<%@ WebHandler Language="C#" Class="ImgCode" %>
using System;
using System.Web;
using System.Web.SessionState;
using System.Collections;
using System.IO;
using System.Drawing;
using System.Drawing.Imaging;

public class ImgCode : IHttpHandler, IRequiresSessionState
{
    
    public void ProcessRequest (HttpContext context) 
    {
        string chkCode = string.Empty;

        //颜色列表，用于验证码、噪线、噪点
        Color[] color = { Color.Black, Color.Red, Color.Blue, Color.Green, Color.Orange, Color.Brown, Color.DarkBlue };

        //字体列表，用于验证码
        string[] font = { "Times New Roman", "MS Mincho", "Book Antiqua", "Gungsuh", "PMingLiU", "Impact" };

        //验证码的字符集，去掉了一些容易混淆的字符
        char[] character = { '2', '3', '4', '5', '6', '8', '9', 'A', 'B', 'D', 'E', 'F', 'H', 'J', 'K', 'L', 'M', 'N', 'P', 'R', 'S', 'T', 'W', 'X', 'Y' };

        Random rnd = new Random();

        //生成验证码字符串
        for (int i = 0; i < 4; i++)
        {
            chkCode += character[rnd.Next(character.Length)];
        }
        Bitmap bmp = new Bitmap(100, 30);

        Graphics g = Graphics.FromImage(bmp);

        g.Clear(Color.White);

        //画噪线
        for (int i = 0; i < 2; i++)
        {
            Color clr = color[rnd.Next(color.Length)];
            g.DrawLine(new Pen(clr), rnd.Next(bmp.Width), rnd.Next(bmp.Height), rnd.Next(bmp.Width), rnd.Next(bmp.Height));
        }

        //画验证码字符串
        for (int i = 0; i < chkCode.Length; i++)
        {
            string fnt = font[rnd.Next(font.Length)];
            Font ft = new Font(fnt, 16);
            Color clr = color[rnd.Next(color.Length)];
            g.DrawString(chkCode[i].ToString(), ft, new SolidBrush(clr), (float)i * 20 + 20, (float)6);
        }

        //画噪点
        for (int i = 0; i < 20; i++)
        {
            int x = rnd.Next(bmp.Width);
            int y = rnd.Next(bmp.Height);
            Color clr = color[rnd.Next(color.Length)];
            bmp.SetPixel(x, y, clr);
        }
        //清除该页输出缓存，设置该页无缓存
        context.Response.Buffer = true;
        context.Response.ExpiresAbsolute = System.DateTime.Now.AddMilliseconds(0);
        context.Response.Expires = 0;
        context.Response.CacheControl = "no-cache";
        context.Response.AppendHeader("Pragma", "No-Cache");

        //将验证码图片写入内存流，并将其以"image/Png" 格式输出
        MemoryStream ms = new MemoryStream();

        try
        {
            bmp.Save(ms, ImageFormat.Png);
            context.Response.ClearContent();
            context.Response.ContentType = "image/Png";
            context.Response.BinaryWrite(ms.ToArray());

            context.Session["imgcode"] = chkCode.ToLower();
            
        }
        finally
        {
            //显式释放资源
            bmp.Dispose();
            g.Dispose();
        }
    }
 
    public bool IsReusable {
        get {
            return false;
        }
    }

}