Interface vs Base class

Question

When should I use an interface and when should I use a base class?

Should it always be an interface if I don\'t want to actually define a base implementation of the

Answer 1

Juan,

I like to think of interfaces as a way to characterize a class. A particular dog breed class, say a YorkshireTerrier, may be a descended of the parent dog class, but it is also implements IFurry, IStubby, and IYippieDog. So the class defines what the class is but the interface tells us things about it.

The advantage of this is it allows me to, for example, gather all the IYippieDog's and throw them into my Ocean collection. So now I can reach across a particular set of objects and find ones that meet the criteria I am looking at without inspecting the class too closely.

I find that interfaces really should define a sub-set of the public behavior of a class. If it defines all the public behavior for all the classes that implement then it usually does not need to exist. They do not tell me anything useful.

This thought though goes counter to the idea that every class should have an interface and you should code to the interface. That's fine, but you end up with a lot of one to one interfaces to classes and it makes things confusing. I understand that the idea is it does not really cost anything to do and now you can swap things in and out with ease. However, I find that I rarely do that. Most of the time I am just modifying the existing class in place and have the exact same issues I always did if the public interface of that class needs changing, except I now have to change it in two places.

So if you think like me you would definitely say that Cat and Dog are IPettable. It is a characterization that matches them both.

The other piece of this though is should they have the same base class? The question is do they need to be broadly treated as the same thing. Certainly they are both Animals, but does that fit how we are going to use them together.

Say I want to gather all Animal classes and put them in my Ark container.

Or do they need to be Mammals? Perhaps we need some kind of cross animal milking factory?

Do they even need to be linked together at all? Is it enough to just know they are both IPettable?

I often feel the desire to derive a whole class hierarchy when I really just need one class. I do it in anticipation someday I might need it and usually I never do. Even when I do, I usually find I have to do a lot to fix it. That’s because the first class I am creating is not the Dog, I am not that lucky, it is instead the Platypus. Now my entire class hierarchy is based on the bizarre case and I have a lot of wasted code.

You might also find at some point that not all Cats are IPettable (like that hairless one). Now you can move that Interface to all the derivative classes that fit. You will find that a much less breaking change that all of a sudden Cats are no longer derived from PettableBase.