public Pupil AddPupil(string firstName, string lastName) { if (firstName == null || lastName == null || firstName.Length < 2 || lastName.Length < 2) { throw new ArgumentException(); } if (_pupils.FirstOrDefault(p => p.FirstName == firstName && p.LastName == lastName) != null) { throw new ArgumentException(); } var newOne = new Pupil(this, firstName, lastName); _pupils.Add(newOne); return(newOne); }