public IPhoneBookEntry GetPhoneBook(int id)
{
var unvalidatedEntry = database[id]; //e.g. "+39 122-34-56-78"
return(PhoneBookEntry.Get(unvalidatedEntry)); //from db
// if db has a null or invalid entry, we will get an InvalidPhoneBookEntry object!
// that we cannot add back or perform any valid phone book entry operation on it!
// that's type safety!!!
}
public int AddEntry(PhoneBookEntry validatedPhoneBookEntry)
{
int id = database.Count;
database.Add(id, validatedPhoneBookEntry.Entry);
return(id);
// we know is valid!
// because the object PhoneBookEntry only has a private constructor!
// accessed via it's static creational factory method which also validates its arguments!
}
