Is there an alternative to bastard injection? (AKA poor man's injection via default constructor)

Question

I most commonly am tempted to use "bastard injection" in a few cases. When I have a "proper" dependency-injection constructor:

public class ThingMaker {
    ...
    public ThingMaker(IThingSource source){
        _source = source;
    }

But then, for classes I am intending as public APIs (classes that other development teams will consume), I can never find a better option than to write a default "bastard" constructor with the most-likely needed dependency:

    public ThingMaker() : this(new DefaultThingSource()) {} 
    ...
}

The obvious drawback here is that this creates a static dependency on DefaultThingSource; ideally, there would be no such dependency, and the consumer would always inject whatever IThingSource they wanted. However, this is too hard to use; consumers want to new up a ThingMaker and get to work making Things, then months later inject something else when the need arises. This leaves just a few options in my opinion:

1. Omit the bastard constructor; force the consumer of ThingMaker to understand IThingSource, understand how ThingMaker interacts with IThingSource, find or write a concrete class, and then inject an instance in their constructor call.
2. Omit the bastard constructor and provide a separate factory, container, or other bootstrapping class/method; somehow make the consumer understand that they don't need to write their own IThingSource; force the consumer of ThingMaker to find and understand the factory or bootstrapper and use it.
3. Keep the bastard constructor, enabling the consumer to "new up" an object and run with it, and coping with the optional static dependency on DefaultThingSource.

Boy, #3 sure seems attractive. Is there another, better option? #1 or #2 just don't seem worth it.

Answer 1

As far as I understand, this question relates to how to expose a loosely coupled API with some appropriate defaults. In this case, you may have a good Local Default, in which case the dependency can be regarded as optional. One way to deal with optional dependencies is to use Property Injection instead of Constructor Injection - in fact, this is sort of the poster scenario for Property Injection.

However, the real danger of Bastard Injection is when the default is a Foreign Default, because that would mean that the default constructor drags along an undesirable coupling to the assembly implementing the default. As I understand this question, however, the intended default would originate in the same assembly, in which case I don't see any particular danger.

In any case you might also consider a Facade as described in one of my earlier answers: Dependency Inject (DI) "friendly" library

BTW, the terminology used here is based on the pattern language from my book.

Answer 2

My trade-off is a spin on @BrokenGlass:

1) Sole constructor is parameterized constructor

2) Use factory method to create a ThingMaker and pass in that default source.

public class ThingMaker {
  public ThingMaker(IThingSource source){
    _source = source;
  }

  public static ThingMaker CreateDefault() {
    return new ThingMaker(new DefaultThingSource());
  }
}

Obviously this doesn't eliminate your dependency, but it does make it clearer to me that this object has dependencies that a caller can deep dive into if they care to. You can make that factory method even more explicit if you like (CreateThingMakerWithDefaultThingSource) if that helps with understanding. I prefer this to overriding the IThingSource factory method since it continues to favor composition. You can also add a new factory method when the DefaultThingSource is obsoleted and have a clear way to find all the code using the DefaultThingSource and mark it to be upgraded.

You covered the possibilities in your question. Factory class elsewhere for convenience or some convenience within the class itself. The only other unattractive option would be reflection-based, hiding the dependency even further.

Answer 3

One alternative is to have a factory method CreateThingSource() in your ThingMaker class that creates the dependency for you.

For testing or if you do need another type of IThingSource you would then have to create a subclass of ThingMaker and override CreateThingSource() to return the concrete type you want. Obviously this approach only is worth it if you mainly need to be able to inject the dependency in for testing, but for most/all other purposes do not need another IThingSource

Answer 4

I vote for #3. You'll be making your life--and the lives of other developers--easier.

Answer 5

If you have to have a "default" dependency, also known as Poor Man’s Dependency Injection, then you have to initialize and "wire" the dependency somewhere.

I will keep the two constructors but have a factory just for the initialization.

public class ThingMaker
{
    private IThingSource _source;

    public ThingMaker(IThingSource source)
    {
        _source = source;
    }

    public ThingMaker() : this(ThingFactory.Current.CreateThingSource())
    {
    }
}

Now in the factory create the default instance and allow the method to be overrided:

public class ThingFactory
{
    public virtual IThingSource CreateThingSource()
    {
        return new DefaultThingSource();
    }
}

Update:

Why using two constructors: Two constructors clearly show how the class is intended to be used. The parameter-less constructor states: just create an instance and the class will perform all of it's responsibilities. Now the second constructor states that the class depends of IThingSource and provides a way of using an implementation different than the default one.

