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Developers exposed to ASP.NET are likely to be familiar with the so-called Provider pattern. You see it a lot in that part of the BCL: Role Provider, Membership Provider, Profile Provider, etc. Lots of text has already been written about Providers, but the reason I want to add yet another blog post on the topic is because once in a while I get the question on how it relates to Dependency Injection (DI).

Is Provider a proper way to do DI?

No, it has nothing to do with DI, but as it tries to mimic loose coupling I can understand the confusion.

First things first. Let's start with the name. Is it a pattern at all? Regular readers of this blog may get the impression that I'm fond of calling everything and the kitchen sink an anti-pattern. That's not true because I only make that claim when I'm certain I can hold that position, so I'm not going to denounce Provider as an anti-pattern. On the contrary I will make the claim that Provider is not a pattern at all.

A design pattern is not invented - it's discovered as a repeated solution to a commonly recurring problem. Providers, on the other hand, were invented by Microsoft, and I've rarely seen them used outside their original scope. Secondly I'd also dispute that they solve anything.

That aside, however, I want to explain why Provider is bad design:

• It uses the Constrained Construction anti-pattern
• It hides complexity
• It prevents proper lifetime management
• It's not testable

In the rest of this post I will explain each point in detail, but before I do that we need an example to look at. The old OrderProcessor example suffices, but instead of injecting IOrderValidator, IOrderCollector, and IOrderShipper this variation uses Providers to provide instances of the Services:

public SuccessResult Process(Order order)
{
    IOrderValidator validator = 
        ValidatorProvider.Validator;
    bool isValid = validator.Validate(order);
    if (isValid)
    {
        CollectorProvider.Collector.Collect(order);
        ShipperProvider.Shipper.Ship(order);
    }
 
    return this.CreateStatus(isValid);
}

The ValidatorProvider uses the configuration system to create and return an instance of IOrderValidator:

public static IOrderValidator Validator
{
    get 
    {
        var section = 
            OrderValidationConfigurationSection
                .GetSection();
        var typeName = section.ValidatorTypeName;
        var type = Type.GetType(typeName, true);
        var obj = Activator.CreateInstance(type);
        return (IOrderValidator)obj;
    }
}

There are lots of details I omitted here. I could have saved the reference for later use instead of creating a new instance each time the property is accessed. In that case I would also have had to make the code thread-safe, so I decided to skip that complexity. The code could also be more defensive, but I'm sure you get the picture.

The type name is defined in the app.config file like this:

<orderValidation 
  type="Ploeh.Samples.OrderModel.UnitTest.TrueOrderValidator,
        Ploeh.Samples.OrderModel.UnitTest" />

Obviously, CollectorProvider and ShipperProvider follow the same… blueprint.

This should be well-known to most .NET developers, so what's wrong with this model?

Constrained Construction #

In my book's chapter on DI anti-patterns I describe the Constrained Construction anti-pattern. Basically it occurs every time there's an implicit constraint on the constructor of an implementer. In the case of Providers the constraint is that each implementer must have a default constructor. In the example the culprit is this line of code:

var obj = Activator.CreateInstance(type);

This constrains any implementation of IOrderValidator to have a default constructor, which obviously means that the most fundamental DI pattern Constructor Injection is out of the question.

Variations of the Provider idiom is to supply an Initialize method with a context, but this creates a temporal coupling while still not enabling us to inject arbitrary Services into our implementations. I'm not going to repeat six pages of detailed description of Constrained Construction here, but the bottom line is that you can't fix it - you have to refactor towards true DI - preferably Constructor Injection.

Hidden complexity #

Providers hide the complexity of their implementations. This is not the same as encapsulation. Rather it's a dishonest API and the problem is that it just postpones the moment when you discover how complex the implementation really is.

When you implement a client and use code like the following everything looks deceptively simple:

IOrderValidator validator = 
    ValidatorProvider.Validator;

However, if this is the only line of code you write it will fail, but you will not notice until run-time. Check back to the implementation of the Validator property if you need to refresh the implementation: there's a lot of things that can go wrong here:

• The appropriate configuration section is not available in the app.config file.
• The ValidatorTypeName is not provided, or is null, or is malformed.
• The ValidatorTypeName is correctly formed, but the type in question cannot be located by Fusion.
• The Type doesn't have a default constructor. This is one of the other problems of Constrained Construction: it can't be statically enforced because a constructor is not part of an abstraction's API.
• The created type doesn't implement IOrderValidator.

I'm sure I even forgot a thing or two, but the above list is sufficient for me. None of these problems are caught by the compiler, so you don't discover these issues until you run an integration test. So much for rapid feedback.

I don't like APIs that lie about their complexity.

