代码如下:

 protected void Page_Load(object sender, EventArgs e)
    {
        if (!IsPostBack)
        {
            HttpCookie cookie = Request.Cookies["loginInfo"];

            if (cookie != null)
            {
                cb_repwd.Checked = true;
                tb_loginName.Text = UIHelper.Decrypt(cookie["loginName"]);
                tb_pwd.Attributes.Add("value", UIHelper.Decrypt(cookie["pwd"]));
            }
            else
            {
                tb_pwd.Attributes.Add("value", "******");
            }

        }

    }

    protected void btn_login_Click(object sender, EventArgs e)
    {

        if (UIHelper.GetString(Session["Code"]).ToUpper()!=this.tb_code.Text.Trim().ToUpper())
        {
            oJavaScript.Alert("验证码输入不正确");
            return;
        }

        NetUserService NerService = ContextFactory.GetObject("NetUserService");
        string sql = string.Format("from NetUser where LoginName='{0}'", tb_loginName.Text.Trim());
        IList list = NerService.Find(sql);
        if (list.Count > 0)
        {
            if (list[0].Status == 2)
            {
                oJavaScript.Alert("帐号已停用，请联系管理员");
                return;
            }
        }

        if (NerService.Login(tb_loginName.Text.Trim(), oSecurity.GetPwdEncrypt(tb_pwd.Text.Trim())))
        {
            HttpCookie cookie = Request.Cookies["loginInfo"];
            if (cb_repwd.Checked)
            {
                if (cookie == null)
                {
                    HttpCookie cookielogin = new HttpCookie("loginInfo");
                    setCookies(cookielogin,7);
                }
                else
                {
                    if (UIHelper.Decrypt(Request.Cookies["loginInfo"]["loginName"]) != tb_loginName.Text.Trim())
                    {
                        setCookies(cookie, 7);
                    }
                }
            }
            else
            {
                setCookieExpires(cookie);
            }

           
            oJavaScript.Redirect("/");
        }
        else
        {
            oJavaScript.Alert( "用户名或密码不正确");
        }
    }

    #region cookie
    ///

    /// 设置cookie
    ///

    /// HttpCookie cookielogin
    /// 有效期
    private void setCookies(HttpCookie cookielogin, int Expires)
    {
        cookielogin.Values["loginName"] = UIHelper.Encrypt(tb_loginName.Text.Trim());
        cookielogin.Values["pwd"] = UIHelper.Encrypt(tb_pwd.Text.Trim());
        cookielogin.Expires = DateTime.Now.AddDays(Expires);
        Response.AppendCookie(cookielogin);
    }

    ///

    /// 设置cookie失效
    ///

    /// HttpCookie cookielogin
    private void setCookieExpires(HttpCookie cookielogin)
    {
        if (cookielogin != null)
        {
            cookielogin.Expires = DateTime.Now.AddDays(-1); ;
            Response.AppendCookie(cookielogin);
        }
    }
    #endregion

设置cookie过期其实就是修改cookie，必须重新向客户端发送cookie

Request.Cookies取cookie, response，cookies添加cookie