string user = "userName";
            //默认的第1种，超时时间是在web.cofig中forms设置的timeout，单位是分钟,生成的cookie和凭证超时时间一样
            FormsAuthentication.RedirectFromLoginPage(user, true);

            //第二种,自定义cookie超时时间,但是即使cookie没超时凭证超时,还是需要重新登录
            HttpCookie AuthCookie = FormsAuthentication.GetAuthCookie(user, false);
            AuthCookie.Expires = DateTime.Now.AddDays(10);
            Response.Cookies.Add(AuthCookie);
            Response.Redirect(FormsAuthentication.GetRedirectUrl(user, true));

            //第3种,可以自定义凭证和cookie超时时间
            var nowDate = DateTime.Now;
            var myTicket = new FormsAuthenticationTicket(1, user, DateTime.Now, DateTime.Now.AddMinutes(60), false, "other data");
            string encryptedTicket = FormsAuthentication.Encrypt(myTicket); //加密用户凭证
            //把用户凭证存入Cookie 
            HttpCookie authCookie = new HttpCookie(FormsAuthentication.FormsCookieName, encryptedTicket);
            //authCookie.Expires = DateTime.Now.AddMinutes(60); //不设置时间即为会话cookie
            Response.Cookies.Add(authCookie);
            Response.Redirect(FormsAuthentication.GetRedirectUrl(user, true));