Hiding complexity does not make an API easier to use; it makes it harder.

An API that hides necessary complexity makes it impossible to discover problems at compile time. It simply creates more friction.

Lifetime management issues #

A Provider exerts too much control over the instances it creates. This is a variation of the Control Freak anti-pattern (also from my book). In the current implementation the Validator property totally violates the Principle of least surprise since it returns a new instance every time you invoke the getter. I did this to keep the implementation simple (this is, after all, example code), but a more normal implementation would reuse the same instance every time.

However, reusing the same instance every time may be problematic in a multi-threaded context (such as a web application) because you'll need to make sure that the implementation is thread-safe. Often, we'd much prefer to scope the lifetime of the Service to each HTTP request.

HTTP request scoping can be built into the Provider, but then it would only work in web applications. That's not very flexible.

What's even more problematic is that once we move away from the Singleton lifestyle (not to be confused with the Singleton design pattern) we may have a memory leak at hand, since the implementation may implement IDisposable. This can be solved by adding a Release method to each Provider, but now we are moving so far into DI Container territory that I find it far more reasonable to just use proper DI instead of trying to reinvent the wheel.

Furthermore, the fact that each Provider owns the lifetime of the Service it controls makes it impossible to share resources. What if the implementation we want to use implements several Role Interfaces each served up by a different Provider? We might want to use that common implementation to share or coordinate state across different Services, but that's not possible because we can't share an instance across multiple providers.

Even if we configure all Providers with the same concrete class, each will instantiate and serve its own separate instance.

Testability #

The Control Freak also impacts testability. Since a Provider creates instances of interfaces based on XML configuration and Activator.CreateInstance, there's no way to inject a dynamic mock.

It is possible to use hard-coded Test Doubles such as Stubs or Fakes because we can configure the XML with their type names, but even a Spy is problematic because we'll rarely have an object reference to the Test Double.

In short, the Provider idiom is not a good approach to loose coupling. Although Microsoft uses it in some of their products, it only leads to problems, so there's no reason to mimic it. Instead, use Constructor Injection to create loosely coupled components and wire them in the application's Composition Root using the Register Resolve Release pattern.

Comments

Implementor < Service < Provider (or ServiceProvider)

Model < StorageService < MySQLProvider

It gives me great pleasure to announce that AutoFixture 2.1 beta 1 is now available for download. Compared to version 2.0 this release mostly contains added features as well as a few bug fixes. The release page contains a list of the new features.

The general availability of beta 1 of AutoFixture 2.1 marks the beginning of a trial period. If no new issues are reported within the next few weeks, a final version 2.1 will be released. If too many issues are reported, a new beta version may be necessary.

Please report any issues you find.

NuGet packages will follow the RTW version.

When AutoFixture creates complex objects it invokes the constructors of the types in question. When a class exposes more than one constructor, the default behavior is to pick the constructor with the fewest number  of arguments. This is the exact opposite of most DI Containers that select the greediest constructor.

For a DI Container it makes more sense to select the greediest constructor because that maximizes the chance that all dependencies are properly being auto-wired.

The reason that AutoFixture's default behavior is different is that it maximizes the chance that an object graph can be created at all. If a convenience overload is available, this gives AutoFixture a better chance to compose the object graph.

This works well until a class comes along and introduces the wrong kind of ambiguity. Consider this case of Bastard Injection (Dependency Injection in .NET, section 5.2):

public class Bastard
{
    private readonly IFoo foo;
 
    public Bastard()
        : this(new DefaultFoo())
    {
    }
 
    public Bastard(IFoo foo)
    {
        if (foo == null)
        {
            throw new ArgumentNullException("foo");
        }
 
        this.foo = foo;
    }
 
    public IFoo Foo
    {
        get { return this.foo; }
    }
}

By default AutoFixture will use the default constructor which means that the Foo will always be the instance of DefaultFoo owned by the Bastard class itself. This is problematic if we wish to inject and freeze a Test Double because even if we freeze an IFoo instance, it will never be injected because the default constructor is being invoked.

var fixture = new Fixture();
fixture.Register<IFoo>(
    fixture.CreateAnonymous<DummyFoo>);
var b = fixture.CreateAnonymous<Bastard>();
Assert.IsAssignableFrom<DefaultFoo>(b.Foo);

As the above unit test demonstrates, even though the Register method call defines a mapping from IFoo to DummyFoo, the Foo property is an instance of DefaultFoo. The DummyFoo instance is never injected into the Bastard instance since the default constructor is used.

Your first reaction in such a case should be to get rid of all convenience constructors to make the class' constructor unambiguous. However, it's also possible to change AutoFixture's behavior.

