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This post describes a generalized convention for Castle Windsor that handles AppSettings primitives.

In my previous post I explained how Convention over Configuration is the preferred way to use a DI Container. Some readers asked to see some actual convention implementations (although I actually linked to them in the post). In fact, I've previously showcased some simple conventions expressed with Castle Windsor's API.. In this post I'm going to show you another convention, which is completely reusable. Feel free to copy and paste :)

Most conventions are really easy to implement. Actually, sometimes it takes more effort to express the specification than it actually takes to implement it.

This convention deals with Primitive Dependencies. In my original post on the topic I included an AppSettingsConvention class as part of the code listing, but that implementation was hard-coded to only deal with integers. This narrow convention can be generalized:

The AppSettingsConvention should map AppSettings .config values into Primitive Dependencies.

• If a class has a dependency, the name of the dependency is assumed to be the name of the constructor argument (or property, for that matter). If, for example, the name of a constructor argument is top, this is the name of the dependency.
• If there's an appSettings key with the same name in the .config, and if there's a known conversion from string to the type of the dependency, the .config value is converted and used.

Example requirement: int top #

Consider this constructor:

public DbChartReader(int top, string chartConnectionString)

In this case the convention should look after an AppSettings key named top as well as check whether there's a known conversion from string to int (there is). Imagine that the .config file contains this XML fragment:

<appSettings>
  <add key="top" value="40" />
</appSettings>

The convention should read "40" from the .config file and convert it to an integer and inject 40 into a DbChartReader instance.

Example requirement: Uri catalogTrackBaseUri #

Consider this constructor:

public CatalogApiTrackLinkFactory(Uri catalogTrackBaseUri)

In this case the convention should look after an AppSettings key named catalogTrackBaseUri and check if there's a known conversion from string to Uri. Imagine that the .config file contains this XML fragment:

<appSettings>
  <add key="catalogTrackBaseUri" value="http://www.ploeh.dk/foo/img/"/>
  <add key="foo" value="bar"/>
  <add key="baz" value="42"/>
</appSettings>

The convention should read "http://www.ploeh.dk/foo/img/" from the .config file and convert it to a Uri instance.

Implementation #

By now it should be clear what the conventions should do. With Castle Windsor this is easily done by implementing an ISubDependencyResolver. Each method is a one-liner:

public class AppSettingsConvention : ISubDependencyResolver
{
    public bool CanResolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        return ConfigurationManager.AppSettings.AllKeys
                .Contains(dependency.DependencyKey)
            && TypeDescriptor
                .GetConverter(dependency.TargetType)
                .CanConvertFrom(typeof(string));
    }
 
    public object Resolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        return TypeDescriptor
            .GetConverter(dependency.TargetType)
            .ConvertFrom(
                ConfigurationManager.AppSettings[dependency.DependencyKey]);
    }
}

The ISubDependencyResolver interface is an example of the Tester-Doer pattern. Only if the CanResolve method returns true is the Resolve method invoked.

The CanResolve method performs two checks:

• Is there an AppSettings key in the configuration which is equal to the name of the dependency?
• Is there a known conversion from string to the type of the dependency?

The Resolve method simply reads the .config value and converts it to the appropriate type and returns it.

Adding the convention to an IWindsorContainer instance is easy:

container.Kernel.Resolver.AddSubResolver(
    new AppSettingsConvention());            

Summary #

The AppSettingsConvention is a completely reusable convention for Castle Windsor. With it, Primitive Dependencies are automatically wired the appropriate .config values if they are defined.

Comments

This post explains why a DI Container is useful with Convention over Configuration while Poor Man's DI might be a better fit for a more explicit Composition Root.

Note (2018-07-18): Since I wrote this article, I've retired the term Poor Man's DI in favour of Pure DI.

It seems to me that lately there's been a backlash against DI Containers among alpha geeks. Many of the software leaders that I myself learn from seem to dismiss the entire concept of a DI Container, claiming that it's too complex, too 'magical', that it isn't a good architectural pattern, or that the derived value doesn't warrant the 'cost' (most, if not all, DI Containers are open source, so they are free in a monetary sense, but there's always a cost in learning curve etc.).

This must have caused Krzysztof Koźmic to write a nice article about what sort of problem a DI Container solves. I agree with the article, but want to provide a different perspective here.

In short, it makes sense to me to illustrate the tradeoffs of Poor Man's DI versus DI Containers in a diagram like this:

The point of the diagram is that Poor Man's DI can be valuable because it's simple, while a DI Container can be either valuable or pointless depending on how it's used. However, when used in a sufficiently sophisticated way I consider a DI Container to offer the best value/cost ratio. When people criticize DI Containers as being pointless I suspect that what really happened was that they gave up before they were out of the Trough of Disillusionment. Had they continued to learn, they might have arrived at a new Plateau of Productivity.

There are other, less important advantages and disadvantages of each approach, but here I'm focusing on three main axes that I consider important:

• How easy is it to understand and learn?
• How soon will you get feedback if something is not right?
• How easy is it to maintain?

The major advantage of Poor Man's DI is that it's easy to learn. You don't have to learn the API of any DI Container (Unity, Autofac, Ninject, StructureMap, Castle Windsor, etc.) and while individual classes still use DI, once you find the Composition Root it'll be evident what's going on and how object graphs are constructed. No 'magic' is involved.

The second big advantage of Poor Man's DI is often overlooked: it's strongly typed. This is an advantage because it provides the fastest feedback about correctness that you can get. However, strong typing cuts both ways because it also means that every time you refactor a constructor, you will break the Composition Root. If you are sharing a library (Domain Model, Utility, Data Access component, etc.) between more than one application (unit of deployment), you may have more than one Composition Root to maintain. How much of a burden this is depends on how often you refactor constructors, but I've seen projects where this happens several times each day (keep in mind that constructor are implementation details).

If you use a DI Container, but explicitly Register each and every component using the container's API, you lose the rapid feedback from strong typing. On the other hand, the maintenance burden is also likely to drop because of Auto-wiring. Still, you'll need to register each new class or interface when you introduce them, and you (and your team) still has to learn the specific API of that container. In my opinion, you lose more advantages than you gain.

Ultimately, if you can wield a DI Container in a sufficiently sophisticated way, you can use it to define a set of conventions. These conventions define a rule set that your code should adhere to, and as long as you stick to those rules, things just work. The container drops to the background, and you rarely need to touch it. Yes, this is hard to learn, and is still weakly typed, but if done right, it enables you to focus on code that adds value instead of infrastructure. An additional advantage is that it creates a positive feedback mechanism forcing a team to produce code that is consistent with the conventions.

