验证码

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

        //一般处理程序中使用Session必须实现IRequiresSessionState接口
        public void ProcessRequest(HttpContext context)
        {
            context.Response.ContentType = "text/html";
            Common.ValidateCode validatecode = new Common.ValidateCode();
            string validateLength=validatecode.CreateValidateCode(5);
            context.Session["validatecode"] = validateLength;
            validatecode.CreateValidateGraphic(validateLength,context);
        }

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

public void ProcessRequest(HttpContext context)
    {
            context.Response.ContentType = "text/plain";
            Common.ValidateCode validatecode = new Common.ValidateCode();
            string validateLength=validatecode.CreateValidateCode(5);
            validatecode.CreateValidateGraphic(validateLength,context);
        }

    protected bool CheckValidateCode()
        {
            //定一个变量
            bool isSucess = false;
            //校验Session的时候一定校验是否为空，因为S有过期时间,ToString()的时候会报异常
            if(Session["validatecode"] != null)
            {
                string txtCode = Request.Form["txtVcode"];
                string sysCode = Session["validatecode"].ToString();
                if(sysCode.Equals(txtCode,StringComparison.InvariantCultureIgnoreCase))
                {
                    isSucess = true;
                    //一定要清空，安全问题。如果不清空 暴力破解 不断猜测随机生成用户名和密码
                    Session["validatecode"] = null;
                }
            }
            return isSucess;
        }

    <script src="../Js/jquery-1.7.1.js"></script>
    <script type ="text/javascript">
        $(function () {
            $("#kanbuqing").click(function () {
                $("#imgVCode").attr("src","ValidateImageCode.ashx?date=" + new Date().getSeconds());
            });
        });
    </script>

        vcode:<input type="text" name="txtVcode" /><img src="ValidateImageCode.ashx" id="imgVCode" /><a href="javascript:void(0)" id="kanbuqing">看不清?</a><Br />