People who have to listen to me talk about programming know that I’m a big proponent of Inversion of Control (IoC) containers and what they can do to clean up your code. Most people get introduced to IoC containers via Dependency Injection (DI). DI is another great way to clean up your code. It’s hard to argue against it, really. It decouples your code in a big way. Not only does this make it more testable, but because it aids in separating concerns/responsibilities, this also makes it easier for you to track down bugs when they do show up. But people rightly point out that you don’t need an IoC container to use DI and get these benefits.
I’m usually both happy and sad to hear that argument. On the positive side, it means that people are acknowledging the benefits of DI, which is great. The more people are using DI, the less god classes full of spaghetti code there are out there for me to unearth in future maintenance efforts. Another reason I’m happy to hear that argument is because it means that people aren’t confusing the means for the end. DI is a good thing, for the reasons I established above, not because, as some people seem to think, IoC containers are good and DI is what IoC containers do.
That last sentence there leads into the reason that I’m sad to hear people dismiss IoC containers as being unnecessary for DI. The problem is, DI isn’t the only benefit of IoC containers. DI isn’t what IoC containers do. IoC containers, as their name would indicate, invert control. They take a number of concerns that have traditionally been assigned to the class under consideration, and extract them out to the context in which that class is used. That context may be the immediate consumers of the class, or it may be coordinating code, infrastructure, data access, or any number of other locations in the application. But the point is that the responsibilities are removed from the class itself and given to other classes whose business it is to know what should be created, when, with what initialization, and how it should be disposed of.
A good IoC container does more than just wire up constructor dependencies. It goes beyond that and lives up to the breadth of this definition of IoC. And that is why I laud and evangelize the glories of IoC containers.
Lets take a look at one particular container that I’ve come to know and love: Autofac. It’s an amazing framework that offers an answer to nearly everything that the principles of IoC ask of it. Autofac has features for lifecycle control, object ownership, and deterministic disposal. It has features for nested scoping, contextual dependency resolution, and varied construction mechanisms. These are all concerns that are a function not of the consumer of a dependency, but of the cloud of code and functionality that surrounds it, of the nature and design of your application as a composition. And Autofac gives you ways to deal with them on those terms.
Autofac boasts strong support for robust lifecycle control. In the project wiki you’ll find it laid out under the topic “Deterministic Disposal”, but it’s about creation as much as it’s about disposal. When you register a module with an Autofac container, you have the opportunity to specify a scope strategy. This strategy will determine whether a new instance is created upon the request, or whether an existing one is pulled from the container. Furthermore, it will also determine when references to the instance or instances are released and Dispose called on IDisposables. I’ll be doing some explaining, but if you want to do your own reading on the available strategies, you can do so here: http://code.google.com/p/autofac/wiki/InstanceScope
The two simple lifetime scopes that everyone tends to be conceptually familiar with are found n Autofac’s SingleInstance and InstancePerDependency scopes. The former is roughly equivalent to the function of a singleton pattern implementation, while the latter corresponds to a factory pattern implementation. Autofac goes well beyond this, however, and gives you two more scoping strategies that let you manage creation and disposal of your components in a more nuanced and more powerful way.
Both of the two more nuanced scope strategies depend on Autofac’s support for “nested containers”. A nested container is essentially a scoping mechanism, similar to a method or class definition, or a using block. Nested containers are useful for establishing the architectural layer boundaries of your application. For example, in a desktop application you may have many windows coming in and out of existence, all operating on the same domain objects, persisting them via the same handful of repository classes. These windows may be created in nested container contexts that are spun up as needed, and disposed when the windows are closed. Some objects will be created new each time this happens, while others are unique and shared across the entire UI. The nested container is what allows Autofac to make the appropriate distinction.
Imagine you are writing a file diff’ing application. You have to show two documents at once in side-by-side windows. They are the same thing, in terms of functionality and data, and so could just be two different instances of the same object.... But they will share some dependencies, and have references to their very own copies of certain others.
Lets pick out a few pieces of this puzzle and tie them to Autofac’s features. You will probably have a file access service that allows you to operate on the files that you have loaded. There’s no reason to have more than a single copy of this in the entire app, so it can effectively be a singleton. The way you would express this to Autofac is via the registry.
