<%@ WebHandler Language="C#" Class="YZM" %>

using System;
using System.Web;

public class YZM : IHttpHandler, System.Web.SessionState.IRequiresSessionState
{

    public void ProcessRequest (HttpContext context) {
        context.Response.ContentType = "image/JPEG";
        using (System.Drawing.Bitmap bitImage = new System.Drawing.Bitmap(130, 100))
        {
            //设置画布
            using (System.Drawing.Graphics g = System.Drawing.Graphics.FromImage(bitImage))
            {

                //随机数字
                Random my_random = new Random();
                int num_01 = my_random.Next(100);
                int num_02 = my_random.Next(100);

                int result = num_01 + num_02;
                string num_string01 = num_01.ToString();
                string num_string02 = num_02.ToString();
                string result_string = result.ToString();
                //保存到服务器的sessionid中
                HttpContext.Current.Session["YZM"] = result_string;
                //设置文字
                g.DrawString(num_string01 +"+"+ num_string02+"?", new System.Drawing.Font("宋体", 20), System.Drawing.Brushes.Red, new System.Drawing.PointF(0, 0));
                //保存到输出流中
                bitImage.Save(context.Response.OutputStream, System.Drawing.Imaging.ImageFormat.Jpeg);
            }
        }
    }

    public bool IsReusable {
        get {
            return false;
        }
    }

}

<form id="form1" runat="server">
    <div>
    <%--单击刷新验证码--%>
        <img src="YZM.ashx" alt="请输入验证码" onclick="this.src='YZM.ashx?aaa='+new Date()" />
    </div>
    <asp:TextBox ID="TextBox1" runat="server" text=""></asp:TextBox>
    <asp:Button ID="Button1" runat="server" onclick="Button1_Click" Text="Button" />
    </form>