Is dependency injection really just passing an argument? A brief review.

This is the first article in a small article series called from dependency injection to dependency rejection.

In a talk at the 2012 Northeast Scala Symposium, Rúnar Bjarnason casually remarked that dependency injection is "really just a pretentious way to say 'taking an argument'". Given that I've written a 500+ pages book about dependency injection, you might expect me to disagree with that. Yet, there's some truth to that statement, although it's not quite as simple as that.

In this article, I'll show you some simple examples and explain why, on the one hand, Rúnar Bjarnason is right, but also, on the other hand, why there's a bit more to it.

Restaurant reservation example #

Like the other articles in this series, the example scenario is on-line restaurant reservation. Imagine that you've been asked to develop an HTTP-based API that accepts JSON documents containing restaurant reservations. Furthermore, assume that you're using ASP.NET Web API with C# for the job, and that you're aspiring to use domain-driven design.

In order to handle the incoming POST request, you could write an action method like this:

public IHttpActionResult Post(ReservationRequestDto dto)
    var validationMsg = validator.Validate(dto);
    if (validationMsg != "")
        return this.BadRequest(validationMsg);
    var r = mapper.Map(dto);
    var id = maîtreD.TryAccept(r);
    if (id == null)
        return this.StatusCode(HttpStatusCode.Forbidden);
    return this.Ok();

This method follows a simple and familiar path: validate input, map to a domain model, delegate to said model, examine posterior state, and return a result.

You may have noticed, though, that this method doesn't do all the work itself. It delegates some of the work to collaborators: validator, mapper, and maîtreD. Where do these collaborators come from?

They are dependencies. Could you make the Post method take them as arguments?

Unfortunately, you can't. The Post method constitutes part of the boundary of the HTTP API. ASP NET Web API routes and dispatches incoming HTTP requests by convention, and action methods must follow that convention. You can't just make the function take any argument you'd like, so you have to find another place to pass those dependencies to the object.

The second-best option (after the Post method itself) is via the constructor:

public ReservationsController(
    IValidator validator,
    IMapper mapper,
    IMaîtreD maîtreD)
    this.validator = validator;
    this.mapper = mapper;
    this.maîtreD = maîtreD;

This is the application of a design pattern called constructor injection. It captures the dependencies in class fields, making them available for members (like Post) of the class.

This turns out to be a regular pattern.

Turtles all the way down #

You could argue that the Post method is a special case, since it's part of the boundary of the system, and therefore must adhere to specific rules. On the other hand, these rule don't apply deeper in the implementation, so could you implement other objects by simply passing in dependencies as arguments?

Consider, as an example, the implementation of IMaîtreD.TryAccept:

public int? TryAccept(Reservation reservation)
    var reservedSeats = reservationsRepository
        .Sum(r => r.Quantity);
    if (reservedSeats + reservation.Quantity <= capacity)
        reservation.IsAccepted = true;
        return reservationsRepository.Create(reservation);
    return null;

This method has another collaborator: reservationsRepository. It's another dependency. Where does it come from?

Could you make the TryAccept method take reservationsRepository as an argument?

Unfortunately, that's not possible either, because the method is defined by the IMaîtreD interface:

public interface IMaîtreD
    int? TryAccept(Reservation reservation);

You may recall that the above Post method is programmed against the IMaîtreD interface, and not the concrete class. It'd be a leaky abstraction to add IReservationsRepository as an argument to IMaîtreD.TryAccept, because not all implementations of the interface may need that dependency. Or perhaps another implementation has another dependency. Should we add that to the parameter list of IMaîtreD.TryAcceptas well?

Surely, that's not a tenable design principle. On the other hand, by using constructor injection, you can decouple implementation details from your abstractions:

public MaîtreD(int capacity, IReservationsRepository reservationsRepository)
    this.capacity = capacity;
    this.reservationsRepository = reservationsRepository;

This constructor not only takes an IReservationsRepository object, but also an integer that represents the capacity of the restaurant in question. This demonstrates that dependencies can also be primitive values.

Summary #

Dependency injection is, in a sense, only a specific way for objects to take arguments. Often, however, objects have roles defined by the interfaces they implement. Such objects may need collaborators that are not available via the APIs defined by these interfaces, so you'll have to supply dependencies via members that belong to the concrete class in question. Passing dependencies via a class' constructor is the best way to do that.

Next: Partial application is dependency injection.


Hey Mark. You may want to update the article slightly to cover the use of the [FromServices] attribute, which allows you to inject any service into a controller action as method-level injection. The main article on it is this one.

I'm personally not a fan of using it, but it is an option that transforms standard injection into parameter-passing for controllers.

2020-02-23 15:11 UTC

Juliano, thank you for writing. I wasn't aware of that particular capability, so thank you for bringing it to my attention.

My article attempts to explain a general software design problem, as well as a possible solution. While it uses ASP.NET as an example context, the main point is independent of any particular framework, or language, for that matter.

2021-02-24 14:09 UTC

Wish to comment?

You can add a comment to this post by sending me a pull request. Alternatively, you can discuss this post on Twitter or somewhere else with a permalink. Ping me with the link, and I may respond.


Friday, 27 January 2017 09:27:00 UTC


"Our team wholeheartedly endorses Mark. His expert service provides tremendous value."
Hire me!
Published: Friday, 27 January 2017 09:27:00 UTC