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POObject Oriented Best Practices - Inheritance v Composition v Interfaces
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copied!<p>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.</p> <p>Suppose I am writing a video store application whose database has a <code>Person</code> table, with <code>PersonId</code>, <code>Name</code>, <code>DateOfBirth</code> and <code>Address</code> fields. It also has a <code>Staff</code> table, which has a link to a <code>PersonId</code>, and a <code>Customer</code> table which also links to <code>PersonId</code>.</p> <p>A simple object oriented approach would be to say that a <code>Customer</code> "is a" <code>Person</code> and therefore create classes a bit like this:</p> <pre><code>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; } } </code></pre> <p>Now we can write a function say to send emails to all customers:</p> <pre><code>static void SendEmailToCustomers(IEnumerable<Person> everyone) { foreach(Person p in everyone) if(p is Customer) SendEmail(p); } </code></pre> <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 <code>everyone</code> list to have the same person in twice, once as a <code>Customer</code> and once as a <code>Staff</code>, do we make an arbitrary choice between:</p> <pre><code>class StaffCustomer : Customer { ... </code></pre> <p>and </p> <pre><code>class StaffCustomer : Staff { ... </code></pre> <p>Obviously only the first of these two would not break the <code>SendEmailToCustomers</code> function.</p> <p>So what would you do? </p> <ul> <li>Make the <code>Person</code> class have optional references to a <code>StaffDetails</code> and <code>CustomerDetails</code> class? </li> <li>Create a new class that contained a <code>Person</code>, plus optional <code>StaffDetails</code> and <code>CustomerDetails</code>? </li> <li>Make everything an interface (e.g. <code>IPerson</code>, <code>IStaff</code>, <code>ICustomer</code>) and create three classes that implemented the appropriate interfaces? </li> <li>Take another completely different approach?</li> </ul>

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