The following is a description of a feature that will be a available in AutoFixture 2.1. It's not available in AutoFixture 2.0, but is already available in the code repository. Thus, if you can't wait for AutoFixture 2.1 you can download the source and build it.

As always, AutoFixture's very extensible interface makes it possible to change this behavior. Constructor selection is guided by an interface called IConstructorQuery, and while ModestConstructorQuery is the default implementation, there's also an implementation called GreedyConstructorQuery.

To change the behavior specifically for the Bastard class the Fixture instance must be customized. The following unit test demonstrates how to do that.

var fixture = new Fixture();
fixture.Customize<Bastard>(c => c.FromFactory(
    new MethodInvoker(
        new GreedyConstructorQuery())));
fixture.Register<IFoo>(
    fixture.CreateAnonymous<DummyFoo>);
var b = fixture.CreateAnonymous<Bastard>();
Assert.IsAssignableFrom<DummyFoo>(b.Foo);

Notice that the only difference from before is an additional call to the Customize method where Bastard is modified to use greedy constructor selection. This changes the factory for the Bastard type, but not for any other types.

If you'd rather prefer to completely change the overall constructor selection behavior for AutoFixture you can add the GreedyConstructorQuery wrapped in a MethodInvoker to the Fixture's Customizations:

var fixture = new Fixture();
fixture.Customizations.Add(
    new MethodInvoker(
        new GreedyConstructorQuery()));
fixture.Register<IFoo>(
    fixture.CreateAnonymous<DummyFoo>);
var b = fixture.CreateAnonymous<Bastard>();
Assert.IsAssignableFrom<DummyFoo>(b.Foo);

In this unit test, the only change from the previous is that instead of assigning the GreedyConstructorQuery specifically to Bastard, it's now assigned as the new default strategy. All specimens created by AutoFixture will be created by invoking their most greedy constructor.

As always I find the default behavior more attractive, but the option to change behavior is there if you need it.

Comments

namespace JustATest
{
	using Ploeh.AutoFixture;
	using Ploeh.AutoFixture.Kernel;
 
	using Xunit;
 
	public class JustATest {
 
		[Fact]
		public void GreedyConstructorAndOmitAutoProps() {
			var fixture = new Fixture();
			fixture.Customize<Foo>(c => c.FromFactory(
				new MethodInvoker(
					new GreedyConstructorQuery())));
			fixture.Customize<Foo>(c => c.OmitAutoProperties());
 
			var foo = fixture.Create<Foo>();
			Assert.NotNull(foo.myStr);
		}
	}
 
	public class Foo {
		public Foo(string myStr) {
			this.myStr = myStr;
		}
 
		public Foo() {
		}
 
		public string myStr { get; set; }
	}
}

I just got a question about AutoFixture's way of dealing with enumerables, and since I suspect this is something that might surprise a few people I found it reasonable to answer in a blog post.

In short, this unit test succeeds:

var fixture = new Fixture();
var expected = fixture.CreateMany<string>();
Assert.False(expected.SequenceEqual(expected));

If this doesn't surprise you, then read the assertion again. The sequence is expected to not equal itself!

This behavior is by design.

(I've always wanted to write that.) The reason is that the return type of the CreateMany method is IEnumerable<T>, and that interface makes no guarantee that it will return the same sequence every time you iterate over it. The implementation may be a Generator.

According to its principle of Constrained Non-determinism, AutoFixture goes to great lengths to ensure that since IEnumerable<T> might be non-deterministic, it will be. This can be very useful in flushing out incorrect assumptions made by the SUT.

This behavior is carried forward by the MultipleCustomization. This unit test also succeeds:

var fixture = new Fixture()
    .Customize(new MultipleCustomization());
var expected =
    fixture.CreateAnonymous<IEnumerable<string>>();
Assert.False(expected.SequenceEqual(expected));

The behavior is the same because with the MultipleCustomization IEnumerable<T> acts as a relationship type. The class responsible for this behavior is called FiniteSequenceRelay, but AutoFixture 2.1 will also ship with an alternative StableFiniteSequenceRelay which wraps the sequence in a List<T>.

To change the default behavior you can add the StableFiniteSequenceRelay to a Fixture's customizations:

var fixture = new Fixture();
var stableRelay = new StableFiniteSequenceRelay();
fixture.Customizations.Add(stableRelay);
 
var expected =
    fixture.CreateMany<string>();
Assert.True(expected.SequenceEqual(expected));

The above unit test succeeds. Notice that the assertion now verifies that the sequence of strings is equal to itself.

Just like MultipleCustomization the StableFiniteSequenceRelay class will be available in AutoFixture 2.1, but is not availale in 2.0.

