<%@ WebHandler Language="C#" Class="Handler" %>
using System;
using System.Web;
using System.Drawing.Imaging;
using System.Drawing;
using System.Text;
public class Handler : IHttpHandler, System.Web.SessionState.IRequiresSessionState
{
public void ProcessRequest (HttpContext context)
{
context.Request.ContentType="image/gif";
Bitmap bitmap = new Bitmap(100, 30);
Graphics graphic = Graphics.FromImage(bitmap);
graphic.FillRectangle(new SolidBrush(Color.White), 0, 0, 100, 30);
Font font = new Font(FontFamily.GenericSansSerif, 24, FontStyle.Bold, GraphicsUnit.Pixel);
Random random = new Random();
string letters = "ABCDEFGHIJKLMNPQRSTUVWXYZ";
string letter;
StringBuilder s = new StringBuilder();
for (int i = 0; i < 5; i++)
{
letter = letters.Substring(random.Next(0,letters.Length-1), 1);
s.Append(letters);
graphic.DrawString(letter, font, new SolidBrush(Color.Red), i * 19, random.Next(0, 15));
}
float penFloat = new float();
penFloat = float.Parse("0.5"); ;
Pen linePen = new Pen(new SolidBrush(Color.Purple), penFloat);
for (int x = 0; x < 20; x++)
graphic.DrawLine(linePen, new Point(random.Next(0, 199), random.Next(0, 59)), new Point(random.Next(0, 199), random.Next(0, 59)));
bitmap.Save(context.Response.OutputStream, ImageFormat.Gif);
context.Session["CheckCode"] = s.ToString();   //如果没有实现IRequiresSessionState，则这里会出错，也无法生成图片
        context.Response.End();
}
public bool IsReusable
{
get {
return false;
}
}
}

<img src="Handler.ashx" alt="图片验证码" />
<img src="Handler.ashx" alt="图片验证码" />