dependency injection - Why not use an IoC container to resolve dependencies for entities/business objects?

Question

I understand the concept behind DI, but I'm just learning what different IoC containers can do. It seems that most people advocate using IoC containers to wire up stateless services, but what about using them for stateful objects like entities?

Whether it's right or wrong, I normally stuff my entities with behavior, even if that behavior requires an outside class. Example:

public class Order : IOrder
{

    private string _ShipAddress;
    private IShipQuoter _ShipQuoter;

    public Order(IOrderData OrderData, IShipQuoter ShipQuoter)
    {
        // OrderData comes from a repository and has the data needed 
        // to construct order
        _ShipAddress = OrderData.ShipAddress;  // etc.
        _ShipQuoter = ShipQuoter;

    }

    private decimal GetShippingRate()
    {
        return _ShipQuoter.GetRate(this);
    }
}

As you can see, the dependencies are Constructor Injected. Now for a couple of questions.

1. Is it considered bad practice to have your entities depend on outside classes such as the ShipQuoter? Eliminating these dependencies seems to lead me towards an anemic domain, if I understand the definition correctly.

2. Is it bad practice to use an IoC container to resolve these dependencies and construct an entity when needed? Is it possible to do this?

Thanks for any insight.

Answer 1

The first question is the most difficult to answer. Is it bad practice to have Entities depend on outside classes? It's certainly not the most common thing to do.

If, for example, you inject a Repository into your Entities you effectively have an implementation of the Active Record pattern. Some people like this pattern for the convenience it provides, while others (like me) consider it a code smell or anti-pattern because it violates the Single Responsibility Principle (SRP).

You could argue that injecting other dependencies into Entities would pull you in the same direction (away from SRP). On the other hand you are certainly correct that if you don't do this, the pull is towards an Anemic Domain Model.

I struggled with all of this for a long time until I came across Greg Young's (abandonded) paper on DDDD where he explains why the stereotypical n-tier/n-layer architecture will always be CRUDy (and thus rather anemic).

Moving our focus to modeling Domain objects as Commands and Events instead of Nouns seems to enable us to build a proper object-oriented domain model.

The second question is easier to answer. You can always use an Abstract Factory to create instances at run-time. With Castle Windsor you can even use the Typed Factory Facility, relieving you of the burden of implementing the factories manually.