For me, life is greatly simplified when dependencies are inverted and constructor injection is used to provide a clean mechanism to introduce dependencies to a class. It's explicit and easy to test.
Others however argue that some dependencies are private implementation details to the class and external callers shouldn't know about them. The argument is that exposing these dependencies through the constructor violates encapsulation and introduces coupling. This argument is usually coupled with resistance to using interfaces or classes with virtual methods.
From my experience, there are times when composition in the constructor makes sense but there's a fine line when constructor injection should be used. I want to use this post to elaborate on these arguments and provide my perspective on this debate.
Does Constructor Injection violate Encapsulation?
Does exposing the internal dependencies of a class in the constructor expose the implementation details of a class? If you are allowing callers to construct the class directly, then you are most certainly breaking encapsulation as the callers must posses the knowledge of how to construct your class. However, regardless of the constructor arguments if callers know how to construct your class you are also coupling to a direct implementation and will undoubtedly create a test impediment elsewhere. There are some simple fixes for this (including more constructor injection or another inversion of control technique) which I'll outline later.
So if constructor injection violates encapsulation, when is it safe to use composition in the constructor?
It really depends on the complexity and relationship of the subject to its dependencies. If the internal dependencies are simple and contain no external dependencies there is likely no harm in using composition to instantiate them in the constructor (the same argument can be made for small static utility methods). For instance, a utility class that performs some translation or other processing activity with limited outcomes may work as a private implementation detail. Problems arise as soon as these dependencies take on further dependencies or produce output that influences the conditional logic of the subject under test. When this happens the argument to keep these as internally controlled dependencies becomes flawed and it may be necessary to upgrade the "private implementation detail" to an inverted dependency.
Before:
This example shows a class that has a private implementation detail that influences logic of the subject under test. The consequence of this design is that the internal details of the validator leak into the test specifications. This leads to very brittle tests as tests must concern themselves with object construction and validation rules -- any change to the object or validation logic will break the tests.
public class MyModelTranslator
{
public MyModelTranslator()
{
_validator = new MyModelValidator();
}
public MyViewModel CreateResult(MyModelObject model)
{
var result = new MyViewModel();
if (_validator.Validate( model ));
{
result.Id = model.Id;
}
else {
result.State = MyResultState.Invalid;
}
return result;
}
}
[Test]
public void WhenCreatingAResult_FromAVaidModelObejct_ShouldHaveSameId()
{
var subject = new MyModelTranslator();
// create model object with deep understanding how Id's
// and expiry dates are related
var model = new MyModelObject()
{
Id = "S1402011"
Expiry = Datetime.Parse("12/31/2011");
};
var result = subject.CreateResult( model );
Assert.AreEqual("S1402011", result.Id);
}
After:
This example shows the above example corrected to use a constructor injected dependency that can be mocked. In doing so, the test is not encumbered with object creation or validation details and can easily simulate validate states without introducing brittleness.
public class MyModelTranslator
{
public MyModelTranslator(MyModelValidator validator);
{
_validator = validator;
}
// ...
}
[Test]
public void WhenCreatingAResult_FromAVaidModelObejct_ShouldHaveSameId()
{
var validatorMock = new Mock<MyModelValidator>();
var subject = new MyModelTranslator(validatorMock.Object);
var model = new MyModelObject()
{
Id = "Dummy"
};
// we no longer care how validation is done,
// but we expect validation to occur and can easily control
// and test outcomes
ValidatorMock.Setup( x => x.Validate( model )).Returns( true );
var result = subject.CreateResult( model );
Assert.AreEqual("Dummy", result.Id);
}
Combatting Coupling with Inversion of Control
As the above points out, if we expose the dependencies of a class in the constructor we move the coupling problems out of the class and into the callers. This is not good but it can easily resolved.
An example that shows that our controller class now knows about the MyModelValidator:
public class MyController
{
public MyController()
{
_translator = new MyModelTranslator( new MyModelValidator() );
}
}
More Constructor Injection
Ironically, the solution for solving coupling and construction problems is more constructor injection. This creates a Russian Doll where construction logic is deferred and pushed higher and higher up the stack resulting in a top level component which is responsible for constructing the entire object graph. While this provides an intuitive API that clearly outlines dependencies, it's tedious to instantiate by hand. This is where dependency injection frameworks like Unity, StructureMap and others come in: they can do the heavy lifting for you. If done right, your application should only have well defined points where the dependency container is used.
public class MyController : IController
{
public MyController(MyModelTranslator translator)
{
_translator = translator;
}
}
Note that this assumes that the constructor arguments are abstractions -- either interfaces, abstract classes or classes with virtual methods. In effect, by exposing the constructor arguments we trade off the highly sealed encapsulated black box for an open for extensibility model.
Factories / Builders
In some cases, using Constructor Injection everywhere might not be a good fit. For example, if you had a very large and complex object graph, creating everything upfront using constructor injection might represent a performance problem. Likewise, not all parts of the object graph will be used immediately and you may need to defer construction until needed. In these cases, the good ol' Gang of Four Factory pattern is a handy mechanism to lazy load components.
So rather that construct the entire object graph and pass resolved dependencies as constructor arguments, pass the factory instead and create the lower-level sub-dependencies when needed.
public class MyControllerFactory : IControllerFactory
{
private IUnityContainer _container;
public MyControllerFactory(IUnityContainer container)
{
_container = container;
}
public IController Create()
{
_container.Resolve<MyController>();
}
}
Service Location
While Service Location is an effective mechanism for introducing inversion of control, I'm listing it last for a reason. Unlike Constructor Injection, Service Location couples all your code to an intermediate provider. Depending on your application and relative complexity this might be acceptable, but from personal experience it's a very slippery slope -- actually, it would be more appropriate to call it a cliff because removing or replacing the container from an existing codebase can be a massive undertaking.
I see service location as a useful tool when refactoring legacy code towards Constructor Injection. The lower "leaf-nodes" of the object graph can take advantage of constructor injection and the higher nodes can temporarily use service location to create the "leaves". As you refactor, the service locator is removed and replaced with constructor injection, which results in an easier to test and discoverable API. This also provides a pragmatic bottom up refactor approach.
Summary
- Use "new" when dependencies have no external dependencies or don't influence conditional flow of the subject. Conversely, use constructor injection when dependencies have additional dependencies.
- Use an IoC container to construct objects that use constructor injection, but only use the IoC container in high level components that are responsible for construction of the object graph. If constructing the entire object graph is a performance concern, consider encapsulating the container in a Factory that can be used to resolve remaining parts of the object graph when needed.
- To prevent violating encapsulation concerns, use inversion of control to promote using abstractions instead of concrete dependencies.
