Rich domain objects with DivineInject

DivineInject is a .net dependency injection framework, designed to be simple to use and easy to understand.

You can find the source for DivineInject on github. In this second part in the series we’ll look at creating rich domain objects, the first part covers getting started with Divine Inject.

Simple Domain Objects

As an example, imagine I have a simple shopping basket in my application. The shopping basket is encapsulated by a Basket object, for which the interface looks like this:

interface IBasket
{
    IList<IBasketItem> GetBasketContents();
    void AddItemToBasket(IBasketItem item);
}

The contents of the basket are actually backed by a service to provide persistence:

interface IBasketService
{
    IList<IBasketItem> GetBasketContents(Guid basketId);
    void AddItemToBasket(Guid basketId, IBasketItem item);
}

I can create a very simple implementation of my Basket:

class Basket : IBasket
{
    private readonly IBasketService _basketService;
    private readonly Guid _basketId = Guid.NewGuid();

    public Basket(IBasketService basketService)
    {
        _basketService = basketService;
    }

    public IList<IBasketItem> GetBasketContents()
    {
        return _basketService.GetBasketContents(_basketId);
    }

    public void AddItemToBasket(IBasketItem item)
    {
        _basketService.AddItemToBasket(_basketId, item);
    }
}

Since I’m using dependency injection, IBasketService sounds like a dependency, so how do I go about creating an instance of Basket? I don’t want to create it myself, I need the DI framework to create it for me, passing in dependencies.

I want to do something with how the Basket is created, so let’s start with a simple interface for creating baskets:

interface IBasketFactory
{
    IBasket Create();
}

When I’m creating a basket I don’t care about IBasketService or other dependencies; calling code just wants to be able to create a new, empty basket on demand. How would we implement this interface? Well, I could do the following – although I shouldn’t.

class BadBasketFactory : IBasketFactory
{
    public IBasket Create()
    {
        // DON'T DO THIS - just an example
        return new Basket(
            DivineInjector.Current.Get<IBasketService>());
    }
}

Now I’d never suggest actually doing this, explicitly calling the dependency injector. The last thing I want from my DI framework is to have references to it smeared all over the application. However, what this class does is basically what I want to happen.

DivineInject however can generate a class like this for you; this is configured at the same time you define the rest of your bindings:

DivineInjector.Current
    .Bind<IBasketFactory>().AsGeneratedFactoryFor<Basket>();

This generates an IBasketFactory implementation, which can create new IBasket implementations on demand (they will all actually be instances of Basket); all without having references to the DI framework smeared across my code. If I want to use the IBasketFactory, for example from my Session class, I declare it as a constructor arg the same as I would any other dependency:

public Session(IAuthenticationService authService, 
               IBasketFactory basketFactory)
{
    _authService = authService;
    _basketFactory = basketFactory;
}

The DI framework takes care of sorting out dependencies and I get a Session class with no references to DivineInject. When I need a new basket I just call _basketFactory.Create(). Since I have nice interfaces everywhere, everything is easy to mock so I can TDD everything.

Rich Domain Objects

Now what happens as my domain object becomes more complex? Say, for example, I want to be able to pass in some extra arguments to my constructor. Returning to our basket example: as well as starting a new, empty basket – isn’t there a possibility that I want to continue using an existing basket? E.g. in case of load balanced servers or fail-over. What do I do then?

I start by changing Basket, to allow me to pass in an existing basket id:

private readonly IBasketService _basketService;
private readonly Guid _basketId;

public Basket(IBasketService basketService)
{
    _basketService = basketService;
    _basketId = Guid.NewGuid();
}

public Basket(IBasketService basketService, Guid basketId)
{
    _basketService = basketService;
    _basketId = basketId;
}

I now have two constructors, one of which accepts a basket id. Since all Basket instances are created by an IBasketFactory, I need to change the factory interface, too:

interface IBasketFactory
{
    IBasket Create();
    IBasket UseExisting(Guid id);
}

I now have a new method on my IBasketFactory, if I was hand-coding the factory class I’d expect this second method to call the second constructor, passing in the basket id.

What do we need to tell DivineInject to make it generate this more complex IBasketFactory implementation? Nothing! That’s right, absolutely nothing – DivineInject will already generate a suitable IBasketFactory. Our original declaration above, is still sufficient:

DivineInjector.Current
    .Bind<IBasketFactory>().AsGeneratedFactoryFor<Basket>();

This generates an IBasketFactory implementation, returning a Basket instance for each method it finds on the interface. Since one of these methods takes a Guid, it tries to find a matching constructor which also takes a Guid, plus any dependencies it knows about. DivineInject can automatically wire up the right factory method to the right constructor, using the arguments it finds in each. Now, when a session wants to re-use an existing basket it just calls:

_basketFactory.UseExisting(existingBasketId)

This creates a new Basket instance, with dependencies wired up, passing in the basket id. Everything is still using interfaces so all your collaborations can be unit tested. Behind the scenes DivineInject generates the IL code to implement your factory interfaces, leaving you free to worry about your design.

