It took me two years to write this post. Each time I started writing it, it suffered from gradual and seemingly unavoidable feature creep: should I mention dependency inversion principle? What about SOLID principles in general? What about testability? The list goes on and on. But it was a recent interesting discussion with a friend about the use (and abuse) of dependency injection that prompted me to finally finish it.
I have been using Unity IoC container for a while now and in the process I have managed to ”abuse” it in every possible way so I hope this post covers some of the more advanced and intricate usage scenarios you may ever come across.
Trivial case
In case you have never used Unity before, the behaviour of the container is pretty trivial: call Register<T,TImpl>() on a number of types then make a call to Resolve<T> and with a bit of luck you will get a instance of it:
[TestFixture]
class TrivialTests
{
[Test]
public void TrivialOne()
{
var container = new UnityContainer();
container.RegisterType<IBar, Bar>();
var instance = container.Resolve<IBar>();
Assert.NotNull(instance);
}
}
internal class Bar : IBar {}
internal interface IBar {}
Implementing a factory with Unity
Unity is not only able to resolve an implementation of a particular interface; it may also help in resolving dependencies when the type to be instantiated is known (even if it has not been registered with the container):
[TestFixture]
public class ObjectBuildupTest
{
[Test]
public void ObjectBuildup()
{
var container = new UnityContainer();
container.RegisterType<ISomeService, SomeService>();
var anInstance = container.Resolve<ObjectWithDependencies>();
Assert.NotNull(anInstance);
}
}
public class ObjectWithDependencies
{
public ObjectWithDependencies(ISomeService someService)
{
if (someService == null) throw new ArgumentNullException("someService");
}
}
public interface ISomeService {}
public class SomeService : ISomeService {}
As you can see from the code above, even though the type “ObjectWithDependencies” has not been registered with the container, Unity was able to correctly instantiate it and inject it with previously registered dependency. Such functionality may come very useful when implementing a factory: we know what we want to instantiate but not necessarily want to be bothered with instantiating all the dependencies. The following sample illustrates this approach.
[TestFixture]
public class FactoryTest
{
[Test]
public void UnityBasedFactoryTest()
{
var container = new UnityContainer();
container.RegisterType<ISomeService, SomeService>();
container.RegisterType<ISomeFactory, SomeFactory>();
var factory = container.Resolve<ISomeFactory>();
Assert.NotNull(factory.ManufactureAnObject("A"));
}
}
public interface ISomeFactory
{
ICommonInterface ManufactureAnObject(string anArgument);
}
public class SomeFactory : ISomeFactory
{
private readonly IUnityContainer _container;
public SomeFactory(IUnityContainer container)
{
_container = container;
}
public ICommonInterface ManufactureAnObject(string anArgument)
{
switch(anArgument)
{
case "A":
return _container.Resolve<TypeA>();
case "B":
return _container.Resolve<TypeB>();
default:
throw new NotSupportedException();
}
}
}
public interface ICommonInterface {}
public class TypeB : ICommonInterface
{
public TypeB(ISomeService someService)
{
}
}
public class TypeA : ICommonInterface
{
public TypeA(ISomeService someService)
{
}
}
As you have probably noticed our factory requires an instance of IUnityContainer, luckily the container auto-registers itself, so whenever and instance of IUnityContainer is required, one will be injected without any fuss. Purists may frown upon direct use of the container, but in my opinion this is one of the very rare cases where it can be justified.
Multiple implementations of the same contract
The following example illustrates how to register multiple implementations of the same interface. Each registration is done with a particular key (quite a common feature among IoC containers) and then when resolving it we need to specify the key to get a particular implementation:
[TestFixture]
public class MultipleImplementationsTests
{
[Test]
public void MultipleImplementations_ResolveCorrectly()
{
var container = new UnityContainer();
container.RegisterType<IFooBar, SomeFooBar>("first");
container.RegisterType<IFooBar, OtherFooBar>("second");
Assert.NotNull(container.Resolve<IFooBar>("first"));
Assert.NotNull(container.Resolve<IFooBar>("second"));
Assert.Throws<ResolutionFailedException>(() => container.Resolve<IFooBar>());
}
}
public interface IFooBar {}
public class OtherFooBar : IFooBar {}
public class SomeFooBar : IFooBar {}
This functionality can again be used to implement a factory of sorts but in general it is not that interesting until we get to the next point.
