Is it the best practice to extract an interface for every class?

Question

I have seen code where every class has an interface that it implements.

Sometimes there is no common interface for them all.

They are just there and they are

Answer 1

I like interfaces on things that could be implemented two different ways, either in time or space, i.e. either it could be implemented differently in the future, or there are 2 different code clients in different parts of the code which may want a different implementation.

The original writer of your code might have just been robo coding, or they were being clever and preparing for version resilience, or preping for unit testing. More likely the former because version resilience an uncommon need-- (i.e. where the client is deployed and can't be changed and a component will be deployed that must be compatible with the existing client)

I like interfaces on things that are dependencies worth isolation from some other code I plan to test. If these interfaces weren't created to support unit tests either, then I'm not sure they're such a good idea. Interface have a cost to maintain and when it comes time to make an object swappable with another, you might want to have an interface apply to only a few methods (so more classes can implement the interface), it might be better to use an abstract class (so that default behaviors can be implemented in an inheritance tree).

So pre-need interfaces is probably not a good idea.

Answer 2

It might seem silly, but the potential benefit of doing it this way is that if at some point you realize there's a better way to implement a certain functionality, you can just write a new class that implements the same interface, and change one line to make all of your code use that class: the line where the interface variable is assigned.

Doing it this way (writing a new class that implements the same interface) also means you can always switch back and forth between old and new implementations to compare them.

It may end up that you never take advantage of this convenience and your final product really does just use the original class that was written for each interface. If that's the case, great! But it really didn't take much time to write those interfaces, and had you needed them, they would've saved you a lot of time.

Answer 3

Let me quote OO guru, Martin Fowler, to add some solid justification to the most common answer in this thread.

This excerpt comes from the "Patterns of Enterprise Application Architecture" (enlisted in the "classics of programming" and\or the "every dev must read" book category).

[Pattern] Separated Interface

(...)

When to Use It

You use Separated Interface when you need to break a dependency between two parts of the system.

(...)

I come across many developers who have separate interfaces for every class they write. I think this is excessive, especially for application development. Keeping separate interfaces and implementations is extra work, especially since you often need factory classes (with interfaces and implementations) as well. For applications I recommend using a separate interface only if you want to break a dependency or you want to have multiple independent implementations. If you put the interface and implementation together and need to separate them later, this is a simple refactoring that can be delayed until you need to do it.

Answering your question: no

I've seen some of the "fancy" code of this type myself, where developer thinks he's SOLID, but instead is unintelligible, difficult to extend and too complex.

Answer 4

Having the interface and coding to the interface makes it a ton easier to swap out implementations. This also applies with unit testing. If you are testing some code that uses the interface, you can (in theory) use a mock object instead of a concrete object. This allows your test to be more focused and finer grained.

It is more common from what I have seen to switch out implementations for testing (mocks) then in actual production code. And yes it is wroth it for unit testing.

Answer 5

If is a part of the Dependency Inversion principle. Basically code depends on the interfaces and not on the implementations.

This allows you to easy swap the implementations in and out without affecting the calling classes. It allows for looser coupling which makes maintenance of the system much easier.

As your system grows and gets more complex, this principle keeps making more and more sense!

Answer 6

_{After revisiting this answer, I've decided to amend it slightly.}

No, it's not best practice to extract interfaces for every class. This can actually be counterproductive. However, interfaces are useful for a few reasons:

• Test support (mocks, stubs).
• Implementation abstraction (furthering onto IoC/DI).
• Ancillary things like co- and contra-variance support in C#.

For achieving these goals, interfaces are considered good practice (and are actually required for the last point). Depending on the project size, you will find that you may never need talk to an interface or that you are constantly extracting interfaces for one of the above reasons.

We maintain a large application, some parts of it are great and some are suffering from lack of attention. We frequently find ourselves refactoring to pull an interface out of a type to make it testable or so we can change implementations whilst lessening the impact of that change. We also do this to reduce the "coupling" effect that concrete types can accidentally impose if you are not strict on your public API (interfaces can only represent a public API so for us inherently become quite strict).

That said, it is possible to abstract behaviour without interfaces and possible to test types without needing interfaces, so they are not a requirement to the above. It is just that most frameworks / libraries that you may use to support you in those tasks will operate effectively against interfaces.

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_{I'll leave my old answer for context.}

Interfaces define a public contract. People implementing interfaces have to implement this contract. Consumers only see the public contract. This means the implementation details have been abstracted away from the consumer.

An immediate use for this these days is Unit Testing. Interfaces are easy to mock, stub, fake, you name it.

Another immediate use is Dependency Injection. A registered concrete type for a given interface is provided to a type consuming an interface. The type doesn't care specifically about the implementation, so it can abstractly ask for the interface. This allows you to change implementations without impacting lots of code (the impact area is very small so long as the contract stays the same).

For very small projects I tend not to bother, for medium projects I tend to bother on important core items, and for large projects there tends to be an interface for almost every class. This is almost always to support testing, but in some cases of injected behaviour, or abstraction of behaviour to reduce code duplication.