True- but in this case the classes looked relatively simple (i.e. they were carrying data, but not operating on it) Taking the interface approach further, your

True- but in this case the classes looked relatively simple (i.e. they were carrying data, but not operating on it) Taking the interface approach further, your classes that operate on your data would take in objects by interface e.g.: public class IsinValidator ... public bool Validate(IIsinSecurity security) {}