看不懂：asp.net网站登陆图片验证代码

protected string GetCode()
        {//随机生成数（4个）
            string strCode = "";
            Random rand = new Random();

            string[] strs = new string[36] { "0", "1", "2", "3", "4", "5", "6", "7", "8", "9", "a", "b", "c", "d", "e", "f", "g", "h", "i", "j", "k", "l", "m", "n", "o", "p", "q", "r", "s", "t", "u", "v", "w", "x", "y", "z" };
            string nNum = "";
            for (int nI = 0; nI < 4; nI++)
            {
                nNum = strs[rand.Next(0, 35)];
                strCode += nNum.ToString();
            }
            return strCode;
        }

        protected void DrawCode(string strCode)
        {//绘画图片
            Session["Code"] = strCode; //页面传值
            Bitmap bmp = new Bitmap(70, 22);//画布大小
            Graphics g = Graphics.FromImage(bmp);//构造图片
            g.Clear(Color.Green);//背景色
            /*
             //线条
            Pen blackPen = new Pen(Color.Red, 1);
            Rectangle rect = new Rectangle(10, 0, 100, 20);
            float startAngle = 45.0F;
            float sweepAngle = 270.0F;
           g.DrawArc(blackPen, rect, startAngle, sweepAngle);
            */
            Random random = new Random();
            for (int i = 0; i < 135; i++)
            {
                int x = random.Next(bmp.Width);
                int y = random.Next(bmp.Height);
                bmp.SetPixel(x, y, Color.FromArgb(0, 0, 0));
            }
            g.DrawRectangle(new Pen(Color.Silver), 0, 0, bmp.Width - 1, bmp.Height - 1);//画图片的边框线
            g.DrawString(strCode, new Font("宋体", 18), new SolidBrush(Color.Black), 8, 0);
            //Response. ; //字符串,字体 大小,字体色,位置
            bmp.Save(Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            //打印这个图片
        }
    }