Example: Poor Man's DI #

The following example is part of my Booking sample application. It shows the state of the Ploeh.Samples.Booking.Daemon.Program class as it looks in the git tag total-complexity (git commit ID 64b7b670fff9560d8947dd133ae54779d867a451).

var queueDirectory = 
    new DirectoryInfo(@"..\..\..\BookingWebUI\Queue").CreateIfAbsent();
var singleSourceOfTruthDirectory = 
    new DirectoryInfo(@"..\..\..\BookingWebUI\SSoT").CreateIfAbsent();
var viewStoreDirectory = 
    new DirectoryInfo(@"..\..\..\BookingWebUI\ViewStore").CreateIfAbsent();
 
var extension = "txt";
 
var fileDateStore = new FileDateStore(
    singleSourceOfTruthDirectory,
    extension);
 
var quickenings = new IQuickening[]
{
    new RequestReservationCommand.Quickening(),
    new ReservationAcceptedEvent.Quickening(),
    new ReservationRejectedEvent.Quickening(),
    new CapacityReservedEvent.Quickening(),
    new SoldOutEvent.Quickening()
};
 
var disposable = new CompositeDisposable();
var messageDispatcher = new Subject<object>();
disposable.Add(
    messageDispatcher.Subscribe(
        new Dispatcher<RequestReservationCommand>(
            new CapacityGate(
                new JsonCapacityRepository(
                    fileDateStore,
                    fileDateStore,
                    quickenings),
                new JsonChannel<ReservationAcceptedEvent>(
                    new FileQueueWriter<ReservationAcceptedEvent>(
                        queueDirectory,
                        extension)),
                new JsonChannel<ReservationRejectedEvent>(
                    new FileQueueWriter<ReservationRejectedEvent>(
                        queueDirectory,
                        extension)),
                new JsonChannel<SoldOutEvent>(
                    new FileQueueWriter<SoldOutEvent>(
                        queueDirectory,
                        extension))))));
disposable.Add(
    messageDispatcher.Subscribe(
        new Dispatcher<SoldOutEvent>(
            new MonthViewUpdater(
                new FileMonthViewStore(
                    viewStoreDirectory,
                    extension)))));
 
var q = new QueueConsumer(
    new FileQueue(
        queueDirectory,
        extension),
    new JsonStreamObserver(
        quickenings,
        messageDispatcher));
 
RunUntilStopped(q);

Yes, that's a lot of code. I deliberately chose a non-trivial example to highlight just how much stuff there might be. You don't have to read and understand all of this code to appreciate that it might require a bit of maintenance. It's a big object graph, with some shared subgraphs, and since it uses the new keyword to create all the objects, every time you change a constructor signature, you'll need to update this code, because it's not going to compile until you do.

Still, there's no 'magical' tool (read: DI Container) involved, so it's pretty easy to understand what's going on here. As Dan North put it once I saw him endorse this technique: 'new' is the new 'new' :) Once you see how Explicit Register looks, you may appreciate why.

Example: Explicit Register #

The following example performs exactly the same work as the previous example, but now in a state (git tag: controllers-by-convention; commit ID: 13fc576b729cdddd5ec53f1db907ec0a7d00836b) where it's being wired by Castle Windsor. The name of this class is DaemonWindsorInstaller, and all components are explictly registered. Hang on to something.

container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\Queue").CreateIfAbsent())
    .Named("queueDirectory"));
container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\SSoT").CreateIfAbsent())
    .Named("ssotDirectory"));
container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\ViewStore").CreateIfAbsent())
    .Named("viewStoreDirectory"));            
 
container.Register(Component
    .For<IQueue>()
    .ImplementedBy<FileQueue>()
    .DependsOn(
        Dependency.OnComponent("directory", "queueDirectory"),
        Dependency.OnValue("extension", "txt")));
 
container.Register(Component
    .For<IStoreWriter<DateTime>, IStoreReader<DateTime>>()
    .ImplementedBy<FileDateStore>()
    .DependsOn(
        Dependency.OnComponent("directory", "ssotDirectory"),
        Dependency.OnValue("extension", "txt")));
container.Register(Component
    .For<IStoreWriter<ReservationAcceptedEvent>>()
    .ImplementedBy<FileQueueWriter<ReservationAcceptedEvent>>()
    .DependsOn(
        Dependency.OnComponent("directory", "queueDirectory"),
        Dependency.OnValue("extension", "txt")));
container.Register(Component
    .For<IStoreWriter<ReservationRejectedEvent>>()
    .ImplementedBy<FileQueueWriter<ReservationRejectedEvent>>()
    .DependsOn(
        Dependency.OnComponent("directory", "queueDirectory"),
        Dependency.OnValue("extension", "txt")));
container.Register(Component
    .For<IStoreWriter<SoldOutEvent>>()
    .ImplementedBy<FileQueueWriter<SoldOutEvent>>()
    .DependsOn(
        Dependency.OnComponent("directory", "queueDirectory"),
        Dependency.OnValue("extension", "txt")));
 
container.Register(Component
    .For<IChannel<ReservationAcceptedEvent>>()
    .ImplementedBy<JsonChannel<ReservationAcceptedEvent>>());
container.Register(Component
    .For<IChannel<ReservationRejectedEvent>>()
    .ImplementedBy<JsonChannel<ReservationRejectedEvent>>());
container.Register(Component
    .For<IChannel<SoldOutEvent>>()
    .ImplementedBy<JsonChannel<SoldOutEvent>>());
 
container.Register(Component
    .For<ICapacityRepository>()
    .ImplementedBy<JsonCapacityRepository>());
 
container.Register(Component
    .For<IConsumer<RequestReservationCommand>>()
    .ImplementedBy<CapacityGate>());
container.Register(Component
    .For<IConsumer<SoldOutEvent>>()
    .ImplementedBy<MonthViewUpdater>());
 
container.Register(Component
    .For<Dispatcher<RequestReservationCommand>>());
container.Register(Component
    .For<Dispatcher<SoldOutEvent>>());
 
container.Register(Component
    .For<IObserver<Stream>>()
    .ImplementedBy<JsonStreamObserver>());
container.Register(Component
    .For<IObserver<DateTime>>()
    .ImplementedBy<FileMonthViewStore>()
    .DependsOn(
        Dependency.OnComponent("directory", "viewStoreDirectory"),
        Dependency.OnValue("extension", "txt")));
container.Register(Component
    .For<IObserver<object>>()
    .UsingFactoryMethod(k =>
    {
        var messageDispatcher = new Subject<object>();
        messageDispatcher.Subscribe(k.Resolve<Dispatcher<RequestReservationCommand>>());
        messageDispatcher.Subscribe(k.Resolve<Dispatcher<SoldOutEvent>>());
        return messageDispatcher;
    }));
 
container.Register(Component
    .For<IQuickening>()
    .ImplementedBy<RequestReservationCommand.Quickening>());
container.Register(Component
    .For<IQuickening>()
    .ImplementedBy<ReservationAcceptedEvent.Quickening>());
container.Register(Component
    .For<IQuickening>()
    .ImplementedBy<ReservationRejectedEvent.Quickening>());
container.Register(Component
    .For<IQuickening>()
    .ImplementedBy<CapacityReservedEvent.Quickening>());
container.Register(Component
    .For<IQuickening>()
    .ImplementedBy<SoldOutEvent.Quickening>());
 
container.Register(Component
    .For<QueueConsumer>());
 
container.Kernel.Resolver.AddSubResolver(new CollectionResolver(container.Kernel));

This is actually more verbose than before - almost double the size of the Poor Man's DI example. To add spite to injury, this is no longer strongly typed in the sense that you'll no longer get any compiler errors if you change something, but a change to your classes can easily lead to a runtime exception, since something may not be correctly configured.

This example uses the Registration API of Castle Windsor, but imagine the horror if you were to use XML configuration instead.

Other DI Containers have similar Registration APIs (apart from those that only support XML), so this problem isn't isolated to Castle Windsor only. It's inherent in the Explicit Register style.

I can't claim to be an expert in Java, but all I've ever heard and seen of DI Containers in Java (Spring, Guice, Pico), they don't seem to have Registration APIs much more sophisticated than that. In fact, many of them still seem to be heavily focused on XML Registration. If that's the case, it's no wonder many software thought leaders (like Dan North with his 'new' is the new 'new' line) dismiss DI Containers as being essentially pointless. If there weren't a more sophisticated option, I would tend to agree.