As always, it's best to encapsulate this behavior change into a Customization:

public class StableFiniteSequenceCustomization :
    ICustomization
{
    public void Customize(IFixture fixture)
    {
        var stableRelay = 
            new StableFiniteSequenceRelay();
        fixture.Customizations.Add(stableRelay);
    }
}

This makes it easier to bundle it with the MultipleCustomization if we want all the other benefits of conventions for multiples:

public class StableMultipeCustomization : 
    CompositeCustomization
{
    public StableMultipeCustomization()
        : base(
            new StableFiniteSequenceCustomization(),
            new MultipleCustomization())
    {
    }
}

With the StableMultipleCustomization the following unit test now passes:

var fixture = new Fixture()
    .Customize(new StableMultipeCustomization());
var expected =
    fixture.CreateAnonymous<IEnumerable<string>>();
Assert.True(expected.SequenceEqual(expected));

I still prefer the default behavior for its potential to act as an early warning system, but as I realize that this behavior may be problematic in some scenarios, the StableFiniteSequenceRelay provides an easy way to change that behavior.

Update (2011.8.10): StableFiniteSequenceCustomization is now part of AutoFixture and will be available in AutoFixture 2.2.

Comments

My latest MSDN Magazine article, this time about CQRS on Windows Azure, is now available at the April MSDN Magazine web site.

It's mostly meant as an introduction to CQRS as well as containing some tips and tricks that are specific to applying CQRS on Windows Azure.

As an added bonus the code sample download contains lots of idiomatic unit tests written with AutoFixture's xUnit.net extensions, so if you'd like to see the result of my TDD work with AutoFixture, there's a complete code base to look at there.

Comments

A few months back I wrote a (somewhat theoretical) post on composable interfaces. A major point of that post was that Role Interfaces  with a single Command method (i.e. a method that returns no value) is a very versatile category of abstraction.

Some of my readers asked for examples, so in this post I will provide a few. Consider this interface that fits the above description:

public interface IMessageConsumer<T>
{
    void Consume(T message);
}

This is a very common type of interface you will tend to encounter a lot in distributed, message-based architectures such as CQRS or Udi Dahan's view of SOA. Some people would call it a message subscriber instead...

In the rest of this post I will examine how we can create compositions out of the IMessageConsumer<T> interface using (in order of significance) Decorator, Null Object, Composite, and other well-known programming constructs.

Decorator #

Can we create a meaningful Decorator around the IMessageConsumer<T> interface? Yes, that's easy - I've earlier provided various detailed examples of Decorators, so I'm not going to repeat them here.

I've yet to come up with an example of an interface that prevents us from applying a Decorator, so it's a valid falsifiable claim that we can always Decorate an interface. However, I have yet to prove that this is true, so until now we'll have to call it a conjecture.

However, since it's so easy to apply a Decorator to an interface, it's not a particularly valuable trait when evaluating the composability of an interface.

Null Object #

It can be difficult to implement the Null Object pattern when the method(s) in question return a value, but for Commands it's easy:

public class NullMessageConsumer<T> : IMessageConsumer<T>
{
    public void Consume(T message)
    {
    }
}

The implementation simply ignores the input and does nothing.

Once again my lack of formal CS education prevents me from putting forth a formal proof, but I strongly suspect that it's always possibly to apply the Null Object pattern to a Command (keep in mind that out parameters count as output, so any interface with one or more of these are not Commands).

It's often valuable to be able to use a Null Object, but the real benefit comes when we can compose various implementations together.

Composite #

To be truly composable, an interface should make it possible to create various concrete implementations that each adhere to the Single Responsibility Principle and then compose those together in a complex implementation. A Composite is a general-purpose implementation of this concept, and it's easy to create a Composite out of a Command:

public class CompositeMessageConsumer<T> : 
    IMessageConsumer<T>
{
    private readonly IEnumerable<IMessageConsumer<T>> 
        consumers;
 
    public CompositeMessageConsumer(
        params IMessageConsumer<T>[] consumers)
    {
        if (consumers == null)
        {
            throw new ArgumentNullException("consumers");
        }
 
        this.consumers = consumers;
    }
 
    public IEnumerable<IMessageConsumer<T>> Consumers
    {
        get { return this.consumers; }
    }
 
    #region IMessageConsumer<T> Members
 
    public void Consume(T message)
    {
        foreach (var consumer in this.Consumers)
        {
            consumer.Consume(message);
        }
    }
 
    #endregion
}

The implementation of the Consume method simply loops over each composed IMessageConsumer<T> and invokes its Consume method.

I can, for example, implement a sequence of actions that will take place by composing the individual concrete implementations. First we have a guard that protects against invalid messages, followed by a consumer that writes the message to a persistent store, completed by a consumer that raises a Domain Event that the reservation request was accepted.

var c = new CompositeMessageConsumer<MakeReservationCommand>(
    guard, 
    writer, 
    acceptRaiser);

Consumers following the Liskov Substitution Principle will not notice the difference, as all they will see is an implementation of IMessageConsumer<MakeReservationCommand>.