Building a composite
Having a multiple implementations of a particular contract registered with the container is the stepping stone to building a composite, as the following sample illustrates:
[TestFixture]
public class CompositeTests
{
[Test]
public void BuildComposite()
{
var container = new UnityContainer();
container.RegisterType<IFoo, SomeFoo>("first");
container.RegisterType<IFoo, AnotherFoo>("second");
container.RegisterType<IFoo, CompositeFoo>();
var instanceOfFoo = container.Resolve<IFoo>();
Assert.IsInstanceOf<CompositeFoo>(instanceOfFoo);
}
}
public class CompositeFoo : IFoo
{
public CompositeFoo(IFoo[] others)
{
Debug.Assert(others != null);
Debug.Assert(others.Any());
}
}
public class AnotherFoo : IFoo {}
public class SomeFoo : IFoo {}
public interface IFoo {}
Composite in this case “consumes” an array of child objects and in a sense “sucks in” every implementation of IFoo registered with a key. This is an important aspect: if you were to register composite with a key, it would try to instantiate itself leading in immediately to StackOverflowException. The other interesting aspect is the fact that the composite requires an argument of type IChild[]. I am not sure about you but I personally find arrays very much 20th century and would rather have IEnumerable<IChild> injected, sadly Unity does not support it out of the box. There is however a workaround for it which I will share with you later.
Building chain of responsibility
Another interesting use case for naming your registrations is the implementation of Chain Of Responsibility pattern as the following sample illustrates:
[TestFixture]
class ChainOfResponsibilityTests
{
[Test]
public void BuildChain()
{
var container = new UnityContainer();
container.RegisterType<IChainLink, LastLink>("last");
container.RegisterType<IChainLink, SecondLink>("second",
new InjectionConstructor(
new ResolvedParameter<IChainLink>("last")));
container.RegisterType<IChainLink, FirstLink>(new InjectionConstructor(
new ResolvedParameter<IChainLink>("second")));
var firstLink = container.Resolve<IChainLink>();
Assert.IsInstanceOf<FirstLink>(firstLink);
}
}
internal interface IChainLink {}
internal class FirstLink : IChainLink
{
public FirstLink(IChainLink nextLink)
{
if (nextLink == null) throw new ArgumentNullException("nextLink");
}
}
internal class SecondLink : IChainLink
{
public SecondLink(IChainLink nextLink)
{
if (nextLink == null) throw new ArgumentNullException("nextLink");
}
}
internal class LastLink : IChainLink {}
In this case we need to resort to pointing each registered element of the chain to its predecessor which is achieved using ResolvedParameter<T>(name). This way we have full control as to what registration gets injected where. This may come helpful in cases where different implementations of the same contract are to be injected in different places.
Injecting the same object in multiple places in the object graph (semi-singleton)
Unity has a very interesting feature which allows the same object to be injected and shared in multiple places in the object graph. Let’s consider the following object diagram
:
As you can see both the parent and the children contain references to an instance of Service. Standard behaviour of the IoC container is to inject new instance of the dependency into every object in the graph, this however may not be what we want:
In order to solve this conundrum Unity provides PerResolveLifetimeManager which within a single call to Resolve<T>() will create an instance of particular type and reuse it wherever required as the next sample illustrates:
[TestFixture]
public class PerResolutionLifetimeTests
{
[Test]
public void Regiser_WithPerResolveLifetimeManager_ContainerInjectsSameInstance()
{
var container = new UnityContainer();
container.RegisterType<IService, AService>(new PerResolveLifetimeManager());
container.RegisterType<IParent, Parent>();
container.RegisterType<IChild, Child>();
var parent = container.Resolve<IParent>();
Assert.AreSame(parent.Service, parent.Child.Service);
}
}
public interface IService {}
public class AService : IService {}
public interface IChild
{
IService Service { get; set; }
}
public class Child : IChild
{
public IService Service { get; set; }
public Child(IService service)
{
Service = service;
}
}
public interface IParent
{
IChild Child { get; set; }
IService Service { get; set; }
}
public class Parent : IParent
{
public IChild Child { get; set; }