By following this pattern we can create rich domain objects that include both state and dependencies: it becomes possible to create stateful objects that have behaviours (methods) that make sense in the domain. Successfully modelling your domain is critical to creating code that’s easy to understand and easy to change. DivineInject helps you model your domain better.

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Getting started with DivineInject

DivineInject is a .net dependency injection framework, designed to be simple to use and easy to understand.

You can find the source for DivineInject on github.

Why another DI framework?

Because dependency injection is important – but done wrong it can do more harm than good. DivineInject is opinionated about the right way to use dependency injection:

  • Constructor injection or death
    Setter injection is bad for your health, so just say no
  • Dependencies are singletons
    Dependencies are external to your application – your DI framework doesn’t need to know about users or sessions or threads.
  • Domain objects can be rich, too
    Your domain model doesn’t have to be anemic

Constructor Injection

Setter and method injection are much harder to get right – so DivineInject simply doesn’t support them. If you can’t implement your dependencies as constructor arguments, then maybe you should refactor the dependency so you can.

Singleton Dependencies

Dependencies are external to your application. They are external things that your application depends on like databases and web services; these things are generally used across your application. Hint: if a dependency is only used in one or two places, it isn’t an application-wide dependency.

Things like users and sessions are domain concepts in your domain, not in the domain of dependency injectors. All dependency injection frameworks get wrapped up in different scopes, which makes the frameworks harder to use. DivineInject simply doesn’t support them — if you need something user-scoped or session-scoped, then implement the logic yourself. It isn’t hard, and if you ever want to understand the lifecycle of your objects it’s in your code, not mine — which will make reasoning about your code or debugging it a million times easier.

Rich Domain Objects

DivineInject borrows an idea from Google Guice – with Guice it is called “assisted injection”, in DivineInject we call it generated factory injection. The idea is the same — providing a simple way to create objects with constructors which accept runtime arguments as well as dependencies to inject. This allows you to create rich, stateful domain objects which also have dependencies.

Getting Started

So how do you get started with DivineInject? I’ll assume you’ve not been living under a rock and already know what dependency injection is. In which case basic usage of DivineInject boils down to three steps:

  • Add the dependency
  • Configure bindings
  • Create your root object

Add the Dependency

DivineInject is available as a NuGet package:

Install-Package DivineInject

Configure Bindings

When DivineInject creates a new instance of a class it calls a constructor, each of the constructor arguments is a dependency of the class — something external to the class, for which an implementation must be provided. But how do we know which value to pass for each dependency? This is controlled by the bindings.

Your bindings must be configured near the start of your application — e.g. in the main method or global.asax.

There are basically two ways to configure bindings with DivineInject.

1) bind an interface to a concrete type. DivineInject will pass the same (singleton) instance of the given concrete type whenever it encounters a constructor argument of the interface type.

DivineInjector.Current
	.Bind<IOrdersService>().To<OrdersService>();

2) bind an interface to a specific instance. DivineInject will pass the given instance whenever it encounters a constructor argument of the interface type.

var myOrdersService = new OrdersService(...);
DivineInjector.Current
	.Bind<IOrdersService>().ToInstance(myOrdersService);

Create Your Root Object

DivineInject allows you to create a tree of objects — each object has references to dependencies, which in turn reference their own dependencies; forming a tree of objects. This tree is created starting with the root object — e.g. in a WPF application the root would be the outermost ViewModel; in a WCF application it would be the service class.

The root object is created by calling DivineInject — any arguments the root object constructor requires are taken from the bindings. This should be the only time you explicitly call DivineInject to create objects.

There are two ways of creating the root object:

1) by explicit type:

private MainWindowViewModel CreateViewModel()
{
    return DivineInjector.Current.Get<MainWindowViewModel>();
}

2) by passing the type as an argument:

public object GetInstance(InstanceContext instanceContext, 
                          Message message)
{
    return DivineInjector.Current.Get(_instanceType);
}

At this point you now have a very simple set of dependencies configured, which can be wired up by DivineInject. E.g.

public class MainWindowViewModel
{
    public MainWindowViewModel(IOrdersService ordersService)
    {
        ...
    }
}

I can define classes which accept an instance of an interface as a constructor argument, when DivineInject instantiates this class it will pass in the right implementation of the interface, the one configured by the bindings.

In the second part of this series, we’ll look at how we use DivineInject to create classes that aren’t singletons.

Your DI framework is killing your code

I read a really interesting post recently looking at the difference between typical OO code and a more functional style. There’s a lot to be said for the functional style of coding, even in OO languages like Java and C#. The biggest downside I find is always one of code organisation: OO gives you a discoverable way of organising large amounts of code. While in a functional style you might have less code, but it’s harder to organise it clearly.

It’s not Mike’s conclusion that I want to challenge, it’s his starting point: he starts out with what he describes as “typical object oriented C# code”. Unfortunately I think he’s bang on: even in this toy example, it is exactly like almost all so-called “object oriented” code I see in the wild. My issue with code like this is: it isn’t in the least bit object oriented. It’s procedural code haphazardly organised into classes. Just cos you’ve got classes, don’t make it OO.