Why using a factory: 1- Discipline: Creating new instances shouldn't be part of the responsibilities of this class, a factory class is more appropriate. 2- DRY: Imagine that in the same API other classes also depend on IThingSource and do the same. Override once the factory method returning IThingSource and all the classes in your API automatically start using the new instance.

I don't see a problem in coupling ThingMaker to a default implementation of IThingSource as long as this implementation makes sense to the API as a whole and also you provide ways to override this dependency for testing and extension purposes.

Answer 6

You are unhappy with the OO impurity of this dependency, but you don't really say what trouble it ultimately causes.

• Is ThingMaker using DefaultThingSource in any way that does not conform to IThingSource? No.
• Could there come a time where you would be forced to retire the parameterless constructor? Since you are able to provide a default implementation at this time, unlikely.

I think the biggest problem here is the choice of name, not whether to use the technique.

Answer 7

The examples usually related to this style of injection are often extremely simplisitic: "in the default constructor for class B, call an overloaded constructor with new A() and be on your way!"

The reality is that dependencies are often extremely complex to construct. For example, what if B needs a non-class dependency like a database connection or application setting? You then tie class B to the System.Configuration namespace, increasing its complexity and coupling while lowering its coherence, all to encode details which could simply be externalized by omitting the default constructor.

This style of injection communicates to the reader that you have recognized the benefits of decoupled design but are unwilling to commit to it. We all know that when someone sees that juicy, easy, low-friction default constructor, they are going to call it no matter how rigid it makes their program from that point on. They can't understand the structure of their program without reading the source code for that default constructor, which isn't an option when you just distribute the assemblies. You can document the conventions of connection string name and app settings key, but at that point the code doesn't stand on its own and you put the onus on the developer to hunt down the right incantation.

Optimizing code so those who write it can get by without understanding what they are saying is a siren song, an anti-pattern that ultimately leads to more time lost in unraveling the magic than time saved in initial effort. Either decouple or don't; keeping a foot in each pattern diminishes the focus of both.

Answer 8

For what it is worth, all the standard code I've seen in Java does it like this:

public class ThingMaker  {
    private IThingSource  iThingSource;

    public ThingMaker()  {
        iThingSource = createIThingSource();
    }
    public virtual IThingSource createIThingSource()  {
        return new DefaultThingSource();
    }
}

Anybody who doesn't want a DefaultThingSource object can override createIThingSource. (If possible, the call to createIThingSource would be somewhere other than the constructor.) C# does not encourage overriding like Java does, and it might not be as obvious as it would be in Java that the users can and perhaps should provide their own IThingSource implementation. (Nor as obvious how to provide it.) My guess is that #3 is the way to go, but I thought I would mention this.

Answer 9

Just an idea - perhaps a bit more elegant but sadly doesn't get rid of the dependency:

• remove the "bastard constructor"
• in the standard constructor you make the source param default to null
• then you check for source being null and if this is the case you assign it "new DefaultThingSource()" otherweise whatever the consumer injects

Answer 10

Have an internal factory (internal to your library) that maps the DefaultThingSource to IThingSource, which is called from the default constructor.

This allows you to "new up" the ThingMaker class without parameters or any knowledge of IThingSource and without a direct dependency on DefaultThingSource.

Answer 11

For truly public APIs, I generally handle this using a two-part approach:

1. Create a helper within the API to allow an API consumer to register "default" interface implementations from the API with their IoC container of choice.
2. If it is desirable to allow the API consumer to use the API without their own IoC container, host an optional container within the API that is populated the same "default" implementations.

The really tricky part here is deciding when to activate the container #2, and the best choice approach will depend heavily on your intended API consumers.

Answer 12

I support option #1, with one extension: make DefaultThingSource a public class. Your wording above implies that DefaultThingSource will be hidden from public consumers of the API, but as I understand your situation there's no reason not to expose the default. Furthermore, you can easily document the fact that outside of special circumstances, a new DefaultThingSource() can always be passed to the ThingMaker.