When do you use dependency injection?

Question

I've been using StructureMap recently and have enjoyed the experience thoroughly. However, I can see how one can easily get carried away with interfacing everything out and end up with classes that take in a boatload of interfaces into their constructors. Even though that really isn't a huge problem when you're using a dependency injection framework, it still feels that there are certain properties that really don't need to be interfaced out just for the sake of interfacing them.

Where do you draw the line on what to interface out vs just adding a property to the class?

Answer 1

The main problem with dependency injection is that, while it gives the appearance of a loosely coupled architecture, it really doesn't.

What you're really doing is moving that coupling from the compile time to the runtime, but still if class A needs some interface B to work, an instance of a class which implements interface B needs still to be provided.

Dependency injection should only be used for the parts of the application that need to be changed dynamically without recompiling the base code.

Uses that I've seen useful for an Inversion of Control pattern:

• A plugin architecture. So by making the right entry points you can define the contract for the service that must be provided.
• Workflow-like architecture. Where you can connect several components dynamically connecting the output of a component to the input of another one.
• Per-client application. Let's say you have various clients which pays for a set of "features" of your project. By using dependency injection you can easily provide just the core components and some "added" components which provide just the features the client have paid.
• Translation. Although this is not usually done for translation purposes, you can "inject" different language files as needed by the application. That includes RTL or LTR user interfaces as needed.