Example: Convention over Configuration #

This is still the same example as before, but now in a state (git tag: services-by-convention-in-daemon; git commit ID: 0a7e6f246cacdbefc8f6933fc84b024774d02038) where almost the entire configuration is done by convention.

container.AddFacility<ConsumerConvention>();
 
container.Register(Component
    .For<IObserver<object>>()
    .ImplementedBy<CompositeObserver<object>>());
 
container.Register(Classes
    .FromAssemblyInDirectory(new AssemblyFilter(".").FilterByName(an => an.Name.StartsWith("Ploeh.Samples.Booking")))
    .Where(t => !(t.IsGenericType && t.GetGenericTypeDefinition() == typeof(Dispatcher<>)))
    .WithServiceAllInterfaces());
 
container.Kernel.Resolver.AddSubResolver(new ExtensionConvention());
container.Kernel.Resolver.AddSubResolver(new DirectoryConvention(container.Kernel));
container.Kernel.Resolver.AddSubResolver(new CollectionResolver(container.Kernel));
 
#region Manual configuration that requires maintenance
container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\Queue").CreateIfAbsent())
    .Named("queueDirectory"));
container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\SSoT").CreateIfAbsent())
    .Named("ssotDirectory"));
container.Register(Component
    .For<DirectoryInfo>()
    .UsingFactoryMethod(() =>
        new DirectoryInfo(@"..\..\..\BookingWebUI\ViewStore").CreateIfAbsent())
    .Named("viewStoreDirectory"));
#endregion

It's pretty clear that this is a lot less verbose - and then I even left three explicit Register statements as a deliberate decision. Just because you decide to use Convention over Configuration doesn't mean that you have to stick to this principle 100 %.

Compared to the previous example, this requires a lot less maintenance. While you are working with this code base, most of the time you can concentrate on adding new functionality to the software, and the conventions are just going to pick up your changes and new classes and interfaces. Personally, this is where I find the best tradeoff between the value provided by a DI Container versus the cost of figuring out how to implement the conventions. You should also keep in mind that once you've learned to use a particular DI Container like this, the cost goes down.

Summary #

Using a DI Container to compose object graphs by convention presents an unparalled opportunity to push infrastructure code to the background. However, if you're not prepared to go all the way, Poor Man's DI may actually be a better option. Don't use a DI Container just to use one. Understand the value and cost associated with it, and always keep in mind that Poor Man's DI is a valid alternative.

Comments

This post describes how to compose Controllers with Castle Windsor in the ASP.NET Web API

In my previous post I described how to use Dependency Injection (DI) in the ASP.NET Web API using Poor Man's DI. It explained the basic building blocks, including the relevant extensibility points in the Web API. Poor Man's DI can be an easy way to get started with DI and may be sufficient for a small code base, but for larger code bases you may want to adopt a more convention-based approach. Some DI Containers provide excellent support for Convention over Configuration. One of these is Castle Windsor.

Composition Root #

Instead of the PoorMansCompositionRoot from the example in the previous post, you can create an implementation of IHttpControllerActivator that acts as an Adapter over Castle Windsor:

public class WindsorCompositionRoot : IHttpControllerActivator
{
    private readonly IWindsorContainer container;
 
    public WindsorCompositionRoot(IWindsorContainer container)
    {
        this.container = container;
    }
 
    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
    {
        var controller =
            (IHttpController)this.container.Resolve(controllerType);
 
        request.RegisterForDispose(
            new Release(
                () => this.container.Release(controller)));
 
        return controller;
    }
 
    private class Release : IDisposable
    {
        private readonly Action release;
 
        public Release(Action release)
        {
            this.release = release;
        }
 
        public void Dispose()
        {
            this.release();
        }
    }
}

That's pretty much all there is to it, but there are a few points of interest here. First of all, the class implements IHttpControllerActivator just like the previous PoorMansCompositionRoot. That's the extensibility point you need to implement in order to create Controller instances. However, instead of hard-coding knowledge of concrete Controller classes into the Create method, you delegate creation of the instance to an injected IWindsorContainer instance. However, before returning the IHttpController instance created by calling container.Resolve, you register that object graph for disposal.

With Castle Windsor decommissioning is done by invoking the Release method on IWindsorContainer. The input into the Release method is the object graph originally created by IWindsorContainer.Resolve. That's the rule from the Register Resolve Release pattern: What you Resolve you must also Release. This ensures that if the Resolve method created a disposable instance (even deep in the object graph), the Release method signals to the container that it can now safely dispose of it. You can read more about this subject in my book.

The RegisterForDispose method takes as a parameter an IDisposable instance, and not a Release method, so you must wrap the call to the Release method in an IDisposable implementation. That's the little private Release class in the code example. It adapts an Action delegate into a class which implements IDisposable, invoking the code block when Dispose is invoked. The code block you pass into the constructor of the Release class is a closure around the outer variables this.container and controller so when the Dispose method is called, the container releases the controller (and the entire object graph beneath it).

Configuring the container #

With the WindsorCompositionRoot class in place, all that's left is to set it all up in Global.asax. Since IWindsorContainer itself implements IDisposable, you should create and configure the container in the application's constructor so that you can dispose it when the application exits:

private readonly IWindsorContainer container;
 
public WebApiApplication()
{
    this.container =
        new WindsorContainer().Install(new DependencyConventions());
}
 
public override void Dispose()
{
    this.container.Dispose();
    base.Dispose();
}

Notice that you can configure the container with the Install method directly in the constructor. That's the Register phase of the Register Resolve Release pattern.

In Application_Start you tell the ASP.NET Web API about your WindsorCompositionRoot instead of PoorMansCompositionRoot from the previous example:

GlobalConfiguration.Configuration.Services.Replace(
    typeof(IHttpControllerActivator),
    new WindsorCompositionRoot(this.container));

Notice that the container instance field is passed into the constructor of WindsorCompositionRoot, so that it can use the container instance to Resolve Controller instances.

Summary #

Setting up DI in the ASP.NET Web API with a DI Container is easy, and it would work very similarly with other containers (not just Castle Windsor), although the Release mechanisms tend to be a bit different from container to container. You'll need to create an Adapter from IHttpControllerActivator to your container of choice and set it all up in the Global.asax.

Comments

This post describes how to properly use Dependency Injection in the ASP.NET Web API, including proper Lifetime Management.

The ASP.NET Web API supports Dependency Injection (DI), but the appropriate way to make it work is not the way it's documented by Microsoft. Even though the final version of IDependencyResolver includes the notion of an IDependencyScope, and thus seemingly supports decommissioning (the release of IDisposable dependencies), it's not very useful.

The problem with IDependencyResolver #

The main problem with IDependencyResolver is that it's essentially a Service Locator. There are many problems with the Service Locator anti-pattern, but most of them I've already described elsewhere on this blog (and in my book). One disadvantage of Service Locator that I haven't yet written so much about is that within each call to GetService there's no context at all. This is a general problem with the Service Locator anti-pattern, not just with IDependencyResolver. Glenn Block originally pointed this out to me. The problem is that in an implementation, all you're given is a Type instance and asked to return an object, but you're not informed about the context. You don't know how deep in a dependency graph you are, and if you're being asked to provide an instance of the same service multiple times, you don't know whether it's within the same HTTP request, or whether it's for multiple concurrent HTTP requests.

In the ASP.NET Web API this issue is exacerbated by another design decision that the team made. Contrary to the IDependencyResolver design, I find this other decision highly appropriate. It's how context is modeled. In previous incarnations of web frameworks from Microsoft, we've had such abominations as HttpContext.Current, HttpContextBase and HttpContextWrapper. If you've ever tried to work with these interfaces, and particularly if you've tried to do TDD against any of these types, you know just how painful they are. That they are built around the Singleton pattern certainly doesn't help.

