asp.net验证码实现

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;

public partial class GreadCoad : System.Web.UI.Page
{
    protected void Page_Load(object sender, EventArgs e)
    {
        this.CreateCheckCodeImage(GenerateCheckCode());
    }

    private string GenerateCheckCode()
 {
  int number;
  char code;
  string checkCode = String.Empty;

  System.Random random = new Random();

  for(int i=0; i<5; i++)
  {
   number = random.Next();

   if(number % 2 == 0)
    code = (char)('0' + (char)(number % 10));
   else
    code = (char)('A' + (char)(number % 26));

   checkCode += code.ToString();
  }

  Response.Cookies.Add(new HttpCookie("CheckCode", checkCode));

  return checkCode;
 }

 private void CreateCheckCodeImage(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
  {
   //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", 12, (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();
  }
 }
}