More advanced programming constructs #

The Composite pattern only describes a single, general way to compose implementations, but with a Command interface we can do more. As Domain-Driven Design explains, a successful interface is often characterized by making it possible to apply well-known arithmetic or logical operators. As an example, in the case of the IMessageConsumer<T> interface, we can easily mimic the well-known ?! ternary operator from C#:

public class ConditionalMessageConsumer<T> : 
    IMessageConsumer<T>
{
    private Func<T, bool> condition;
    private IMessageConsumer<T> first;
    private IMessageConsumer<T> second;
 
    public ConditionalMessageConsumer(
        Func<T, bool> condition, 
        IMessageConsumer<T> first, 
        IMessageConsumer<T> second)
    {
        if (condition == null)
        {
            throw new ArgumentNullException("condition");
        }
        if (first == null)
        {
            throw new ArgumentNullException("first");
        }
        if (second == null)
        {
            throw new ArgumentNullException("second");
        }
 
        this.condition = condition;
        this.first = first;
        this.second = second;
    }
 
    public void Consume(T message)
    {
        (this.condition(message) ? this.first : this.second)
            .Consume(message);
    }
}

This is more verbose than the ?! operator because C# doesn't allow us to define operator overloads for interfaces, but apart from that, it does exactly the same thing. Notice particularly that the Consume method uses the ?! operator to select among the two alternatives, and then subsequently invokes the Consume method on the selected consumer.

We can use the ConditionalMessageConsumer to define branches in the consumption of messages. As an example, we can encapsulate the previous CompositeMessageConsumer<MakeReservationCommand> into a conditional branch like this:

var consumer = 
    new ConditionalMessageConsumer<MakeReservationCommand>(
        guard.HasCapacity, c, rejectRaiser);

Notice that I use the method group syntax to supply the condition delegate. If the HasCapacity method returns true, the previous composite (c) is being invoked, but if the result is false we instead use a consumer that raises the Domain Event that the reservation request was rejected.

Concluding thoughts #

Apart from the direct purpose of providing examples of the immensely powerful composition options a Command interface provides I want to point out a couple of things:

• Each of the design pattern implementations (Null Object, Composite - even Conditional) are generic. This is a strong testament to the power of this particular abstraction.
• The dynamic mocks I'm familiar with (Moq, Rhino Mocks) will by default try to create Null Object implementations for interfaces without explicit setups. Since it's trivial to implement a Null Command, they just emit them by default. If you use an Auto-mocking Container with your unit tests, you can refactor to your heart's content, adding and removing dependencies as long as they are Command interfaces, and your testing infrastructure will just take care of things for you. It'll just work.
• Did you notice that even with these few building blocks we have implemented a large part of a sequential workflow engine? We can execute consumers in sequence as well as branch between different sequences. Obviously, more building blocks are needed to make a full-blown workflow engine, but not that many. I'll leave the rest as an exercise to the reader :)

As I originally sketched, a Command interface is the ultimate in composability. To illustrate, the application from where I took the above examples is a small application with 48 types (classes and interfaces) in the production code base (that is, excluding unit tests). Of these, 9 are implementations of the IMessageConsumer<T> interface. If we also count the interface itself, it accounts for more than 20 percent of the code base. According to the Reused Abstractions Principle (RAP) I consider this a very successful abstraction.

AutoFixture is designed around the 80-20 principle. If your code is well-designed I'd expect a default instance of Fixture to be able to create specimens of your classes without too much trouble. There are cases where AutoFixture needs extra help:

• If a class consumes interfaces a default Fixture will not be able to create it. However, this is easily fixed through one of the AutoMocking Customizations.
• If an API has circular references, Fixture might enter an infinite recursion, but you can easily customize it to cut off one the references.
• Some constructors may only accept arguments that don't fit with the default specimens created by Fixture. There are ways to deal with that as well.

I could keep on listing examples, but let's keep it at that. The key assumption underlying AutoFixture is that these special cases are a relatively small part of the overall API you want to test - preferably much less than 20 percent.

Still, to address those special cases you'll need to customize a Fixture instance in some way, but you also want to keep your test code DRY.

As the popularity of AutoFixture grows, I'm getting more and more glimpses of how people address this challenge: Some derive from Fixture, others create extension methods or other static methods, while others again wrap creation of Fixture instances in Factories or Builders.