Given the class and interface definition:
You would register the component as a singleton like this:
The run-time behavior you would see based on this registration is that only one instance of the FileAccess component will ever be produced by the container. Subsequent requests will just return the one that’s already constructed.
The next layer on top of that is the UI. You decide that you may want to be able to have multiple comparison windows open at once, without having to run separate instances of the app. But each of those windows is still essentially a full instance of your apps interface. They’re not exactly singletons, but they should be unique within their separate stacks. Whatever sub-context they are in, there should be only one.
Given the class and interface definition:
You would register the component using the InstancePerMatchingLifetimeScope strategy, like this:
One weakness of Autofac for this use case is that in order to establish the proper resolution contexts, you have to refer directly to the container. In this case it’s probably okay, since you can be fairly certain you’ll only ever need a Left context and a Right context. So you can probably create the lifetime scopes up front, store them away somewhere, and explicitly resolve these objects from the separate contexts when needed. A sample setup for this can be seen below.
Then you’d need to make sure that the LeftContainer and RightContainer were used explicitly to resolve the left and right ComparisonWindow components.
This works for us in this situation. But it’s not at all difficult to imagine a scenario, maybe even in this same app, where the contexts aren’t predetermined and static. For example, you may want have a worker thread pool, where each thread has its own resolution context. In fact this is a situation addressed explicitly in the Autofac wiki. Even there, it seems to be accepted that an explicit container reference in the thread pool is necessary. It doesn’t seem like there’s a great solution to this challenge at this time, though I will surely be keeping an eye out for one. This is messy, concern-leaking infrastructure code that I would really prefer not to have to write.
There’s another scoping type that’s related to this one. In fact it’s use is a bit simpler. This is the InstancePerLifetimeScope strategy. Note the subtle lack of the “Matching” adjective in the name. What this indicates is that the context is implied rather than explicit. The behavior specified by this strategy is that at most one instance will be created within the resolution context where the resolution happens, at whatever level of nesting that happens to be. Functionally, this differs from InstancePerMatchingLifetimeScope in that it doesn’t search the context stack for one particular context in which to do the resolution.
This strategy can be effective when you have a set architecture with a trivial tree structure. Well-defined layering is crucial. All the leaf nodes of your context tree need to be at the same depth in order to be certain when a new instance will be created and when not. For example, a website where you have a base layer for the whole web app, and a leaf layer nested within for each individual web requests. In our diffing app, if we can be certain that we need no more deeply nested containers beyond our LeftContainer and our RightContainer, and all significant service resolution will happen at those layers, then we may have a use for this strategy for the dependencies of our Left and Right windows and controllers
The registration for this strategy is very similar to the others. Given a class and interface definition such as this:
The registration would look like this:
The final scoping strategy is InstancePerDependency. As noted before, the behavior is roughly equivalent to an implementation of a factory pattern. Every resolution request for a service registered as InstancePerDependency will result in the creation of a new instance. In our diffing app, we may find use for this strategy with something like an alert dialog. There’s no need to keep an alert around when it’s not being shown, and in fact it should almost certainly *not* carry any state from one alert to the next.
So given a class and interface such as this:
The registration would look like this:
That covers most all of Autofac’s lifecycle control functionality. There are four strategies available: SingleInstance which approximates Singleton, InstancePerDependency which approximates Factory, and the more subtle and, honestly, difficult to use, InstancePerLifetimeScope and InstancePerMatchingLifetimeScope which are heavily contextual. I really wish that these last two were more manageable and directable. If they were, I think that Autofac could claim to easily address most any lifecycle control need with very little overhead and requiring few concessions to the framework. This would be a very noble goal. But as it is, their behavior will tend to be circumstantial rather than controlled and intentional. And the only hope of improving that situation lies in taking great pains to organize your design to account for the container’s shortcomings and then go on to break the rule of not referencing the container.
Despite these shortcomings, I believe there are many benefits to be found in relying on Autofac for lifecycle control where it’s possible and not overly problematic. Certainly I’ve saved myself some headaches in doing so. And we haven’t even begun to explore the dependency relationship patterns that Autofac supports out of the box. We'll dive into to those next time!