Question

When using IoC for dependency injection the most recommended use is constructor injection (as told by many articles), my question is :
Is it better to inject using the constructor parameters or passing the IoC container through the constructor to inject the classes needed, and what is more useful for unit testing ?

Answer 1

Although passing the container through the constructor is better than making the container accessible as a singleton for the complete application, it is still a form of Service Locator (anti-pattern), and is not recommended. This has clear disadvantages:

1. It makes your code much harder to follow.
2. It makes it much less obvious when the Single Responsibility Principle is violated (since the class hides which things it depends on).
3. It makes it much hard to test, since you need to pass a configured container and you need to look in the code to see what the test need.
4. When requesting instances from the container directly from within every class, you will disable many features that IOC containers give you, because you don’t allow the container to build up the object graph. Depending on the framework of choice, certain lifestyles and features like context based injection will not be usable.
5. All your tests use a container instance, making your tests come complex, and all your test have a dependency on the DI framework, which makes it very expensive to switch to another framework when needed.

Just inject the dependencies into the constructor, never* the container itself.

_{*Exception to this rule is when such class is located inside the appliation’s Composition Root. In that case it’s not considered to be the Simple Locator pattern, because that class is simply an infrastructure component.}