There really is no need to do that. AutoFixture has an idiomatic way to encapsulate Customizations. It's called… well… a Customization, which is just another way to say that there's an ICustomization interface that you can implement. This concept corresponds closely to the modularization APIs for several well-known DI Containers. Castle Windsor has Installers, StructureMap has Registries and Autofac has Modules.

The ICustomization interface is simple and has a very gentle learning curve:

public interface ICustomization
{
    void Customize(IFixture fixture);
}

Anything you can do with a Fixture instance you can also do with the IFixture instance passed to the Customize method, so this is the perfect place to encapsulate common Customizations to AutoFixture. Note that the AutoMocking extensions as well as several other optional behaviors for AutoFixture are already defined as Customizations.

Using a Customization is also easy:

var fixture = new Fixture()
    .Customize(new DomainCustomization());

You just need to invoke the Customize method on the Fixture. That's no more difficult than calling a custom Factory or extension method - particularly if you also use a Visual Studio Code Snippet.

When I start a new unit testing project, one of the first things I always do is to create a new ‘default' customization for that project. It usually doesn't take long before I need to tweak it a bit - if nothing else, then for adding the AutoMoqCustomization. To apply separation of concerns I encapsulate each Customization in its own class and compose them with a CompositeCustomization:

public class DomainCustomization : CompositeCustomization
{
    public DomainCustomization()
        : base(
            new AutoMoqCustomization(),
            new FuncCustomization())
    {
    }
}

Whenever I need to make sweeping changes to my Fixtures I can simply modify DomainCustomization or one of the Customizations it composes.

In fact, these days I rarely explicitly create new Fixture instances, but rather encapsulate them in a custom AutoDataAttribute like this:

public class AutoDomainDataAttribute : AutoDataAttribute
{
    public AutoDomainDataAttribute()
        : base(new Fixture()
            .Customize(new DomainCustomization()))
    {
    }
}

This means that I can reuse the DomainCustomization across normal, imperative unit tests as well as the declarative, xUnit.net-powered data theories I normally prefer:

[Theory, AutoDomainData]
public void CanReserveReturnsTrueWhenQuantityIsEqualToRemaining(
    Capacity sut, Guid id)
{
    var result = sut.CanReserve(sut.Remaining, id);
    Assert.True(result);
}

Using Customizations to encapsulate your own specializations makes it easy to compose and manage them in an object-oriented fashion.

A while back I posed the challenge of resolving closed types with MEF. I received some responses, but I also wanted to provide an alternative outline for a solution. In case you don't remember the problem statement, it revolved around using the Managed Extensibility Framework (MEF) to compose classes in those cases where it's impossible to annotate those classes with the MEF attributes. In the given example I want to compose the Mayonnaise class from EggYolk and OliveOil, but all three classes are sealed and cannot be recompiled.

As I describe in my book, a general solution to this type of problem is to create a sort of adapter the exports the closed type via a read-only attribute, like these EggYolkAdapter and MayonnaiseAdapter classes (the OliveOilAdapter looks just like the EggYolkAdapter):

public class EggYolkAdapter
{
    private readonly EggYolk eggYolk;
 
    public EggYolkAdapter()
    {
        this.eggYolk = new EggYolk();
    }
 
    [Export]
    public virtual EggYolk EggYolk
    {
        get { return this.eggYolk; }
    }
}
 
public class MayonnaiseAdapter
{
    private readonly Mayonnaise mayo;
 
    [ImportingConstructor]
    public MayonnaiseAdapter(
        EggYolk yolk, OliveOil oil)
    {
        if (yolk == null)
        {
            throw new ArgumentNullException("yolk");
        }
        if (oil == null)
        {
            throw new ArgumentNullException("oil");
        }
 
        this.mayo = new Mayonnaise(yolk, oil);
    }
 
    [Export]
    public virtual Mayonnaise Mayonnaise
    {
        get { return this.mayo; }
    }
}

Doing it like this is always possible, but if you have a lot of types that you need to compose, it becomes tedious having to define a lot of similar adapters. Fortunately, we can take it a step further and generalize the idea of a MEF adapter to a small set of generic classes.

The EggYolkAdapter can be generalized as follows:

public class MefAdapter<T> where T : new()
{
    private readonly T export;
 
    public MefAdapter()
    {
        this.export = new T();
    }
 
    [Export]
    public virtual T Export
    {
        get { return this.export; }
    }
}

Notice that I've more or less just replaced the EggYolk class with a type argument (T). However, I also had to add the generic new() constraint, which is often quite restrictive. However, to support a type like Mayonnaise, I can create another, similar generic MEF adapter like this:

public class MefAdapter<T1, T2, TResult>
{
    private readonly static Func<T1, T2, TResult> createExport =
        FuncFactory.Create<T1, T2, TResult>();
    private readonly TResult export;
 