What do I mean procedural code? The typical pattern I see is a codebase made up of two types of classes:

  1. Value objects, holding all the data with no business logic
    Extra credit here if it’s an object from your persistence layer, nhibernate or the like.
  2. Classes with one or two public functions –  they act on the value objects with no state of their own
    These are almost always “noun-verbers”

A noun-verber is the sure-fire smell of poor OO code: OrderProcessor, AccountManager, PriceCalculator. No, calling it an OrderService doesn’t make it any better. Its still a noun-verber you’re hiding by the meaningless word “service”. A noun-verber means its all function and no state, it’s acting on somebody else’s state. It’s almost certainly a sign of feature envy.

The other design smell with these noun-verbers is they’re almost always singletons. Oh you might not realise they’re singletons, because you’ve cleverly hidden that behind your dependency injection framework: but it’s still a singleton. If there’s no state on it, it might as well be a singleton. It’s a static method in all but name. Oh sure its more testable than if you’d actually used the word “static”. But it’s still a static method. If you’d not lied to yourself with your DI framework and written this as a static method, you’d recoil in horror. But because you’ve tarted it up and called it a “dependency”, you think it’s ok. Well it isn’t. It’s still crap code. What you’ve got is procedures, arbitrarily grouped into classes you laughably call “dependencies”. It sure as hell isn’t OO.

One of the properties of good OO design is that code that operates on data is located close to the data. The way the data is actually represented can be hidden behind behaviours. You focus on the behaviour of objects, not the representation of the data. This allows you to model the domain in terms of nouns that have behaviours. A good OO design should include classes that correspond to nouns in the domain, with behaviours that are verbs that act on those nouns: Order.SubmitForPicking(). UserAccount.UpdatePostalAddress(), Basket.CalculatePriceIncludingTaxes().

These are words that someone familiar with the domain but not software would still understand. Does your customer know what an OrderPriceStrategyFactory is for? No, then it’s not a real thing. Its some bullshit you made up.

The unloved value objects are, ironically, where the real answer is to the design problem. These are almost always actual nouns in the domain. They just lack any behaviours which would make them useful. Back to Mike’s example: he has a Customer class – its public interface is just an email address property, a classic value object: all state and no behaviour [if you want to play along at home, I’ve copied Mike’s code into a git repo; as well as my refactored version].

Customer sounds like a good noun in this domain. I bet the customer would know what a Customer was. If only there were some behaviours this domain object could have. What do Customers do in Mike’s world? Well, this example is all about creating and sending a report. A report is made for a single customer, so I think we could ask the Customer to create its own report. By moving the method from ReportBuilder onto Customer, it is right next to the data it operates on. Suddenly the public email property can be hidden – well this seems a useful change, if we change how we contact customers then only the customer needs to change, not also the ReportBuilder. It’s almost like a single change in the design should be contained within a single class. Maybe someone should write a principle about this single responsibility malarkey…

By following a pattern like this, moving methods from noun-verbers onto the nouns on which they operate, we end up with a clearer design. A Customer can CreateReport(), a Report can RenderAsEmail(), and an Email can Send(). In a domain like this, these seem like obvious nouns and obvious behaviours for these nouns to have. They are all meaningful outside of the made up world of software. The design models the domain and it should be clear how the domain model must change in response to each new requirement – since each will represent a change in our understanding of the domain.

So why is this pattern so uncommon? I blame the IoC frameworks. No seriously, they’re completely evil. The first thing I hit when refactoring Mike’s example, even using poor man’s DI, was that my domain objects needed dependencies. Because I’ve now moved the functionality to email a report from ReportingService onto the Report domain object, my domain object now needs to know about Emailer. In the original design it could be injected in, but with an object that’s new’d up, how can I inject the dependency? I either need to pass Emailer into my Report at construction time or on sending as email. When refactoring this I opted to pass in the dependency when it was used, but only because passing in at construction time is cumbersome without support.

Almost all DI frameworks make this a royal pain. If I want to inject dependencies into a class that also has state (like the details of the customer’s report), it’s basically impossible, so nobody does it. It’s better, simpler, quicker to just pull report creation onto a ReportBuilder and leave Customer alone. But it’s wrong. Customer deserves to have some functionality. He wants to be useful. He’s tired of just being a repository for values. If only there was a way of injecting dependencies into your nouns that wasn’t completely bullshit.

For example using NInject – pretty typical of DI frameworks – you can create a domain object that requires injected dependencies and state by string typing. Seriously? In the 21st century, you want me to abandon type safety, and put the names of parameters into strings. No. Just say no.

No wonder when faced with these compromises people settle for noun-verbers. The alternatives are absolutely horrific. The only DI framework I’ve seen which lets you do this properly is Guice‘s assisted injection. Everybody else, as far as I can see, is just plain wrong.

Is your DI framework killing your code? Almost certainly: if you’ve got value objects and noun-verbers, your design is rubbish. And it’s your DI framework’s fault for making a better design too hard.