.net - Dependency Injection and IDisposable

Question

I'm a little bit confused about Dispose() methods in IDisposable implementations with Autofac usage

Say I have a certain depth to my objects:

• Controller depends on IManager;
• Manager depends on IRepository;
• Repository depends on ISession;
• ISession is IDisposable.

This leads to the following object graph:

new Controller(
    new Manager(
        new Repository(
            new Session())));

Do I need to make my Manager and Repository implement IDisposable as well and call Manager.Dispose() in Controller, Repository.Dispose() in Manager, etc. or will Autofac automagically know which objects in my call stack properly need to be disposed? Controller object is already IDisposable as it derives from base ASP.NET Web API controller

Answer 1

The general rule of resources is that:

He who owns the resource is responsible of disposing of it.

This means that if a class owns a resource, it should either dispose of it in the same method that it created it in (in which case the disposable is called an ephemeral disposable), or if that’s not possible, this generally means that the owning class must implement IDisposable, so it can dispose of the resource in its Dispose method.

But it is important to note that, in general, a class should only own a resource if it is responsible of its creation. But when a resource is injected, this means that this resource already existed before the consumer did. The consumer didn't create the resource and should in that case not dispose of it. Although you can pass on the ownership of the resource to the consumer (and communicate this in the class’s documentation that ownership is passed on), in general you should not pass on the ownership, because this complicates your code and makes the application very fragile.

Although the strategy of transferring ownership of objects might make sense in some cases, for instance for types that are part of a reusable API (like System.IO.StreamReader), it is always bad when dealing with components that are part of your object graph (our so called injectables). I’ll explain why below.

So even if your Controller depends on dependencies that need to be disposed of, your controller should not dispose of them:

• Because the consumer did not create such dependency, it has no idea what the expected lifetime of its dependency is. It could be very well that the dependency should outlive the consumer. Letting the consumer dispose of that dependency will in that case cause a bug in your application, because the next controller will get an already disposed of dependency, and this will cause an ObjectDisposedException to be thrown.
• Even if the lifestyle of the dependency equals that of the consumer, disposing is still a bad idea, because this prevents you from easily replacing that component for one that might have a longer lifetime in the future. Once you replace that component for a longer-lived component, you will have to go through all it consumers, possibly causing sweeping changes throughout the application. In other words, you will be violating the Open/closed principle by doing that–it should be possible to add or replace functionality without making sweeping changes instead.
• If your consumer is able to dispose of its dependency, this means that you implement IDisposable on that abstraction. This means this abstraction is leaking implementation details–that’s a violation of the Dependency Inversion Principle. You are leaking implementation details when implementing IDisposable on an abstraction, because it is very unlikely that every implementation of that abstraction needs deterministic disposal and so you defined the abstraction with a certain implementation in mind. The consumer should not have to know anything about the implementation, whether it needs deterministic disposal or not.
• Letting that abstraction implement IDisposable also causes you to violate the Interface Segregation Principle, because the abstraction now contains an extra method (i.e. Dispose), that not all consumers need to call. They might not need to call it, because –as I already mentioned– the resource might outlive the consumer. Letting it implement IDisposable in that case is dangerous, because anyone can call Dispose on it causing the application to break. If you are more strict about testing, this also means you will have to test a consumer for not calling the Dispose method. This will cause extra test code. This is code that needs to be written and maintained.

So instead, you should only let the implementation implement IDisposable. This frees any consumer of the abstraction from the doubts whether it should or shouldn't call Dispose (because there is no Dispose method to call on the abstraction).

Because only the implementation implements IDisposable and only your Composition Root creates this implementation, it is the Composition Root that is responsible of its disposal. In case your DI container creates this resource, it should also dispose it. Autofac will actually do this for you. You can easily verify this. In case you wire your object graphs without the use of a DI Container (a.k.a. Pure DI), it means that you will have to dispose of those objects in your Composition Root yourself.

Note that this design change makes your code simpler. Implementing IDisposable on the abstraction and letting consumers dispose of their dependencies will cause IDisposable to spread out through your system like a virus and pollutes your code base. It spreads out, because for any abstraction, you can always think of an implementation that needs to clean up its resources, so you will have to implement IDisposable on every abstraction. This means that every implementation that takes one or more dependencies will also has to implement IDisposable as well, and this cascades up the object graph. This adds a lot of code and unnecessary complexity to each class in your system.