Why should you ever have to care whether an object reference is an interface or a class?

Question

I often seem to run into the discussion of whether or not to apply some sort of prefix/suffix convention to interface type names, typically adding "I" to the beginning of the name.

Personally I'm in the camp that advocates no prefix, but that's not what this question is about. Rather, it's about one of the arguments I often hear in that discussion:

You can no longer see at-a-glance whether something is an interface or a class.

The question that immediately pops up in my head is: apart from object creation, why should you ever have to care whether an object reference is a class or an interface?

I've tagged this question as language agnostic, but as has been pointed out it may not be. I contend that it is because while specific language implementation details may be interesting, I'd like to keep this on a conceptual level. In other words, I think that, conceptually, you'd never have to care whether an object reference is typed as a class or an interface but I'm not sure, hence the question.

This is not a discussion about IDEs and what they do or don't do when visualizing the different types; caring about the type of an object is certainly a necessity when browsing through code (packages/sources/whatever form). Nor is it a discussion about the pros or cons about either naming convention. I just can't seem to figure out in what scenario, other than object creation, you actually care about wether or not you're referencing a concrete type or an interface.

Answer 1

Most of the time, you probably don't care. But here are some instances that I can think of where you would. There are several, and it does vary a little bit by language. Some languages don't mind as much as others.

In the case of inversion of control (where someone PASSES you a parameter) you probably don't care if it's an interface or an object as far as calling its methods etc. But when dealing with types, it definitely can make a difference.

• In managed languages such as .NET languages, interfaces can usually only inherit one interface, whereas a class can inherit one class but implement many interfaces. The order of classes vs interfaces may also matter in a class or interface declaration. So you need to know which is which when defining a new class or interface.

• In Delphi / VCL, interfaces are reference counted and automatically collected, whereas classes must be explicitly freed, so lifecyle management on the whole is affected, not just the creation.

• Interfaces may not be viable sources for class references.

• Interfaces can be cast to compatible interfaces, but in many languages, they cannot be cast to compatible classes. Classes can be cast to either.

• Interfaces may be passed to parameters of type IID, or IUnknown, whereas classes cannot (without a cast and a supporting interface).

• An interface's implementation is unknown. Its input and output are defined, but the implementation which creates the output is abstracted. In general, ones attitude may be that when working with a class, one may know how the class works. But when working with an interface, no such assumption should be made. In a perfect world, it might make no difference. But in reality, this most certainly can have affect your design.

Answer 2

I agree with you (and thereby do not use an "I" prefix for interfaces). We shouldn't have to care whether it is an abstract class or an interface.

Worth noting that Java needs to have a notion of interface solely because it does not support multiple inheritance. Otherwise, "abstract class" concept would suffice (which may be "all" abstract, or partially abstract, or almost concrete and just 1 tiny bit abstract, whatever).