You have functions void callNumber(string phoneNumber); void associatePhoneNumber(string phoneNumber, Person person); Person lookupPerson(string ph

You have functions void callNumber(string phoneNumber); void associatePhoneNumber(string phoneNumber, Person person); Person lookupPerson(string phoneNumber); Provider getProvider(string phoneNumber); I pass in "555;324+289G". Are you putting validation logic into all of those functions? You could have a validation function you write once and call in all of those functions, but why? Why not just parse the phone number into an already validated type and pass that around? PhoneNumber PhoneNumber(string phoneNumber); void callNumber(PhoneNumber phoneNumber); void associatePhoneNumber(PhoneNumber phoneNumber, Person person); Person lookupPerson(PhoneNumber phoneNumber); Provider getProvider(PhoneNumber phoneNumber); Put all of the validation logic into the type conversion function. Now you only need to validate once from string to PhoneNumber, and you can safely assume it's valid everywhere else.