Question

Recently I have come across a curious pattern in some code. We know that there is a time and a place for everything, especially when it comes to the issue of ABCs and interfaces, but this just seems redundant to me.

  // This describes a person....
  public interface IPerson
  {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int BasePay { get; set; }
    public string Address { get; set; }
  }

  // And so does this, but it also uses the interface....
  public abstract class Person : IPerson
  {
    public string FirstName { get; set; }
    public string LastName { get; set; }
    public int BasePay { get; set; }
    public string Address { get; set; }
  }

  // This uses both ?!
  public class CoalMiner : Person, IPerson
  {
    public CoalMiner()
    {
      BasePay = 10000;
    }
  }

Can anybody think of what the specific advantage of using both and ABC and an interface that define the same members be?

Answer 1

Personally, I feel that using an interface to describe a “noun”, such as a person, it typically a poor design choice.

Interfaces should be used for contracts – all people are always a Person, so an abstract class makes more sense here. Interfaces could be added for behaviors attached to specific types of people, or to allow a Person to be used in a certain way by fulfilling a contract (ie: Person : IComparable<Person>).