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Code Smell 135 - Interfaces With just One Realization

Maxi Contieri on May 26, 2022

Being generic and foreseeing the future is good. TL;DR: Don't over-generalize Problems Speculative Design Complexity Over-Engineeri...

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David Kröll • May 26 '22

I mostly agree with you opition, except a single thing: tests. I almost ever extract an interface for logic classes to write tests.

Maxi Contieri • May 26 '22

And how many realizations do you have ?

David Kröll • May 26 '22

Typically just a single "real" one, the other ones are created by mocking frameworks (like NSubstitue for .NET)

Maxi Contieri • May 27 '22

I don't follow your tests example

Can you add a small piece of code as an example?

David Kröll • May 27 '22

public class Car
{
    private readonly IEngine _engine;
    private bool isRunning;

    public Car(IEngine engine)
    {
        _engine = engine;
    }

    public void Start()
    {
        if (!isRunning)
        {
            _engine.Start();
            isRunning = true;
        }
    }
}

public interface IEngine
{
    void Start();
}

public class DieselEngine : IEngine
{
    public void Start()
    {
        Console.WriteLine("Starting diesel engine");
    }
}

Assume you have a class Car which depends on a class DieselEngine. It would not be possible to write a unit test just for the Car when the concrete type DieselEngine is required from the Car constructor. Instead, the test have then to be written keeping in mind that also the concrete implementation for DieselEngine is going to be tested.

When you extract an interface IEngine, you can write a test for the Car itself again (without relying on concrete DieselEngine). Typically you do not have to create another realization of the IEngine interface - instead you can use a mocking framework which does this for you, and check if the method IEngine.Start is only called if the engine is currently off.

I hope this answers your question

Maxi Contieri • May 27 '22

Thank you very much for the code.

I think the problem is here

"Assume you have a class Car which depends on a class DieselEngine. It would not be possible to write a unit test just for the Car when the concrete type DieselEngine is required from the Car constructor."

Why not test the Car with a DieselEngine ? This is a real business scenario

Also, IMHO , Callling an interface IEngine is another smell. Even dough I know it is a standard practice.
Can I use your code snippet in another smell ?

Stepan Mozyra • May 27 '22

"Why not test the Car with a DieselEngine ?" - This is idea behind unit tests :) to have a single unit under the test.

If you do unit testing in your project - you will always have two implementations of your interfaces.

"This is a real business scenario" - Testing of business scenarios is just an another kind of tests.

Maxi Contieri • May 27 '22

Right !

You have two interfaces realizations. Therefore this code smell does not apply anymore

David Kröll • May 28 '22

Can I use your code snippet in another smell ?

Yes feel free.

It depends on what you call a unit I think, bit you're right. If the DieselEngine is testable at all (could be a real DieselEngine which is not availabe during CI test run, etc).

Maxi Contieri • May 28 '22

Build your abstractions with TDD so they will be testaable from their inception

Jessica Veit • May 26 '22

Additionally - Dependency Injection in general. If you do add a second realization later, you have the first realizations reference everywhere (testability) and must rewrite everything.

Maxi Contieri • May 26 '22

Exactly. That is a valid counterexample. I'll add it

Maxi Contieri • May 27 '22

Done!

Jessica Veit • May 27 '22

Thank you for being that open! 😁

Aman Agrawal • May 27 '22

Testing needs often drive the decisions to put an implementation behind an interface. Main reason being the ability to isolate the system under test from its runtime dependencies by creating appropriate test doubles that follow the same interface. Anymore coupling, and we are talking about integrated tests which though valuable can also be expensive to maintain in the long run as the system scales and can also get flaky over time. Any valuable system with reasonably high essential complexity better be designed with a modular/component architecture and then it's only right that these components be testable in isolation from their runtime dependencies. The way to achieve that then is putting abstractions that you can create test doubles out of.

If a component is testable with its runtime dependencies without increasing the testing effort or cost significantly, then I'd forego interfaces in those narrow cases and test with the real dependencies for e.g. classes that mostly have algorithmic logic executed in memory with other algorithmic dependencies that don't do I/O or interact with external systems.

Reusable libraries is another place where interfaces are very useful as extensibility hooks for consumers to provide their own implementations for.

Indiscriminately creating an interface for every class in your code without critically thinking, is wrong! There I agree with you!

Maxi Contieri • May 27 '22

I agree with you testability is one of the most important quality attributes of a system.
Test doubles decrease the confidence of a system since you are not testing a real one.
If you need to isolate your system you can build an interface and have two implementations: one for real and one for testing. Therefore you don't have just one realization. You have two. And you are no longer in code smell scenario

That said. Your last paragraph is the most important

Aman Agrawal • May 28 '22 • Edited

How is creating an implementation for testing, not a test double?

Maxi Contieri • May 28 '22

You need to use real objects for testing, not mock or test objects

Aman Agrawal • May 28 '22

Interfaces for external dependencies allow you the flexibility of using either a test double or a real implementation without much overhead. Arguing about that is a moot point.

There is a lot to be said about levels and granularity of testing but that's not this post!

Maxi Contieri • May 28 '22

Right! there also a lot of test smells, many on this series and many others to come!

Adam Katora • May 26 '22

This reminds me a lot of the Don Knuth quote, "Premature optimization is the root of all evil"

Maxi Contieri • May 27 '22

actually, this is my favorite quote