This is the second part of my series on abstractions.
The Reused Abstraction Principle is a simple in concept in practice, but oddly rarely followed in typical enterprise development. I myself have been incredibly guilty of this in the past.
Most code bases have a 1:1 mapping of interfaces to implementations. Usually this is the sign of TDD or automated testing being applied badly. The majority of these interfaces are wrong. 1:1 mappings between interfaces and implementations is a code smell.
Such situations are usually the result of extracting an interface from an implementation, rather than having the client drive behaviour.
These interfaces are also often bad abstractions, known as "leaky abstractions". As I've discussed previously, these abstractions tend to offer nothing more than simple indirection.
Apply the "rule of three". If there is only ever one implementation, then you don't need the interface/base class. If you do need to introduce an interface, have the client provide it. Try to resist the urge to extract from an implementation. Any stubs or testing implementations should be treated as valid implementations, despite no use within the production code directly.
In the first example there is a 1:1 mapping. This is clutter and needless indirection. As we have nothing to replace
FooService with, the interface offers no value. The second example shows multiple implementations of
IFoo. Here different implementations have unique responsibilities. We could use a test stub, or use the decorator pattern whenever we use
IFoo. The abstraction is valuable.
If you can introduce a composite or decorator this is probably a sign of a good abstraction at work. Likewise the ability to replace your implementation and have the code still function is a good sign. Such an example would be
SqlRepository replaced with
IRepository is required.
Additionally just because you opt to use dependency injection, there is no rule stating said dependency must be an interface or base class.
The final point is to remember what good dependencies are, everything else can be an implementation detail leading to more flexible and resilient code.