public bool Login(string login, string password)
        {
            var result = Entity.Users?.GetAll().Any(t => login == t.Login && password == t.Password);

            foreach (var t in Entity.Users.GetAll().Where(t => t.Login == login))
            {
                _activeUser = t;
            }

            return (bool)result;
        }
        public bool Register(string login, string password)
        {
            if (Entity.Users != null)
            {
                if (Entity.Users.GetAll().Any(t => t.Login == login))
                {
                    return false;
                }
            }
            Entity.Users?.Add(new User() { Login = login, Password = password });

            foreach (var t in Entity.Users.GetAll().Where(t => t.Login == login))
            {
                _activeUser = t;
            }

            Entity.SaveChanges();
            return true;
        }
 public void AddSubject(User user, Subject subject)
 {
     user.Subjects.Add(subject);
     Entity.SaveChanges();
 }