The ASP.NET Web API does it differently, and that's very fortunate. Everything you need to know about the context is accessible through the HttpRequestMessage class. While you could argue that it's a bit of a God Object, it's certainly a step in the right direction because at least it's a class you can instantiate within a unit test. No more nasty Singletons.

This is good, but from the perspective of DI this makes IDependencyResolver close to useless. Imagine a situation where a dependency deep in the dependency graph need to know something about the context. What was the request URL? What was the base address (host name etc.) requested? How can you share dependency instances within a single request? To answer such questions, you must know about the context, and IDependencyResolver doesn't provide this information. In short, IDependencyResolver isn't the right hook to compose dependency graphs. Fortunately, the ASP.NET Web API has a better extensibility point for this purpose.

Composition within context #

Because HttpRequestMessage provides the context you may need to compose dependency graphs, the best extensibility point is the extensibility point which provides an HttpRequestMessage every time a graph should be composed. This extensibility point is the IHttpControllerActivator interface:

public interface IHttpControllerActivator
{
    IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType);
}

As you can see, each time the Web API framework invokes this method, it will provide an HttpRequestMessage instance. This seems to be the best place to compose the dependency graph for each request.

Example: Poor Man's DI #

As an example, consider a Controller with this constructor signature:

public RootController(IStatusQuery statusQuery)

If this is the only Controller in your project, you can compose its dependency graph with a custom IHttpControllerActivator. This is easy to do:

public class PoorMansCompositionRoot : IHttpControllerActivator
{
    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
    {
        if (controllerType == typeof(RootController))
            return new RootController(
                new StatusQuery());
 
        return null;
    }
}

The Create implementation contains a check on the supplied controllerType parameter, creating a RootController instance if the requested Controller type is RootController. It simply creates the (very shallow) dependency graph by injecting a new StatusQuery instance into a new RootController instance. If the requested Controller type is anything else than RootController, the method returns null. It seems to be a convention in the Web API that if you can't supply an instance, you should return null. (This isn't a design I'm fond of, but this time I'm only on the supplying side, and I can only pity the developers on the receiving side (the ASP.NET team) that they'll have to write all those null checks.)

Some readers may think that it would be better to use a DI Container here, and that's certainly possible. In a future post I'll provide an example on how to do that.

The new PoorMansCompositionRoot class must be registered with the Web API framework before it will work. This is done with a single line in Application_Start in Global.asax.cs:

GlobalConfiguration.Configuration.Services.Replace(
    typeof(IHttpControllerActivator),
    new PoorMansCompositionRoot());

This replaces the default implementation that the framework provides with the PoorMansCompositionRoot instance.

Decommissioning #

Implementing IHttpControllerActivator.Create takes care of composing object graphs, but what about decommissioning? What if you have dependencies (deep within the dependency graph) implementing the IDisposable interface? These must be disposed of after the request has ended (unless they are Singletons) - if not, you will have a resource leak at hand. However, there seems to be no Release hook in IHttpControllerActivator. On the other hand, there's a Release hook in IDependencyResolver, so is IDependencyResolver, after all, the right extensibility point? Must you trade off context for decommissioning, or can you have both?

Fortunately you can have both, because there's a RegisterForDispose extension method hanging off HttpRequestMessage. It enables you to register all appropriate disposable instances for disposal after the request has completed.

Example: disposing of a disposable dependency #

Imagine that, instead of the StatusQuery class from the previous example, you need to use a disposable implementation of IStatusQuery. Each instance must be disposed of after each request completes. In order to accomplish this goal, you can modify the PoorMansCompositionRoot implementation to this:

public class PoorMansCompositionRoot : IHttpControllerActivator
{
    public IHttpController Create(
        HttpRequestMessage request,
        HttpControllerDescriptor controllerDescriptor,
        Type controllerType)
    {
        if (controllerType == typeof(RootController))
        {
            var disposableQuery = new DisposableStatusQuery();
            request.RegisterForDispose(disposableQuery);
            return new RootController(disposableQuery);
        }
 
        return null;
    }
}

Notice that the disposableQuery instance is passed to the RegisterForDispose method before it's injected into the RootController instance and the entire graph is returned from the method. After the request completes, DisposableStatusQuery.Dispose will be called.

If you have a dependency which implements IDisposable, but should be shared across all requests, obviously you shouldn't register it for disposal. Such Singletons you can keep around and dispose of properly when the application exits gracefull (if that ever happens).

Summary #

Proper DI and Lifetime Management with the ASP.NET Web API is easy once you know how to do it. It only requires a few lines of code.

Stay away from the IDependencyResolver interface, which is close to useless. Instead, implement IHttpControllerActivator and use the RegisterForDispose method for decommissioning.

In a future post I will demonstrate how to use a DI Container instead of Poor Man's DI.

Comments

Concrete classes can also be used as dependencies

Usually, when we think about Dependency Injection (DI), we tend to consider that only polymorphic types (interfaces or (abstract) base classes) can act as dependencies. However, in a previous blog post I described how primitives can be used as dependencies. A primitive is about as far removed from polymorphism as it can be, but there's a middle ground too. Sometimes 'normal' concrete classes with non-virtual members can be used as dependencies with to great effect.

While the Liskov Substitution Principle is voided by injecting a concrete type, there can be other good reasons to occasionaly do something like this. Consider how many times you've written an extension method to perform some recurring task. Sometimes it turns out that an extension method isn't the best way to encapsulate a common algorithm. It might start out simple enough, but then you realize that you need to provide the extension method with a control parameter in order to 'configure' it. This causes you to add more arguments to the extension method, or to add more overloads. Soon, you have something like the Object Mother (anti-)pattern on your hand.

A concrete class can sometimes be a better way to encapsulate common algorithms in a way where the behavior can be tweaked or changed in one go. Sometimes the boundary can become blurred. In the previous post I examined constructor arguments such as strings and integers, but what about an Uri instance? It might act as a base URI for creating absolute links from within a Controller. An Uri instance isn't really a primitive, although it basically just encapsulates something which is a string. It's an excellent example of the Value Object pattern, providing a rich API for manipulating and querying URIs.

It can be more complex that that. Consider Hyprlinkr as an example. What it does is to produce URI links to other Controllers in an ASP.NET Web API service in a strongly typed way. It's not really a polymorphic dependency as such, although it does implement an interface. It's more like a reusable component which produces a determinstic result without side-effects. In Functional Programming terminology, it's comparable to a pure function. For a number of reasons, this is a prime candidate for a concrete dependency.

Before I get to that, I want to show you what I mean when I talk about locally scoped methods, including extension methods and such. Then I want to talk about using the RouteLinker class (the main class in Hyprlinkr) as a classic polymorphic dependency, and why that doesn't really work either. Finally, I want to talk about why the best option is to treat RouteLinker as a concrete dependency.

RouteLinker as a local variable #

While Hyprlinkr was always designed with DI in mind, you actually don't have to use DI to use it. From within an ApiController class, you can just create an instance like this:

var linker = new RouteLinker(this.Request);

With this locally scoped variable you can start creating links to other resources:

Href = linker.GetUri<NoDIController>(r => r.Get(id)).ToString()

That seems easy, so why make it hard than that? Well, it's easy as long as you have only a single, default route in your web service. As soon as you add more routes, you'll need to help Hyprlinkr a bit by providing a custom IRouteDispatcher. That goes as the second argument in a constructor overload:

var linker = new RouteLinker(this.Request, ??);

The question is: how do you create an instance of the desired IRouteDispatcher? You could do it inline every time you want to create an instance of RouteLinker:

var linker = new RouteLinker(this.Request, new MyCustomRouteDispatcher());

However, that's starting to look less than DRY. This is where many people might consider creating an extension method which creates a RouteLinker instance from an HttpRequestMessage instance. Now what if you need to supply a configuration value to the custom route dispatcher? Should you pull it from app.config straight from within your extension method? Then what if you need to be able to vary that configuration value from a unit test? This could lead toward an unmaintainable mess quickly. Perhaps it would be better injecting the dependency after all...