public IService Service { get; set; }
public Parent(IChild child, IService service)
{
Child = child;
Service = service;
}
}
Eternal problem: Chicken and Egg
Now there is an interesting chicken and egg problem which may seem to be impossible to solve using IoC container:
public interface IEgg
{
IChicken Chicken { get; set; }
}
public interface IChicken
{
IEgg Egg { get; set; }
}
public class Chicken : IChicken
{
public IEgg Egg { get; set; }
}
public class Egg : IEgg
{
public Egg(IChicken chicken)
{
Chicken = chicken;
}
public IChicken Chicken
{
get;
set;
}
}
As you can see we have a circular dependency going on here (Chicken “contains” an egg, while the egg points to its parent). This may seem to be a hopeless case but there is a solution to it: we need to combine property injection with PerResolveLifetimeManager:
[Test]
public void BuildObjectGraphWithCircularDependency()
{
var container = new UnityContainer();
container.RegisterType<IChicken, Chicken>(new PerResolveLifetimeManager(), new InjectionProperty("Egg"));
container.RegisterType<IEgg, Egg>();
var chicken = container.Resolve<IChicken>();
Assert.AreSame(chicken, chicken.Egg.Chicken);
}
The magic above works as follows: container instantiates an instance of the Chicken class and then tries to resolve the Egg dependency. Egg in turn requires another instance of Chicken, luckily the Chicken class has been registered with PerResolveLifetimeManager which means it that instance of it can be reused in building the Egg. I think the case above is the only one I can think of where the use of property injection could be justified.
IoC and Interface segregation principle
There are some cases where a single type implements multiple contracts (interfaces). If you follow interface segregation principle this may lead to some troublesome cases:
[Test]
public void Resolve_ObjectDependantOnBaseInterface_ResolutionFails()
{
var container = new UnityContainer();
container.RegisterType<IDerived, Derived>();
container.RegisterType<IDependsOnBase, DependsOnBase>();
Assert.Throws<ResolutionFailedException>(() => container.Resolve<IDependsOnBase>());
}
public interface IBase { }
internal interface IDerived : IBase { }
internal class Derived : IDerived { }
public interface IDependsOnBase { }
public class DependsOnBase : IDependsOnBase
{
public DependsOnBase(IBase dependency){}
}
As you can see from the code above, Derived implements IDerived which in turn is an instance of IBase. Inspite of that, resolution of DependsOnBase fails miserably. The reason is obviously the fact that there is no explicit implementation of IBase registered with the container. We could circumvent the problem by registering the Derived as type implementing the IBase interface. This approach would work but what if Derived implements more than one interface? There is however an easy way to work around the issue and this requires a hint to the container that instead of IBase it should search for an implementation of IDerived:
[Test]
public void Resolve_ObjectDependantOnBaseInterface_ResolutionSucceeds()
{
var container = new UnityContainer();
container.RegisterType<IDerived, Derived>();
container.RegisterType<IDependsOnBase, DependsOnBase>(new InjectionConstructor(typeof(IDerived)));
Assert.DoesNotThrow(() => container.Resolve<IDependsOnBase>());
}
This way btw it is the way to construct a composite using IEnumerable<> instead of an array as mentioned previously.
Overriding constructor parameters
The final set of Unity tricks deals with overriding constructor parameters. As you may remember we did this to a certain extent when registering individual elements of the chain of responsibility. There are two ways to do it: we can either instruct container at the time of registration to use specific instance of the parameters, or override parameter value at the time of resolution. Following sample illustrates both techniques.
[TestFixture]
public class ConstructorParameters
{
[Test]
public void OverridingCtorParams()
{
var container = new UnityContainer();
container.RegisterType<SomeType>(new InjectionConstructor("Boo"));
var firstInstance = container.Resolve<SomeType>();
Assert.AreEqual("Boo", firstInstance.Message);
var secondInstance = container.Resolve<SomeType>( new ParameterOverride("message", "Baa"));
Assert.AreEqual("Baa", secondInstance.Message);
}
}
public class SomeType
{
public string Message { get; set; }
public SomeType(string message)
{
Message = message;
}
}