Do you inject dependencies in the constructor even if the class does not own these dependencies, but uses them in methods?

Question

If a School class relies on an EmailSender class you might have to inject this EmailSender through it's constructor with a generic interface like IMessagable or IMessageSender or even IEmailSender so that it can be swapped out for something else in the future.

In order to do this you would create a read-only private field in the School class.

But then you're saying that the School HAS-A MessageSender but surely it just USES it in a few of it's methods??

Answer 1

Even if a class only uses one of many methods in a passed-in object, the entire object is still considered a dependency of the class.

This becomes evident when you change the API (the method signatures) of an object. By handing that object to a class, you essentially guarantee that object's "contract." If you change that object's API, you risk breaking classes that depend on it.

That's why we often use Interfaces to pass dependencies to classes. The interface enforces the contract between the class and its dependency, and encourages decoupling because the passed in object has to conform only to the interface's methods.

The nomenclature IS-A and HAS-A is used primarily to distinguish inheritance from composition.

Answer 2

Any property/member a class declares, is something a class HAS.

The Has-A is here to differentiate from the Is-A of inheritance.

The uses-a property, is more related to class cohesion rather than composition. If the class has-A property but it does not use it, or uses it in a small percentage of functions, it has low cohesion.

Both HAS-A and IS-A are dependencies of the class. That's why we strive to have them both as abstractions, to loosely couple the two.

Any class/interface definition without which your class won't compile, is a dependency. Even if it doesn't have a HAS-A relationship and it's just passed as a function parameter.