aspx" />

学习：

http://hi.baidu.com/rayshow/item/b778f522677f48cba4275a75