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I want to ask a question about how you would approach a simple object-oriented design problem. I have a few ideas of my own about what the best way of tackling this scenario, but I would be interested in hearing some opinions from the Stack Overflow community. Links to relevant online articles are also appreciated. I'm using C#, but the question is not language specific. Suppose I am writing a video store application whose database has a Person table, with PersonId , Name , DateOfBirth and Address fields. It also has a Staff table, which has a link to a PersonId , and a Customer table which also links to PersonId . A simple object oriented approach would be to say that a Customer "is a" Person and therefore create classes a bit like this: class Person { public int PersonId { get; set; } public string Name { get; set; } public DateTime DateOfBirth { get; set; } public string Address { get; set; } } class Customer : Person { public int CustomerId { get; set; } public DateTime JoinedDate { get; set; } } class Staff : Person { public int StaffId { get; set; } public string JobTitle { get; set; } } Now we can write a function say to send emails to all customers: static void SendEmailToCustomers(IEnumerable<Person> everyone) { foreach(Person p in everyone) if(p is Customer) SendEmail(p); } This system works fine until we have someone who is both a customer and a member of staff. Assuming that we don't really want our everyone list to have the same person in twice, once as a Customer and once as a Staff , do we make an arbitrary choice between: class StaffCustomer : Customer { ... and class StaffCust

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