///
  /// 随机字符串
  ///
  /// 字符串长度
  ///
  protected string GenerateCode(int number)
  {
  string checkCode = string.Empty;
  string vChar="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,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[] vcArray = vChar.Split(',');

  int temp = -1;//记录上次随机数值，尽量避免产生几个一样的随机数
  Random rand = new Random();
  for (int i = 1; i < number+1; i++)
  {
  if (temp != -1)
  {
  rand = new Random(i * temp * unchecked((int)DateTime.Now.Ticks));
  }
  int t = rand.Next(vcArray.Length);
  if (temp != -1 && temp == t)
  {
  return GenerateCode(number);
  }
  temp = t;
  checkCode += vcArray[t];
  }
  Response.Cookies.Add(new HttpCookie("CheckCode",checkCode));
 

///
  /// 生成验证码图片
  ///
  /// 随机字符串
  private void CreateCheckImage(string checkCode)
  {
  if (checkCode == null || checkCode.Trim() == string.Empty)
  {
  return;
  }

  System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling((checkCode.Length*12.5)),22);
  Graphics g = Graphics.FromImage(image);
  try
  {
  Random radom = new Random();
  g.Clear(Color.White);
  for (int i = 0; i < 2; i++)
  {
  int x1 = radom.Next(image.Width);
  int x2 = radom.Next(image.Width);
  int y1 = radom.Next(image.Height);
  int y2 = radom.Next(image.Height);
  g.DrawLine(new Pen(Color.Black), x1, y1, x2, y2);
  }
  Font font = new Font("Arial", 12, (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(checkCode, font, brush, 2, 2);

  //画图片的前景噪音点
  for (int i = 0; i < 50; i++)
  {
  int x = radom.Next(image.Width);
  int y = radom.Next(image.Height);
  image.SetPixel(x, y, Color.FromArgb(radom.Next()));
  }

  //画图片的边框线
  g.DrawRectangle(new Pen(Color.Silver), 0, 0, image.Width - 1, image.Height - 1);
  System.IO.MemoryStream ms = new System.IO.MemoryStream();
  image.Save(ms, System.Drawing.Imaging.ImageFormat.Gif);
  Response.ClearContent();
  Response.ContentType = "image/Gif";
  Response.BinaryWrite(ms.ToArray());

  }
  catch { }
  finally
  {
  g.Dispose();
  image.Dispose();
  }
  }