.net 环境下带加减法的验证码

ValidCode.aspx.cs

using System;
using System.Data;
using System.Configuration;
using System.Collections;
using System.Web;
using System.Web.Security;
using System.Web.UI;
using System.Web.UI.WebControls;
using System.Web.UI.WebControls.WebParts;
using System.Web.UI.HtmlControls;
using System.Drawing;

//from http://i.aspx1.com
public partial class Admin_ValidCode : System.Web.UI.Page