#region"自定义"//验证码
  private void CreateCheckCodeImage()
  {
   Session["checkCode"]=null;
   string checkCode=this.CreateCheckCode(4).Trim();
   if(checkCode==null || checkCode==String.Empty)
   {
    return;
   }
   else
    Session["checkCode"]=checkCode;
   System.Drawing.Bitmap image = new System.Drawing.Bitmap((int)Math.Ceiling(((double)checkCode.Length * 13)), 24);
   Graphics g = Graphics.FromImage(image);//http://localhost/dx/Global.asax
   try
   {
    //生成随机生成器
    Random random = new Random();
    //清空图片背景色
    g.Clear(Color.White);
    //画图片的背景噪音线
    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, y1, x2, y2);
    }
    Font font = new System.Drawing.Font("Arial", 13, (System.Drawing.FontStyle.Bold | System.Drawing.FontStyle.Italic));
    System.Drawing.Drawing2D.LinearGradientBrush brush = new System.Drawing.Drawing2D.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<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);

    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());
   }
   finally
   {
    g.Dispose();
    image.Dispose();
   }
  }
  private string CreateCheckCode(int length)
  {
   int number;
   char code=(char)0;
   string checkCode = String.Empty;

   System.Random random = new Random();

   for(int i=0; i<length;i++)
   {
    number = random.Next();

    //if(number % 2 == 0)
    code = (char)('0' + (char)(number % 10));
    //    else
    //     code = (char)('A' + (char)(number % 26));
    checkCode += code.ToString();
   }
   return checkCode;
  }
  #endregion