5

Given the class:

public class Foo : IFoo
{
  private IBarRepository repository

  public Foo(IBarRepository repository)
  {
     this.repository = repository
  }

  public IList<IBar> Bars { get; private set; }
}

My long standing instinct is to initialise the list of IBar in the constructor: this.Bars = new List<IBar>(); but using Constructor Injection, the Single Responsibility of the constructor is for it to set the dependancies for the class.

What is the best way to handle this?

Should I have a collection initialiser that I call in any method before using the collection?

private void InitialiseCollection()
{
  if (this.Bars == null)
  {
     this.Bars = new List<IBar>();
  }
}

public void Add(IBar bar)
{
    this.InitialiseCollection();
    this.Bars.Add(bar)
}
Improve this question

2 Answers 2

Reset to default
13

Well, the simple answer to your question is

public readonly IList<IBar> Bars = new List<IBar>();

But I think you should question your premise.

but using Constructor Injection, the Single Responsibility of the constructor is for it to set the dependancies for the class.

Where have you read that? It's entirely incorrect.

It doesn't matter if you're using Constructor Injection or not. Your constructor's responsibility is to construct an object, fully initialized, with all the dependencies required to operate on that object. Those dependencies can be other services or state or values that distinguish this object from another object of its type.

Improve this answer
6
  • It's from "Dependancy Injection in .Net" by Mark Seemann, Section 4.1.1 (page 99 of my edition) Commented Jan 6, 2014 at 16:05
  • @NikolaiDante: Interesting. Mark used to be around here a lot but hasn't been much recently. With some luck, he'll notice this and be able to explain. I'd be surprised if he meant it as literally as you have taken it here. Commented Jan 6, 2014 at 16:29
  • That would be good. I've also just found a blog post of his on the subject - blog.ploeh.dk/2011/03/03/InjectionConstructorsshouldbesimple which has "It's important to understand that when using Constructor Injection the constructor should contain no additional logic.". Commented Jan 6, 2014 at 16:32
  • 1
    @NikolaiDante: That sentence is correct but, again, literally questionable. Initializing a list is not logic, at least not in the context of that sentence. Logic would be if it were calculating a single item from a list, or conditionally adding items to the list. But, arguably, all code is logic so I can see why you'd misread the intent. Commented Jan 6, 2014 at 16:49
  • @pdr +1 instantiating the list in this case is part of the "construction process" and rightfully belongs in the constructor regardless of whatever else you are injecting into it. Commented Jan 6, 2014 at 19:28
4

The Single Responsibility principle applies to the class as a whole, not just to the constructor. Also, resist the urge to initialize the Bars variable using that last method - if you are using injection, it should be responsible for giving you a complete and working instance of Foo or none at all if it can't resolve the object to pass to the constructor.

Improve this answer

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Start asking to get answers

Find the answer to your question by asking.

Ask question

Explore related questions

See similar questions with these tags.