I've been working with Jeremy D Miller's StructureMap IoC container lately. Over the weekend I also did some research on Castle Windsor, in order to properly offer an opinion to Krysztof Kozmic about whether he should remove nested containers from it. Previous to all this, the only IoC I'd really had the opportunity to use on a real project was Autofac. Maybe it's just the bias of first exposure, but I still like Autofac the best. But I can point to one big tangible reason that I like Autofac better, which the other two options don't have: lifetime scopes.
Lifetime scopes are the primary unit of context in Autofac. When the framework has to resolve a dependency, it doesn't pull it from a container. Rather, it pulls it from a lifetime scope. A lifetime scope is different from a container in that you can't change or query its configuration at runtime. It is a purely resolution-oriented interface. But more importantly, in concert with a few other features, it offers a very convenient way to establish what I call "resolution contexts" within your application.
A resolution context is a "bubble" within your application with some or all of its own resolution rules. You can dictate that within a given bubble, certain objects are guaranteed unique, and will be cleaned up when the context is disposed of. Or you can ensure that dependencies are resolvable inside the bubble, but not outside of it. By nesting bubbles that do this, you can enforce inter-layer dependency rules ensuring, for example, that the domain services layer does not depend on the UI layer. You can see the obvious reason that the word "scope" is used in the name: scoping is their explicit purpose.
The features that combine with lifetime scope to make it a truly powerful idiom are tags, and tagged registration. You can tag a lifetime scope with an object that identifies that scope. This value can also be specified in a given type registration to mark it as being unique within a scope having that tag.
Combined, these features allow you to tie instances of objects to a scope, and ensure that anything resolved within that scope gets a particular object. This can be particularly useful for managing things such as database transactions, or domain units of work. Or really any transaction-like operation consisting of a set of sub-operations involving resources which need only, or should only, exist for the duration of the parent operation.
Without this feature, it falls to the developer to explicitly write an object whose responsibility is to manage the lifetime or reach of these things, and another whose responsibility it is to generate and dispose of these scoping objects. This is essentially simple, but for an inexperienced dev it is a tough problem. For an experienced dev, it's annoyingly repetitive.
The thing I love about Autofac is that it supports tagged scopes which make it trivially easy to solve this problem. The thing I hate about Autofac is that you can't define as part of the registration process where you want these nested lifetime scopes to be generated. Instead, you must use a hook, a direct framework reference, a relationship type or some other extension mechanism, to dictate when nested lifetime scopes are introduced.
The reason I have these conflicting feelings is that Autofac approaches, but falls short of what I have come to consider the ideal role of an IoC container: to be the composer of the application. The use to which most of us put IoC containers today diverges from the implications of the name "container". While they do serve as containers in the sense that they hang onto shared (singleton) instances of objects, they also fill a factory role. And with Autofac's nested lifetime scopes, they can draw boundaries around self-contained operations and object graphs. Windsor prides itself on powerful, flexible "release" mechanisms, for cleaning up objects that are no longer needed. Again, this goes far beyond the role of a "container".
Yet despite this, the configuration API of most IoC frameworks still is highly container-oriented. We "register" types and interfaces, attaching bits of metadata here and there as needed. These are "stored" in the container, which can be queried or overridden at run-time by adding or removing registrations from the container.
But an IoC is at its most effective when it is acting not as a container, but as a rules engine, to regulate when new objects are created, and when existing ones are reused. Or how one object is chosen from many that implement a common interface. Or when and how object graphs are disposed of and resources cleaned up. These roles all build toward an overarching theme of "composing" the application. That is to say, building it up out of different blocks of functionality, fitting them together according to a pattern which is clearest when viewed as a whole.
This is why the ServiceLocator block from Microsoft makes me angry. While you can still configure the composition of your application using your IoC of choice, ServiceLocator exposes it as if it were a simple abstract-to-concrete mapping operation. And beyond that, the base idea of a centralized "service locator" completely undermines the goal of IoC in the first place, which is to invert the control. Don't ask for what you think you want. Just expose the dependency declaratively, and let it be provided to you as deemed appropriate by someone whose whole job is to know what you need.
I say we should embrace the composer idiom. I would love to see an IoC framework that took this role to heart, and wore it on its sleeve, so to speak, by expressing it naturally in its API. Autofac comes closest of any container I've used or researched, but it still shows some artifacts of its container heritage. One big improvement that could be made is a declarative configuration-time way of establishing when new scopes should be spun up. I posted a gist to this effect over the weekend and shared it with Krysztof Kozmic, hoping he'd take some inspiration from it for Windsor. But in truth, Windsor also still has a lot of other aspects that are tied to its identity as a "container".
Maybe it's time for someone to take another crack at the IoC problem. The ideal functionality is mostly all available out there. All that's lacking is an API that makes its true role obvious and natural.