IResourceLinker as a polymorphic dependency #

The RouteLinker class actually implements an interface (IResourceLinker) so would it be worthwhile to inject it as a polymorphic interface? This is possible, but actually leads to more trouble. The problem is that due to its signature, it's damn hard to unit test. The interface looks like this:

public interface IResourceLinker
{
    Uri GetUri<T>(Expression<Action<T>> method);
}

That may at first glance look innocuous, but is actually quite poisonous. The first issue is that it's impossible to define proper setups when using dynamic mocks. This is because of the Expression parameter. The problem is that while the following Moq setup compiles, it can never work:

linkerStub
    .Setup(x => x.GetUri<ArtistController>(c => c.Get(artistId)))
    .Returns(uri);

The problem is that the expression passed into the Setup method isn't the same as the expression used in the SUT. It may look like the same expression, but it's not. Most of the expression tree actually is the same, but the problem is the leaf of the tree. The leaf of the expression tree is the reference to the artistId variable. This is a test variable, while in the SUT it's a variable which is internal to the SUT. While the values of both variables are expected to be the same, the variables themselves aren't.

It might be possible to write a custom equality comparer that picks expression trees apart in order to compare the values of leaf nodes, but that could become messy very quickly.

The only option seems to define Setups like this:

linkerStub
    .Setup(x => x.GetUri(It.IsAny<Expression<Action<ArtistController>>>()))
    .Returns(uri);

That sort of defies the purpose of a dynamic Test Double...

That's not the only problem with the IResourceLinker interface. The other problem is the return type. Since Uri doesn't have a default constructor, it's necessary to tell Moq what to return when the GetUri method is called. While the default behavior of Moq is to return null if no matching Setups were found, I never allow null in my code, so I always change Moq's behavior to return something proper instead. However, this has the disadvantage that if there's no matching Setup when the SUT attempts to invoke the GetUri method, Moq will throw an exception because there's no default constructor for Uri and it doesn't know what else to return.

This leads to Fixture Setup code like this:

linkerStub
    .Setup(x => x.GetUri(It.IsAny<Expression<Action<ArtistController>>>()))
    .Returns(uri);
linkerStub
    .Setup(x => x.GetUri(It.IsAny<Expression<Action<ArtistAlbumsController>>>()))
    .Returns(uri);
linkerStub
    .Setup(x => x.GetUri(It.IsAny<Expression<Action<ArtistTracksController>>>()))
    .Returns(uri);
linkerStub
    .Setup(x => x.GetUri(It.IsAny<Expression<Action<SimilarArtistsController>>>()))
    .Returns(uri);

...and that's just to prevent the unit test from crashing. Each and every unit test that hits the same method must have this Setup because the SUT method internally invokes the GetUri method four times with four different parameters. This is pure noise and isn't even part of the test case itself. The tests become very brittle.

If only there was a better way...

RouteLinker as a concrete dependency #

What would happen if you inject the concrete RouteLinker class into other classes? This might look like this:

private readonly RouteLinker linker;
 
public HomeController(
    RouteLinker linker)
{
    this.linker = linker;
}

Creating links from within the Controller is similar to before:

Href = this.linker.GetUri<HomeController>(r => r.GetHome()).ToString(),

What about unit testing? Well, since the GetUri method is strictly deterministic, given the same input, it will always produce the same output. Thus, from a unit test, you only have to ask the instance of RouteLinker injected into the SUT what it would return if invoked with a specific input. Then you can compare this expected output with the actual output.

[Theory, AutoUserData]
public void GetHomeReturnsResultWithCorrectSelfLink(
    [Frozen]RouteLinker linker,
    HomeController sut)
{
    var actual = sut.GetHome();
 
    var expected = new AtomLinkModel
    {
        Href = linker.GetUri<HomeController>(r => r.GetHome()).ToString(),
        Rel = "self"
    }.AsSource().OfLikeness<AtomLinkModel>();
    Assert.True(actual.Links.Any(expected.Equals));
}

In this test, you Freeze the RouteLinker instance, which means that the linker variable is the same instance as the RouteLinker injected into the SUT. Next, you ask that RouteLinker instance what it would produce when invoked in a particular way, and since AtomLinkModel doesn't override Equals, you produce a Likeness from the AtomLinkModel and verify that the actual collection of links contains the expected link.

That's much more precise than those horribly forgiving It.IsAny constraints. The other advantage is also that you don't have to care about Setups of methods you don't care about in a particular test case. The SUT can invoke the GetUri method as many times as it wants, with as many different arguments as it likes, and the test is never going to break because of that. Since the real implementation is injected, it always works without further Setup.

Granted, strictly speaking these aren't unit tests any longer, but rather Facade Tests.

This technique works because the GetUri method is deterministic and has no side-effects. Thus, it's very similar to Function Composition in Functional languages.

This post answers a FAQ about ordering of AutoFixture Customizations

With AutoFixture you can encapsulate common Customizations using the Customize method and the ICustomization interface. However, customizations may 'compete' for the same requests in the sense that more than one customization is able to handle a request.

As an example, consider a request for something as basic as IEnumerable<T>. By default, AutoFixture can't create instances of IEnumerable<T>, but more than one customization can.

As previously described the MultipleCustomization handles requests for sequences just fine:

var fixture = new Fixture().Customize(new MultipleCustomization());
var seq = fixture.CreateAnonymous<IEnumerable<int>>();

However, the AutoMoqCustomization can also (sort of) create sequences:

var fixture = new Fixture().Customize(new AutoMoqCustomization());
var seq = fixture.CreateAnonymous<IEnumerable<int>>();

However, in this case, the implementation of IEnumerable<int> is a dynamic proxy, so it's not much of a sequence after all.

Mocking IEnumerable<T> #

Here I need to make a little digression on why that is, because this seems to confuse a lot of people. Consider what a dynamic mock object is: it's a dynamic proxy which implements an interface (or abstract base class). It doesn't have a lot of implemented behavior. Dynamic mocks do what we tell them through their configuration APIs (such as the Setup methods for Moq). If we don't tell them what to do, they must fall back on some sort of default implementation. When the AutoMoqCustomization is used, it sets Mock<T>.DefaultValue to DefaultValue.Mock, which means that the default behavior is to return a new dynamic proxy for reference types.

Here's how an unconfigured dymamic proxy of IEnumerable<T> will behave: the interface only has two (very similar) methods:

public interface IEnumerable<out T> : IEnumerable
{
    IEnumerator<T> GetEnumerator();
}

Via IEnumerable the interface also defines the non-generic GetEnumerator method, but it's so similar to the generic GetEnumerator method that the following discussion applies for both.

When you iterate over IEnumerable<T> using foreach, or when you use LINQ, the first thing that happens is that the GetEnumerator method is called. An unconfigured dynamic mock will respond by returning another dynamic proxy implementing IEnumerator<T>. This interface directly and indirectly defines these methods:

T Current { get; }
 
object IEnumerator.Current { get; }
 
bool MoveNext();
 
void Reset();
 
void Dispose();

Iterating over a sequence will typically start by invoking the MoveNext method. Since the dynamic proxy is unconfigured, it has to fall back to default behavior. For booleans the default value is false, so the return value of a call to MoveNext would be false. This means that there are no more elements in the sequence. Iteration stops even before it begins. In effect, such an implementation would look like an empty sequence.

