public ISession Login(string username, string password) {
            if (username == null)
                throw new ArgumentNullException("username", "il nome utente non deve essere null");
            if (password == null)
                throw new ArgumentNullException("password", "la passsword non deve essere null");
            Context.Refresh(RefreshMode.StoreWins, this);
            if (this.EntityState == System.Data.EntityState.Detached)
                throw new InvalidOperationException("il sito � stato cancellato");
            /*using (var context = new DatabaseEntities(connectionString))
            {*/
            /* foreach (var session in Sessions){
                if(session.User.username == username && )
            }*/
            //Context.Refresh(RefreshMode.StoreWins, Users);
            var existingUsers = from user in Users
                       where /*user.Site == this
                             &&*/ user.Username == username
                             && user.password == password
                       select user;
            if (!existingUsers.Any())
                return null;
            User myUser = existingUsers.First();
            /*var existingSessions = from session in Sessions
                                   where session.User.Username ==
                                         && session.User.password == password
                                   select session;*/

            var validSessions = from session in myUser.Sessions
                                where session.IsValid()
                                select session;
            if (validSessions.Any())
                return Sessions.First();
            Session newsession = Session.CreateSession(0);
            newsession.User = myUser;
            //myUser.Sessions.Add(newsession);
            newsession.Site = this;
            newsession.AlarmClock = this.AlarmClock;
            newsession.Context = this.Context;
            newsession.Starting = this.AlarmClock.Now;
            newsession.Time = new TimeSpan(0, 0, SessionExpirationInSeconds);
            //this.Sessions.Add(newsession);
            Context.AddToSessions(newsession);
            Context.SaveChanges();
            return newsession;
        }