On NetTuts.com today there's a new tutorial introducing dependency injection and how it compares to other injection types (like setter and constructor). It shows how to apply the "Inversion of Control" methodology to create a container that can be reused across the entire application.

Chances are, at some point in your learning, you’ve come across the term, “dependency injection.” If you’re still relatively early in your learning, you likely formed a confused expression and skipped over that part. Still, this is an important aspect of writing maintainable (and testable) code. In this article, I’ll explain it in as simple a way as I’m capable of.

He starts with an example of setter/constructor injection and talks about how they can make it difficult to use the class as things get more complex. His solution is to create a simple DIC (dependency injection container) that stores instances of objects or closures that can be extracted an reused.