建一个web网站 新建一般处理程序直接贴代码：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.Web.SessionState;
using System.Drawing;
using System.Text;
using System.Drawing.Imaging;

namespace HandlerStudy
{
    /// <summary>
    /// ImageHandler 的摘要说明
    /// </summary>
    public class ImageHandler : IHttpHandler,IRequiresSessionState
    {

        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "image/gif";
            Bitmap basemap = new Bitmap(200, 60);
            Graphics graph = Graphics.FromImage(basemap);
            graph.FillRectangle(new SolidBrush(Color.White), 0, 0, 200, 60);
            Font font = new Font(FontFamily.GenericSerif, 48, FontStyle.Bold, GraphicsUnit.Pixel);
            Random r = new Random();
            string letters = "1234567890ABCDEFGHIJKLMNOPQRSTUVWXYZ";
            string letter = string.Empty;
            StringBuilder s = new StringBuilder();
            //添加随机的五个验证码
            for (int x = 0; x < 5; x++)
            {
                letter = letters.Substring(r.Next(0, letters.Length - 1), 1);
                s.Append(letter);
                graph.DrawString(letter, font, new SolidBrush(Color.Black), x * 38, r.Next(0, 15));
            }

            //混淆背景

            Pen linePen = new Pen(new SolidBrush(Color.Black), 2);

            for (int x = 0; x < 8; x++)
            {
                graph.DrawLine(linePen, new Point(r.Next(0, 199), r.Next(0, 59)), new Point(r.Next(0, 199), r.Next(0, 59)));
            }


            //将图片保存到输出流中

            basemap.Save(context.Response.OutputStream, ImageFormat.Gif);
            context.Session["CheckCode"] = s.ToString();
            context.Response.End();
        }

        public bool IsReusable
        {
            get
            {
                return false;
            }
        }
    }
}

在网页中这样使用：

效果如下：