c#生成验证码

        /// <summary>
        /// 生成随机的字符串
        /// </summary>
        /// <param name="codecount">验证码长度</param>
        /// <returns></returns>
        public string createrandomcode(int codecount)
        {
            string allchar = "0,1,2,3,4,5,6,7,8,9,a,b,c,d,e,a,b,c,d,e,f,g,h,i,g,k,l,m,n,o,p,q,r,f,g,h,i,g,k,l,m,n,o,p,q,r,s,t,u,v,w,x,y,z,s,t,u,v,w,x,y,z";
            string[] allchararray = allchar.split(',');
            string randomcode = "";
            int temp = -1;
            random rand = new random();
            for (int i = 0; i < codecount; i++)
            {
                if (temp != -1)
                {
                    rand = new random(i * temp * ((int)datetime.now.ticks));
                }
                int t = rand.next(35);
                if (temp == t)
                {
                    return createrandomcode(codecount);
                }
                temp = t;
                randomcode += allchararray[t];
            }
            return randomcode;
        }

        /// <summary>
        /// 创建验证码图片
        /// </summary>
        /// <param name="validatecode"></param>
        /// <returns></returns>
        public void createvalidategraphic(string validatecode, httpcontext context)
        {
            bitmap image = new bitmap((int)math.ceiling(validatecode.length * 16.0), 27);
            graphics g = graphics.fromimage(image);
            try
            {
                //生成随机生成器
                random random = new random();
                //清空图片背景色
                g.clear(color.white);
                //画图片的干扰线
                for (int i = 0; i < 25; i++)
                {
                    int x1 = random.next(image.width);
                    int x2 = random.next(image.width);
                    int y1 = random.next(image.height);
                    int y2 = random.next(image.height);
                    g.drawline(new pen(color.silver), x1, x2, y1, y2);
                }
                font font = new font("arial", 13, (fontstyle.bold | fontstyle.italic));
                lineargradientbrush brush = new lineargradientbrush(new rectangle(0, 0, image.width, image.height), color.blue, color.darkred, 1.2f, true);
                g.drawstring(validatecode, font, brush, 3, 2);

                //画图片的前景干扰线
                for (int i = 0; i < 100; i++)
                {
                    int x = random.next(image.width);
                    int y = random.next(image.height);
                    image.setpixel(x, y, color.fromargb(random.next()));
                }
                //画图片的边框线
                g.drawrectangle(new pen(color.silver), 0, 0, image.width - 1, image.height - 1);

                //保存图片数据 
                image.save(context.response.outputstream, imageformat.jpeg);

            }
            finally
            {
                g.dispose();
                image.dispose();
            }
        }