Greg Young gave a talk at GOTO Aarhus 2011 titled Developers have a mental disorder, which was (semi-)humorously meant, but still addressed some very real concerns about the cognitive biases of software developers as a group. While I have no intention to provide a complete resume of the talk, Greg said one thing that made me think a bit (more) about SOLID code. To paraphrase, it went something like this:

Developers have a tendency to attempt to solve specific problems with general solutions. This leads to coupling and complexity. Instead of being general, code should be specific.

This sounds correct at first glance, but once again I think that SOLID code offers a solution. Due to the Single Responsibility Principle each SOLID concrete (pardon the pun) class will tend to very specifically address a very narrow problem.

Such a class may implement one (or more) general-purpose interface(s), but the concrete type is specific.

The difference between the generality of an interface and the specificity of a concrete type becomes more and more apparent the better a code base applies the Reused Abstractions Principle. This is best done by defining an API in terms of Role Interfaces, which makes it possible to define a few core abstractions that apply very broadly, while implementations are very specific.

As an example, consider AutoFixture's ISpecimenBuilder interface. This is a very central interface in AutoFixture (in fact, I don't even know just how many implementations it has, and I'm currently too lazy to count them). As an API, it has proven to be very generally useful, but each concrete implementation is still very specific, like the CurrentDateTimeGenerator shown here:

public class CurrentDateTimeGenerator : ISpecimenBuilder
    public object Create(object request, 
        ISpecimenContext context)
        if (request != typeof(DateTime))
            return new NoSpecimen(request);
        return DateTime.Now;

This is, literally, the entire implementation of the class. I hope we can agree that it's very specific.

In my opinion, SOLID is a set of principles that can help us keep an API general while each implementation is very specific.

In SOLID code all concrete types are specific.


nelson #
I don't agree with the "Reused Abstractions Principle" article at all. Programming to interfaces provides many benefits even in cases where "one interface, multiple implementations" doesn't apply.

For one, ctor injection for dependencies makes them explicit and increases readability of a particular class (you should be able to get a general idea of what a class does by looking at what dependencies it has in its ctor). However, if you're taking in more than a handful of dependencies, that is an indication that your class needs refactoring. Yes, you could accept dependencies in the form of concrete classes, but in such cases you are voiding the other benefits of using interfaces.

As far as API writing goes, using interfaces with implementations that are internal is a way to guide a person though what is important in your API and what isn't. Offering up a bunch of instantiatable classes in an API adds to the mental overhead of learning your code - whereas only marking the "entry point" classes as public will guide people to what is important.

Further, as far as API writing goes, there are many instances where Class A may have a concrete dependency on Class B, but you wish to hide the methods that Class A uses to talk to Class B. In this case, you may create an interface (Interface B) with all of the public methods that you wish to expose on Class B and have Class B implement them, then add your "private" methods as simple, non-virtual, methods on Class B itself. Class A will have a property of type Interface B, which simply returns a private field of type Class B. Class A can now invoke specific methods on Class B that aren't accessible though the public API using it's private reference to the concrete Class B.

Finally, there are many instances where you want to expose only parts of a class to another class. Let's say that you have an event publisher. You would normally only want to expose the methods that have to do with publishing to other code, yet that same class may include facilities that allow you to register handler objects with it. Using interfaces, you can limit what other classes can and can't do when they accept your objects as dependencies.

These are instances of what things you can do with interfaces that make them a useful construct on their own - but in addition to all of that, you get the ability to swap out implementations without changing code. I know that often times implementations are never swapped out in production (rather, the concrete classes themselves are changed), which is why I mention this last, but in the rare cases where it has to be done, interfaces make this scenario possible.

My ultimate point is that interfaces don't always equal generality or abstraction. They are simply tools that we can use to make code explicit and readable, and allow us to have greater control over method/property accessibility.
2011-10-25 18:15 UTC
The RAP fills the same type of role as unit testing/TDD: theoretically, it's possible to write testable code without writing a single unit test against it, but how do you know?

It's the same thing with the RAP: how can you know that it's possible to exchange one implementation of an interface with another if you've never tried it? Keep in mind that Test Doubles don't count because you can always create a bogus implementation of any interface. You could have an interface that's one big leaky abstraction: even though it's an interface, you can never meaningfully replace that single implementation with any other meaningful production implementation.

Also: using an interface alone doesn't guarantee the Liskov Substitution Principle. However, by following the RAP, you get a strong indication that you can, indeed, replace one implementation with another without changing the correctness of the code.
2011-10-25 18:56 UTC
nelson #
That was my point, though. You can use interfaces as a tool to solve problems that have nothing directly to do with substituting implementations. I think people see this as the only usecase for the language construct, which is sad. These people then turn around and claim that you shouldn't be using interfaces at all, except for cases in which substituting implementation is the goal. I think this attitude disregards many other proper uses for the construct; the most important I think is being able to hide implementation details in the public API.

If an interface does not satisfy RAP, it does not absolutely mean that interface is invalid. Take the Customer and CustomerImpl types specified in the linked article. Perhaps the Customer interface simply exposes a public, readonly, interface for querying information about a customer. The CustomerImpl class, instantiated and acted upon behind the scenes in the domain services, may specify specific details such as OR/mapping or other behavior that isn't intended to be accessible to client code. Although a slightly contrived example (I would prefer the query model to be event sourced, not mapped to a domain model mapped to an RDBMS), I think this use is valid and should not immediately be thrown out because it does not satisfy RAP.
2011-10-25 20:15 UTC
On his bio it says that Greg Young writes for Experts Exchange. Maybe he's the one with the mental disorder :P
2011-10-26 04:16 UTC
Nelson, I think we agree :) To me, the RAP is not an absolute rule, but just another guideline/metric. If none of my interfaces have multiple implementations, I start to worry about the quality of my abstractions, but I don't find it problematic if some of my interfaces have only a single implementation.

Your discussion about interfaces fit well with the Interface Segregation Principle and the concept of Role Interfaces, and I've also previously described how interfaces act as access modifiers.
2011-10-26 08:37 UTC

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Tuesday, 25 October 2011 15:01:15 UTC


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Published: Tuesday, 25 October 2011 15:01:15 UTC