OK, back to AutoFixture.

Ordering Customizations #

Frequently I receive questions like this:

"Creating lists with AutoFixture seems inconsistent. When MultipleCustomization comes before AutoMoqCustomization, lists are popuplated, but the other way around they are empty. Is this a bug?"

No, this is by design. By now, you can probably figure out why.

Still, lets look at the symptoms. Both of these tests pass:

[Fact]
public void OnlyMultipleResolvingSequence()
{
    var fixture = new Fixture().Customize(new MultipleCustomization());
    var seq = fixture.CreateAnonymous<IEnumerable<int>>();
    Assert.NotEmpty(seq);
}
 
[Fact]
public void OnlyAutoMoqResolvingSequence()
{
    var fixture = new Fixture().Customize(new AutoMoqCustomization());
    var seq = fixture.CreateAnonymous<IEnumerable<int>>();
    Assert.Empty(seq);
}

Notice that in the first test, the sequence is not empty, whereas in the second test, the sequence is empty. This is because the MultipleCustomization produces a 'proper' sequence, while the AutoMoqCustomization produces a dynamic proxy of IEnumerable<int> as described above. At this point, this should hardly be surprising.

The same obvervations can be made when both Customizations are in use:

[Fact]
public void WrongOrderResolvingSequence()
{
    var fixture = new Fixture().Customize(
        new CompositeCustomization(
            new AutoMoqCustomization(),
            new MultipleCustomization()));
 
    var seq = fixture.CreateAnonymous<IEnumerable<int>>();
 
    Assert.Empty(seq);
}
 
[Fact]
public void CorrectOrderResolvingSequnce()
{
    var fixture = new Fixture().Customize(
        new CompositeCustomization(
            new MultipleCustomization(),                    
            new AutoMoqCustomization()));
 
    var seq = fixture.CreateAnonymous<IEnumerable<int>>();
 
    Assert.NotEmpty(seq);
}

Both of these tests also pass. In the first test the sequence is empty, and in the second it contains elements. This is because the first Customization 'wins'.

In general, a Customization may potentially be able to handle a lot of requests. For instance, the AutoMoqCustomization can handle all requests for interfaces and abstract base classes. Thus, multiple Customizations may be able to handle a request, so AutoFixture needs a conflict resolution strategy. That strategy is simply that the first Customization which can handle a request gets to do that, and the other Customizations are never invoked. You can use this feature to put specific Customizations in front of more catch-all Customizations. That's essentially what happens when you put MultipleCustomization in front of AutoMoqCustomization.

In my previous post I walked you through stage 1 of the FizzBuzz kata. In this post I'll walk you through stage 2 of the kata, where new requirements are introduced (see the kata itself for details). This makes the implementation much more complex.

Unit test #

In order to meet the new requirements, I first modified and expanded my existing test cases:

[<Theory>]
[<InlineData(1, "1")>]
[<InlineData(2, "2")>]
[<InlineData(3, "Fizz")>]
[<InlineData(4, "4")>]
[<InlineData(5, "Buzz")>]
[<InlineData(6, "Fizz")>]
[<InlineData(7, "7")>]
[<InlineData(8, "8")>]
[<InlineData(9, "Fizz")>]
[<InlineData(10, "Buzz")>]
[<InlineData(11, "11")>]
[<InlineData(12, "Fizz")>]
[<InlineData(13, "Fizz")>]
[<InlineData(14, "14")>]
[<InlineData(15, "FizzBuzz")>]
[<InlineData(16, "16")>]
[<InlineData(17, "17")>]
[<InlineData(18, "Fizz")>]
[<InlineData(19, "19")>]
[<InlineData(20, "Buzz")>]
[<InlineData(30, "FizzBuzz")>]
[<InlineData(31, "Fizz")>]
[<InlineData(32, "Fizz")>]
[<InlineData(33, "Fizz")>]
[<InlineData(34, "Fizz")>]
[<InlineData(35, "FizzBuzz")>]
[<InlineData(36, "Fizz")>]
[<InlineData(37, "Fizz")>]
[<InlineData(38, "Fizz")>]
[<InlineData(39, "Fizz")>]
[<InlineData(50, "Buzz")>]
[<InlineData(51, "FizzBuzz")>]
[<InlineData(52, "Buzz")>]
[<InlineData(53, "FizzBuzz")>]
[<InlineData(54, "FizzBuzz")>]
[<InlineData(55, "Buzz")>]
[<InlineData(56, "Buzz")>]
[<InlineData(57, "FizzBuzz")>]
[<InlineData(58, "Buzz")>]
[<InlineData(59, "Buzz")>]
let FizzBuzzReturnsCorrectResult number expected =
    number
    |> FizzBuzz
    |> should equal expected

This is the same test code as before, only with new or modified test data.

Implementation #

Compared with the stage 1 implementation, my implementation to meet the new requirements is much more complex. First, I'll post the entire code listing and then walk you through the details:

let FizzBuzz number =
    let arithmeticFizzBuzz number =
        seq {
            if number % 3 = 0 then yield "Fizz"
            if number % 5 = 0 then yield "Buzz"
            }
 
    let digitalFizzBuzz digit =
        seq {
            if digit = 3 then yield "Fizz"
            if digit = 5 then yield "Buzz"
            }
 
    let rec digitize number =
        seq {
                yield number % 10
                let aTenth = number / 10
                if aTenth >= 1 then yield! digitize aTenth
            }
 
    let arithmeticFizzBuzzes = number |> arithmeticFizzBuzz
    let digitalFizzBuzzes = number
                            |> digitize
                            |> Seq.collect digitalFizzBuzz
 
    let fizzOrBuzz = arithmeticFizzBuzzes
                     |> Seq.append digitalFizzBuzzes
                     |> Seq.distinct
                     |> Seq.toArray
                     |> Array.sort
                     |> Array.rev
                     |> String.Concat
 
    if fizzOrBuzz = ""
    then number.ToString()
    else fizzOrBuzz

First of all, you may wonder where the original implementation went. According to the requirements, the function must still 'Fizz' or 'Buzz' when a number is divisible by 3 or 5. This is handled by the nested arithmeticFizzBuzz function:

let arithmeticFizzBuzz number =
    seq {
        if number % 3 = 0 then yield "Fizz"
        if number % 5 = 0 then yield "Buzz"
        }

The seq symbol specifies a sequence expression, which means that everything within the curly brackets is expected to produce parts of a sequence. It works a bit like the yield keyword in C#.

Due to F#'s strong type inference, the type of the function is int -> seq<string>, which means that it takes an integer as input and returns a sequence of strings. In C# an equivalent signature would be IEnumerable<string> arithmeticFizzBuzz(int number). This function produces a sequence of strings depending on the input.

• 1 produces an empty sequence.
• 2 produces an empty sequence.
• 3 produces a sequence containing the single string "Fizz".
• 4 produces an empty seqence.
• 5 produces a sequence containing the single string "Buzz".
• 6 produces a sequence containing the single string "Fizz".
• 15 produces a sequence containing the strings "Fizz" and "Buzz" (in that order).

That doesn't quite sound like the original requirements, but the trick will be to concatenate the strings. Thus, an empty sequence will be "", "Fizz" will be "Fizz", "Buzz" will be "Buzz", but "Fizz" and "Buzz" will become "FizzBuzz".

