不出现重复数字，带干扰的验证码

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 1             Bitmap map = new Bitmap(200,80);
 2             Graphics g = Graphics.FromImage(map);
 3             g.Clear(Color.White);
 4             Random r = new Random();
 5             string codestr = null;
 6             for (int i = 0; i < 4; i++)
 7             {
 8                 string rdnum=r.Next(10).ToString ();
 9                 if (codestr != null)
10                 {
11                     if (codestr.IndexOf(rdnum) >= 0)
12                     {
13                         i--;
14                         continue;
15                     }
16                 }
17                 codestr += rdnum;
18             }
19             g.DrawString(codestr, new Font("宋体", 40), Brushes.Red, new PointF(8, 8));
20             //添加噪音线
21             for (int i = 0; i < 20; i++)
22             {
23                 int x1 = r.Next(map.Width);
24                 int y1 = r.Next(map.Height);
25                 int x2 = r.Next(map.Width);
26                 int y2 = r.Next(map.Height);
27                 g.DrawLine(new Pen(Brushes.Plum), x1, y1, x2, y2);
28             }
29             //添加噪音点
30             for (int i = 0; i < 500; i++)
31             {
32                 map.SetPixel(r.Next(map.Width), r.Next(map.Height), Color.FromArgb(r.Next(1)));
33             }
34             MemoryStream ms = new MemoryStream();
35             map.Save (ms,System .Drawing .Imaging .ImageFormat .Jpeg );
36             this .pictureBox1 .Image =Image .FromStream (ms);