[MoreQARespect]

Dependency injection is only useful when you've managed to isolate logic/math intensive code. Some apps dont have any logic intensive code. Many others have very little.

In those cases dependency injection just increases the SLOC with the payoff that you are able a bunch of trivial unit tests that'll probably never catch a bug.

Integration tests as a default have a the best ROI in those cases.

[frankdejonge]

I look more towards optionality myself. Take for example the encapsulation of randomness. Depending on an abstract notion of randomness (an interface) that decouples you from the implementation is both useful for testing as it is for maintenance of a system. For tests, removing randomness entirely makes it deterministic, allowing you to tests for exact matches instead of approximations. For systems, at a smaller scale get away with a reduced amount of randomness, while systems at scale require more sophisticated code for this. You don't want to replace all of that code in all instances, but rather leverage the capability and replace the implementation.

Another angle is the encapsulation of storage. Using in-memory storage for tests makes Ci pipelines very quick and production storage might evolve over time to accommodate scaling requirements (sharding and such).

[Tainnor]

You don't have to inject every dependency. If it's essentially a pure function, then just call that function (despite what old-school Java advocates, free functions not tied to any object are fine).

I like dependency injection for things which have state and/or do IO and/or are expensive to construct.

[taberiand]

This isn't really true. For one, dependency injection isn't specific to testing or processing logic, it's an architectural approach used primarily for managing separation of concerns and modularisation.

[tonyedgecombe]

>it's an architectural approach used primarily for managing separation of concerns and modularisation

That always feels like a post hoc justification to me. If you demand 100% test coverage on a Java/C# project then you will almost certainly end up with some kind of dependency injection at the end. Whether that is useful or not is often debatable.

[taberiand]

Dependency injection where the injected dependencies are interfaces - particularly, interfaces designed to provide the minimal external requirements of the module - has a broad architectural impact on the design and implementation of a system than just enabling unit testing. It enables isolation, decoupling and modularisation and clarifies the system interactions.

[anthony_d]

> It enables isolation, decoupling and modularisation and clarifies the system interactions.

That’s what Spring claims but the truth is that using interfaces enables those things. Dependency injection might encourage the use of interfaces, but it also inserts an abstraction between tests and the things you want to test.

In my experience dependency injection frequently obfuscates system interactions and invalidates any assurances you might get from good unit tests.

Dependency injection is just a tool. It can be useful but is often misused and doesn’t perform magic.

[piva00]

You can do dependency injection without a DI framework/container, just declare the required dependencies for a class as required parameters in the constructor and build your own dependency tree manually when constructing the objects in `main()`-equivalent.

Using a DI container is not required to implement the DI pattern, it's just a tool that facilitates some more complex DI.

I tend to prefer to keep my projects simple enough that manual DI is quite appropriate and readable.

[taberiand]

As the other commenter points out, I believe you are conflating DI implementations with Dependency Injection as a methodology. But I suppose I am too, because really I'm talking about using dependency injection to achieve Dependency Inversion - the D of the SOLID design principles.

There are good and bad implementations of dependency injection (or, perhaps, appropriate and inappropriate depending on requirements), but the principle of dependency inversion applies at an architectural level, before specifics of implementation and things like unit testing come into play

[MoreQARespect]

I didnt say it was specific to testing. Nor did I say it was never appropriate. Nonetheless the pursuit of "unit testability" is probably most often why it's done.