.Net 如何限制用户登录的方法

if (Session["LOGIN_USER_LIST"] != null)
            {
                Dictionary<Guid,UserInfo>  Userlist=  Session["LOGIN_USER_LIST"] as Dictionary<Guid,UserInfo>;
                Guid LoginKey =new Guid();
                Guid.TryParse(CookieHelper.getCookie("LOG_USER_KEY"), out LoginKey);
                if (Userlist.Keys.Contains(LoginKey))
                {
                    //如何用户登录
                }
                else
                {
                    //如果用户没有登录
                }

            }