The digitalFizzBuzz function works in much the same way, but expects only a single digit.

let digitalFizzBuzz digit =
    seq {
        if digit = 3 then yield "Fizz"
        if digit = 5 then yield "Buzz"
        }

• 1 produces an empty sequence.
• 2 produces an empty sequence.
• 3 produces a sequence containing the single string "Fizz".
• 4 produces an empty seqence.
• 5 produces a sequence containing the single string "Buzz".
• 6 produces an empty sequence.

In order to be able to apply the new rule of Fizzing and Buzzing if a digit is 3 or 5, it's necessary to split a number into digits. This is done by the recursive digitize function:

let rec digitize number =
    seq {
            yield number % 10
            let aTenth = number / 10
            if aTenth >= 1 then yield! digitize aTenth
        }

This function works recursively by first yielding the rest of a division by 10, and then calling itself recursively with a tenth of the original number. Since the number is an integer, the division simply still produces an integer. The function produces a sequence of digits, but in a sort of backwards way.

• 1 produces a sequence containing 1.
• 2 produces a sequence containing 2.
• 12 produces a sequence containing 2 followed by 1.
• 23 produces a sequence containing 3 followed by 2.
• 148 produces 8, 4, 1.

This provides all the building blocks. To get the arithmetic (original) FizzBuzzes, the number is piped into the arithmeticFizzBuzz function:

let arithmeticFizzBuzzes = number |> arithmeticFizzBuzz

In order to get the digital (new) FizzBuzzes, the number is first piped into the digitize function, and the resulting sequence of digits is then piped to the digitalFizzBuzz function by way of the Seq.collection function.

let digitalFizzBuzzes = number
                        |> digitize
                        |> Seq.collect digitalFizzBuzz

The Seq.collect function is a built-in function that takes a sequence of elements (in this case a sequence of digits) and for each element calls a method that produces a sequence of elements, and then concatenates all the produced sequences. As an example, consider the number 53.

Calling digitize with the number 53 produces the sequence { 3; 5 }. Calling digitalFizzBuzz with 3 produces the sequence { "Fizz" } and calling digitalFizzBuzz with 5 produces { "Buzz" }. Seq.collect concatenates these two sequences to produce the single sequence { "Fizz"; "Buzz" }.

Now we have two sequences of "Fizz" or "Buzz" strings - one produced by the old, arithmetic function, and one produced by the new, digital function. These two sequences can now be merged and ordered with the purpose of producing a single string:

let fizzOrBuzz = arithmeticFizzBuzzes
                 |> Seq.append digitalFizzBuzzes
                 |> Seq.distinct
                 |> Seq.toArray
                 |> Array.sort
                 |> Array.rev
                 |> String.Concat

First, the Seq.append function simply concatenates the two sequences into a single sequence. This could potentially result in a sequence like this: { "Fizz"; "Buzz"; "Fizz" }. The Seq.distinct function gets rid of the duplicates, but the ordering may be wrong - the sequence may look like this: { "Buzz"; "Fizz" }. This can be fixed by sorting the sequence, but sorting alphabetically would always put "Buzz" before "Fizz" so it's also necessary to reverse the sequence. There's no function in the Seq module which can reverse a sequence, so first the Seq.toArray function is used to convert the sequence to an array. After sorting and reversing the array, the result is one of four arrays: [], [ "Fizz" ], [ "Buzz" ], or [ "Fizz"; "Buzz" ]. The last step is to concatenate these string arrays to a single string using the String.Concat BCL method.

If there were no Fizzes or Buzzes, the string will be empty, in which case the number is converted to a string and returned; otherwise, the fizzOrBuzz string is returned.

if fizzOrBuzz = ""
then number.ToString()
else fizzOrBuzz

To print the FizzBuzz list for numbers from 1 to 100 the same solution as before can be used.

What I like about Functional Programming is that data just flows through the function. There's not state and no mutation - only operations on sequences of data.

In previous posts I've walked through the Bank OCR kata in F#. In this post, I will do the same for the first stage of the very simple FizzBuzz kata. This is a very simple kata, so if you already know F#, there will be nothing new to see here. On the other hand, if you've yet to be exposed to F#, this is a good place to start - I'll attempt to walk you through the code assuming that you don't know F# yet.

Unit test #

Since I developed the solution using Test-Driven Development, I started by writing a single Parameterized Test:

[<Theory>]
[<InlineData(1, "1")>]
[<InlineData(2, "2")>]
[<InlineData(3, "Fizz")>]
[<InlineData(4, "4")>]
[<InlineData(5, "Buzz")>]
[<InlineData(6, "Fizz")>]
[<InlineData(7, "7")>]
[<InlineData(8, "8")>]
[<InlineData(9, "Fizz")>]
[<InlineData(10, "Buzz")>]
[<InlineData(11, "11")>]
[<InlineData(12, "Fizz")>]
[<InlineData(13, "13")>]
[<InlineData(14, "14")>]
[<InlineData(15, "FizzBuzz")>]
[<InlineData(16, "16")>]
[<InlineData(17, "17")>]
[<InlineData(18, "Fizz")>]
[<InlineData(19, "19")>]
[<InlineData(20, "Buzz")>]
let FizzBuzzReturnsCorrectResult number expected =
    number
    |> FizzBuzz
    |> should equal expected

This test uses xUnit.net data theories to provide a set of test data in the form of an integer as input and an expected string.

The number input variable is piped to the FizzBuzz function, using F#'s pipe operator |>. This is just another way of writing

FizzBuzz number

The pipe operator simply takes the data being piped and uses it as the last input parameter to the function being piped. In this case, the number integer variable is the data being piped, so it's used as the last input parameter to the FizzBuzz function, which only takes a single paramter.

The result of invoking the FizzBuzz function is a string. This result is again piped to the should method, which is defined by the FsUnit module. The should method is an assertion function that takes three input parameters. The first two parameters are supplied as part of the function invokation as equal expected, but since the pipe operator is being used, the third and final parameter value is the result of invoking the FizzBuzz function.

In all, the test states that when the FizzBuzz function is called with number, the result should be equal to the expected string.

Implementation #

The FizzBuzz implementation is really simple:

let FizzBuzz number =
    match number with
    | i when i % 3 = 0 && i % 5 = 0 -> "FizzBuzz"
    | i when i % 3 = 0 -> "Fizz"
    | i when i % 5 = 0 -> "Buzz"
    | _ -> number.ToString()

All it does is to use pattern matching against the number input argument. In all cases except the last one, the value of number is matched against any number, but with a condition. The first condition is that the number should be divible by both 3 and 5. If this is the case, the result to the right of the -> operator is returned from the function ("FizzBuzz").

The last line of the match block uses an underscore as the match pattern. This is a catch-all pattern that's being triggered if none of the other patterns are matched. In this case, the number input argument is converted to a string and returned.

Printing all lines #

The kata requires me to print the output for all numbers from 1 to 100. The astute reader may have noticed that the FizzBuzz function doesn't do that - it only converts a single integer to a string. However, printing all numbers fizzed and buzzed is easy:

[1..100]
|> List.map FizzBuzz
|> List.reduce (sprintf "%s\r\n%s")

The first line defines a list of numbers from 1 to 100. The next line pipes this list of integers into the List.map function, which applies the FizzBuzz function to each integer. The output of this function call is another list of strings ["1"; "2"; "Fizz"; "4"; "Buzz"; etc.]. This list of strings is piped into the List.reduce function, which in this case uses the sprintf function to concatenate the strings and add a line break after each element, so that it formats correctly.