    [ImportingConstructor]
    public MefAdapter(T1 arg1, T2 arg2)
    {
        this.export = createExport(arg1, arg2);
    }
 
    [Export]
    public virtual TResult Export
    {
        get { return this.export; }
    }
}

The major difference from the simple MefAdapter<T> is that we need slightly more complicated code to invoke the constructor of TResult, which is expected to take two constructor arguments of types T1 and T2. This work is delegated to a FuncFactory that builds and compiles the appropriate delegate using an expression tree:

internal static Func<T1, T2, TResult>
    Create<T1, T2, TResult>()
{
    var arg1Exp =
        Expression.Parameter(typeof(T1), "arg1");
    var arg2Exp = 
        Expression.Parameter(typeof(T2), "arg2");
 
    var ctorInfo = 
        typeof(TResult).GetConstructor(new[]
        {
            typeof(T1),
            typeof(T2)
        });
    var ctorExp =
        Expression.New(ctorInfo, arg1Exp, arg2Exp);
 
    return Expression.Lambda<Func<T1, T2, TResult>>(
        ctorExp, arg1Exp, arg2Exp).Compile();
}

With a couple of MEF adapters, I can now compose a MEF Catalog almost like I can with a real DI Container, and resolve the Mayonnaise class:

var catalog = new TypeCatalog(
    typeof(MefAdapter<OliveOil>),
    typeof(MefAdapter<EggYolk>),
    typeof(MefAdapter<EggYolk, OliveOil, Mayonnaise>)
    );
var container = new CompositionContainer(catalog);
 
var mayo = container.GetExportedValue<Mayonnaise>();

If you want to change the Creation Policy to NonShared, you can derive from the MefAdapter classes and annotate them with [PartCreationPolicy] attributes.

I'd never voluntarily choose to use MEF like this, but if I was stuck with MEF in a project and had to use it like a DI Container, I'd do something like this.

Comments

The main principle behind the Register Resolve Release pattern is that loosely coupled object graphs should be composed as a single action in the entry point of the application (the Composition Root). For request-based applications (web sites and services), we use a variation where we compose once per request.

It seems to me that a lot of people are apprehensive when they first hear about this concept. It may sound reasonable from an architectural point of view, but isn't it horribly inefficient? A well-known example of such a concern is Jeffrey Palermo's blog post Constructor over-injection anti-pattern. Is it really a good idea to compose a complete object graph in one go? What if we don't need part of the graph, or only need it later? Doesn't it adversely affect response times?

Normally it doesn't, and if it does, there are elegant ways to address the issue.

In the rest of this blog post I will expand on this topic. To keep the discussion as simple as possible, I'll restrict my analysis to object trees instead of full graphs. This is quite a reasonable simplification as we should strive to avoid circular dependencies, but even in the case of full graphs the arguments and techniques put forward below hold.

Consider a simple tree composed of classes from three different assemblies:

All the A classes (blue) are defined in the A assembly, B classes (green) in the B assembly, and the C1 class (red) in the C assembly. In code we create the tree with Constructor Injection like this:

var t =
    new A1(
        new A2(
            new B1(
                new B2()),
            new A3()),
        new C1(
            new B3()));

Given the tree above, we can now address the most common concerns about composing object trees in one go.

Will it be slow? #

Most likely not. Keep in mind that Injection Constructors should be very simple, so not a lot of work is going on during composition. Obviously just creating new object instances takes a bit of time in itself, but we create objects instances all the time in .NET code, and it's often a very fast operation.

Even when using DI Containers, which perform a lot of (necessary) extra work when creating objects, we can create tens of thousand trees per second. Creation of objects simply isn't that big a deal.

But still: what about assembly loading? #

I glossed over an important point in the above argument. While object creation is fast, it sometimes takes a bit of time to load an assembly. The tree above uses classes from three different assemblies, so to create the tree all three assemblies must be loaded.

In many cases that's a performance hit you'll have to take because you need those classes anyway, but sometimes you might be concerned with taking this performance hit too early. However, I make the claim that in the vast majority of cases, this concern is irrelevant.

In this particular context there are two different types of applications: Request-based applications (web) and all the rest (desktop apps, daemons, batch-jobs, etc.).

Request-based applications #

For request-based applications such as web sites and REST services, an object tree must be composed for each request. However, all requests are served by the same AppDomain, so once an assembly is loaded, it sticks around to be available for all subsequent requests. Thus, the first few requests will suffer a performance penalty from having to load all assemblies, but after that there will be no performance impact.

In short, in request-based applications, you can compose object trees with confidence. In only extremely rare cases should you have performance issues from composing the entire tree in one go.

