When to use an Interface and when to use a concrete type

When you first start working with Inversion of Control (might have to write a blog post on that…) & unit testing you’ll probably go interface crazy! Interfaces are great, and they really make it easy to unit test and take full advantage of inversion of control. However, sometimes you don’t want to use one. Fortunately if you’re using design patterns properly, it’s easy to decide if you need to use an interface or not:

  • Is the class just data storage? If so, it should always just be a concrete type
  • Is the class a service that does something? If so it should always have an interface

The problem is when you’ve got a class that’s mostly data storage but also has some logic to it. In this case you’ve broken ‘Separation of Concerns’ and now have a problem! In this case you’ll want to expose an interface simply to make testing easier. However, ideally you want to refactor the logic out into one or more new classes.

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