Code
using System;
using System.Collections.Generic;
using System.Text;
using System.Web;

using System.Web.SessionState;
using System.Web.UI;
using System.Drawing;

namespace ControlValidater
{
    public class CValidater:IHttpHandler,IRequiresSessionState
    {
        HttpResponse response = null;
        HttpSessionState sessionState = null;

        public void ProcessRequest(HttpContext context)
        {
            response = context.Response;
            sessionState = context.Session;

            this.GenImg(this.GenCode(4));
        }
        
        bool System.Web.IHttpHandler.IsReusable
        {
            get { return true; }
        }

        //产生随机字符串
        private string GenCode(int num)
        {
            string[] source ={"0","1","2","3","4","5","6","7","8","9",
                                "A","B","C","D","E","F","G","H","I","J","K","L","M","N",
                                "O","P","Q","R","S","T","U","V","W","X","Y","Z"};
            string code = "";
            Random rd = new Random();
            for (int i = 0; i < num; i++)
            {
                code += source[rd.Next(0, source.Length)];
            }
            return code;
        }

        //生成图片
        private void GenImg(string code)
        {
            Bitmap myPalette = new Bitmap(60, 20);//定义一个画板

            Graphics gh = Graphics.FromImage(myPalette);//在画板上定义绘图的实例

            Rectangle rc = new Rectangle(0, 0, 60, 20);//定义一个矩形

            gh.FillRectangle(new SolidBrush(Color.Silver), rc);//填充矩形

            gh.DrawString(code, new Font("宋体", 16), new SolidBrush(Color.Gray), rc);//在矩形内画出字符串

            myPalette.Save(response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);//将图片显示出来

            sessionState["ValidateCode"] = code;//将字符串保存到Session中，以便需要时进行验证

            gh.Dispose();
            myPalette.Dispose();
        }
    }
}

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<httpHandlers>

      <add path="CValidater.aspx" verb="*" type="ControlValidater.CValidater" />
    </httpHandlers>

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<asp:Image ID="Image1" runat="server" ImageUrl="~/CValidater.aspx"/>

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protected void Button1_Click(object sender, EventArgs e)
    {
        string strIn = this.TextBox1.Text;
        string strSess = Session["ValidateCode"].ToString();
        if (strIn == strSess)
            Button1.Text = "ok";
        else
            Button1.Text = "error";
    }