Long-running applications #

For long-running applications the entire object tree must be composed at start-up. For background services such as daemons and batch processes the start-up time probably doesn't matter much, but for desktop applications it can be of great importance.

In some cases the application requires the entire tree to be immediately available, in which case there's not a lot you can do. Still, once all assemblies have been loaded, actually creating the tree will be very fast.

In other cases an entire branch of the tree may not be immediately required. As an example, if the C1 node in the above graph isn't needed right away, we could improve start-up time if we could somehow defer creating that branch, because this would also defer loading of the entire C assembly.

Deferred branches #

Since object creation is fast, the only case where it makes sense to defer loading of a branch is when creation of that branch causes an assembly to be loaded. If we can defer creation of such a branch, we can also defer loading of the assembly, thus improving the time it takes to compose the initial tree.

Imagine that we wish to defer creation of the C1 branch of the above tree. It will prevent the C assembly from being loaded because that assembly is not used in any other place in the tree. However, it will not prevent the B assembly from being loaded, since that assembly is also being used by the A2 node.

Still, in those rare situations where it makes sense to defer creation of a branch, we can make that cut into a part of the infrastructure of the tree. I originally described this technique as a reaction to the above mentioned post by Jeffrey Palermo, but here's a restatement in the current context.

We can defer creating the C1 node by wrapping it in a lazy implementation of the same interface. The C1 node implements an interface called ISolo<IMarker>, so we can wrap it in a Virtual Proxy that defers creation of C1 until it's needed:

public class LazySoloMarker : ISolo<IMarker>
{
    private readonly Lazy<ISolo<IMarker>> lazy;
 
    public LazySoloMarker(Lazy<ISolo<IMarker>> lazy)
    {
        if (lazy == null)
        {
            throw new ArgumentNullException("lazy");
        }
 
        this.lazy = lazy;
    }
 
    #region ISolo<IMarker> Members
 
    public IMarker Item
    {
        get { return this.lazy.Value.Item; }
    }
 
    #endregion
}

This Virtual Proxy takes a Lazy<ISolo<IMarker>> as input and defers to it to implement the members of the interface. This only causes the Value property to be created when it's first accessed - which may be long after the LazySoloMarker instance was created.

The tree can now be composed like this:

var t =
    new A1(
        new A2(
            new B1(
                new B2()),
            new A3()),
        new LazySoloMarker(
            new Lazy<ISolo<IMarker>>(() => new C1(
                new B3()))));

This retains all the original behavior of the original tree, but defers creation of the C1 node until it's needed for the first time.

The bottom line is this: you can compose the entire object graph with confidence. It's not going to be a performance bottleneck.

Update (2013-08-19 08:09 UTC): For a more detailed treatment of this topic, watch my NDC 2013 talk Big Object Graphs Up Front.

Comments

The Constructor Injection design pattern is a extremely useful way to implement loose coupling. It's easy to understand and implement, but sometime perhaps a bit misunderstood.

The pattern itself is easily described through an example:

private readonly ISpecimenBuilder builder;
 
public SpecimenContext(ISpecimenBuilder builder)
{
    if (builder == null)
    {
        throw new ArgumentNullException("builder");
    }
 
    this.builder = builder;
}

The SpecimenContext constructor statically declares that it requires an ISpecimenBuilder instance as an argument. To guarantee that the builder field is an invariant of the class, the constructor contains a Guard Clause before it assigns the builder parameter to the builder field. This pattern can be repeated for each constructor argument.

It's important to understand that when using Constructor Injection the constructor should contain no additional logic.

An Injection Constructor should do no more than receiving the dependencies.

This is simply a rephrasing of Nikola Malovic's 4th law of IoC. There are several reasons for this rule of thumb:

• When we compose applications with Constructor Injection we often create substantial object graphs, and we want to be able to create these graphs as efficiently as possible. This is Nikola's original argument.
• In the odd (and not recommended) cases where you have circular dependencies, the injected dependencies may not yet be fully initialized, so an attempt to invoke their members at that time may result in an exception. This issue is similar to the issue of invoking virtual members from the constructor. Conceptually, an injected dependency is equivalent to a virtual member.
• With Constructor Injection, the constructor's responsibility is to demand and receive the dependencies. Thus, according to the Single Responsibility Principle (SRP), it should not try to do something else as well. Some readers might argue that I'm misusing the SRP here, but I think I'm simply applying the underlying principle in a more granular context.

There's no reason to feel constrained by this rule, as in any case the constructor is an implementation detail. In loosely coupled code, the constructor is not part of the overall application API. When we consider the API at that level, we are still free to design the API as we'd like.

Please notice that this rule is contextual: it applies to Services that use Constructor Injection. Entities and Value Objects tend not to use DI, so their constructors are covered by other rules.

Comments