不出现重复数字，带干扰的验证码

            Bitmap map = new Bitmap(200,80);
            Graphics g = Graphics.FromImage(map);
            g.Clear(Color.White);
            Random r = new Random();
            string codestr = null;
            for (int i = 0; i < 4; i++)
            {
                string rdnum=r.Next(10).ToString ();
                if (codestr != null)
                {
                    if (codestr.IndexOf(rdnum) >= 0)
                    {
                        i--;
                        continue;
                    }
                }
                codestr += rdnum;
            }
            g.DrawString(codestr, new Font("宋体", 40), Brushes.Red, new PointF(8, 8));
            //添加噪音线
            for (int i = 0; i < 20; i++)
            {
                int x1 = r.Next(map.Width);
                int y1 = r.Next(map.Height);
                int x2 = r.Next(map.Width);
                int y2 = r.Next(map.Height);
                g.DrawLine(new Pen(Brushes.Plum), x1, y1, x2, y2);
            }
            //添加噪音点
            for (int i = 0; i < 500; i++)
            {
                map.SetPixel(r.Next(map.Width), r.Next(map.Height), Color.FromArgb(r.Next(1)));
            }
            MemoryStream ms = new MemoryStream();
            map.Save (ms,System .Drawing .Imaging .ImageFormat .Jpeg );
            this .pictureBox1 .Image =Image .FromStream (ms);