The List.reduce function applies a function to pairwise elements in a list in order to produce a new element of the same type. Consider the beginning of the list of strings ["1"; "2"; "Fizz"; "4"; "Buzz"; etc.]. The List.reduce function starts with "1" and "2" and applies a function in order to produce a new string from those two strings. That function is the sprintf function, which is similar to the more well-known String.Format method in the BCL. In this case, the template is to take the two strings and insert a line break between them. Thus, when applied to "1" and "2", the result is

1
2

(notice the line break). Now, the List.reduce function takes that string and the next string in the list ("Fizz") and applies the funtion again, giving this result:

1
2
Fizz

It now takes this string and the next value ("4") and applies the sprintf function once more, etc. This is how the final list is being printed.

In a future post I'll walk you through stage 2 of the kata.

Comments

This post serves as an announcement that my latest open source project Hyprlinkr is available. It's a very sharply focused helper library for the ASP.NET Web API, enabling you to create type-safe hyperlinks between resources in a RESTful API.

It's basically a reusable package of the RouteLinker class I previously presented as a spike. The original idea wasn't mine, but José F. Romaniello's - I just took the idea and created a project out of it.

The library is mostly useful if you build level 3 RESTful APIs with the ASP.NET Web API.

It's available as a NuGet package.

Apart from that, I'll refer you to the project site for more information.

Comments

Primitives are also dependencies

There are tons of examples of how Dependency Injection (DI) can be used to decouple clients and services. When the subject is DI, the focus tends to be heavily on the Liskov Substitution Principle (LSP), so most people think about dependencies as polymorphic types (interfaces or abstract base classes). Primitive types like strings and integers tend to be ignored or discouraged. It doesn't help that most DI Containers need extra help to deal with such values.

Primitives are dependencies, too. It doesn't really matter whether or not they are polymorphic. In the end, a dependency is something that the client depends on - hence the name. It doesn't really matter whether the dependency is an interface, a class or a primitive type. In most object-oriented languages, everything is an object - even integers and booleans (although boxing occurs).

There are several ways to inject dependencies into clients. My book describes a set of patterns including Constructor Injection and Property Injection. It's important to keep in mind that ultimately, the reason why Constructor Injection should be your preferred DI pattern has nothing to do with polymorphism. It has to do with protecting the invariants of the class.

Therefore, if the class in question requires a primitive value in order to work, that is a dependency too. Primitive constructor arguments can be mixed with polymorphic arguments. There's really no difference.

Example: a chart reader #

Imagine that you're building a service which provides Top 40 music chart data. There's a ChartController which relies on an IChartReader:

public class ChartController
{
    private readonly IChartReader chartReader;
 
    public ChartController(IChartReader chartReader)
    {
        if (chartReader == null)
            throw new ArgumentNullException("chartReader");
 
        this.chartReader = chartReader;
    }
 
    // ...
}

One implementation of IChartReader is based on a database, so it requires a connection string (a primitive). It also requires a configuration value which establishes the size of the chart:

public class DbChartReader : IChartReader
{
    private readonly int top;
    private readonly string chartConnectionString;
 
    public DbChartReader(int top, string chartConnectionString)
    {
        if (top <= 0)
            throw new ArgumentOutOfRangeException(
                "top",
                "Only positive numbers allowed.");
        if (chartConnectionString == null)
            throw new ArgumentNullException("chartConnectionString");
 
        this.top = top;
        this.chartConnectionString = chartConnectionString;
    }
 
    // ...
}

When top has the value 40, the chart is a Top 40 chart; when the value is 10 it's a Top 10 chart; etc.

Unit testing #

Obviously, a class like DbChartReader is easy to wire up in a unit test:

[Fact]
public void UnitTestingExample()
{
    var sut = new DbChartReader(
        top: 10,
        chartConnectionString: "localhost;foo;bar");
 
    // Act goes here...
    // Assert goes here...
}

Hard-coded composition #

When it's time to bootstrap a complete application, one of the advantages of treating primitives as dependencies is that you have many options for how and where you define those values. At the beginning of an application's lifetime, the best option is often to hard-code some or all of the values. This is as easy to do with primitive dependencies as with polymorphic dependencies:

var controller = new ChartController(
    new DbChartReader(
        top: 40,
        chartConnectionString: "foo"));

This code is part of the application's Composition Root.

Configuration-based composition #

If the time ever comes to move the arms of the the Configuration Complexity Clock towards using the configuration system, that's easy to do too:

var topString = ConfigurationManager.AppSettings["top"];
var top = int.Parse(topString);
 
var chartConnectionString = ConfigurationManager
    .ConnectionStrings["chart"].ConnectionString;
 
var controller = new ChartController(
    new DbChartReader(
        top,
        chartConnectionString));

This is still part of the Composition Root.

Wiring a DI Container with primitives #

Most DI Containers need a little help with primitives. You can configure components with primitives, but you often need to be quite explicit about it. Here's an example of configuring Castle Windsor:

container.Register(Component
    .For<ChartController>());
container.Register(Component
    .For<IChartReader>()
    .ImplementedBy<DbChartReader>()
    .DependsOn(
        Dependency.OnAppSettingsValue("top"),
        Dependency.OnValue<string>(
            ConfigurationManager.ConnectionStrings["chart"]
                .ConnectionString)));

This configures the ChartController type exactly like the previous example, but it's actually more complicated now, and you even lost the feedback from the compiler. That's not good, but you can do better.

Conventions for primitives #

A DI Container like Castle Windsor enables you define your own conventions. How about these conventions?

• If a dependency is a string and it ends with "ConnectionString", the part of the name before "ConnectionString" is the name of an entry in the app.config's connectionStrings element.
• If a dependency is a primitive (e.g. an integer) the name of the constructor argument is the key to the appSettings entry.

Here's how those conventions could be configured with Castle Windsor:

container.Register(Classes
    .FromAssemblyInDirectory(new AssemblyFilter(".")
        .FilterByName(an => an.Name.StartsWith("Ploeh")))
    .Pick()
    .WithServiceAllInterfaces());
 
container.Kernel.Resolver.AddSubResolver(
    new ConnectionStringConventions());
container.Kernel.Resolver.AddSubResolver(
    new AppSettingsConvention());            

The Register call scans all appropriate assemblies in the application's root and registers all components according to the interfaces they implement, while the two sub-resolvers each implement one of the conventions described above.

public class ConnectionStringConventions : ISubDependencyResolver
{
    public bool CanResolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        return dependency.TargetType == typeof(string)
            && dependency.DependencyKey.EndsWith("ConnectionString");
    }
 
    public object Resolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        var name = dependency.DependencyKey.Replace("ConnectionString", "");
        return ConfigurationManager.ConnectionStrings[name].ConnectionString;
    }
}

The CanResolve method ensures that the Resolve method is only invoked for string dependencies with names ending with "ConnectionString". If that's the case, the connection string is simply read from the app.config file according to the name.

public class AppSettingsConvention : ISubDependencyResolver
{
    public bool CanResolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        return dependency.TargetType == typeof(int); // or bool, Guid, etc.
    }
 
    public object Resolve(
        CreationContext context,
        ISubDependencyResolver contextHandlerResolver,
        ComponentModel model,
        DependencyModel dependency)
    {
        var appSettingsKey = dependency.DependencyKey;
        var s = ConfigurationManager.AppSettings[appSettingsKey];
        return Convert.ChangeType(s, dependency.TargetType);
    }
}

This other convention can be used to trigger on primitive dependencies. Since this is a bit of demo code, it only triggers on integers, but I'm sure you'll be able to figure out how to make it trigger on other types as well.

Using convention-based techniques like these can turn a DI Container into a very powerful piece of infrastructure. It just sit there, and it just works, and rarely do you have to touch it. As long as all developers follow the conventions, things just work.

Comments