Dependency Injection (DI) is a design pattern that strive to reduce the dependency between components. Dependency Injection is often referred as Inversion of Control (IoC). In fact dependency injection is an application of the Inversion of Control principle.

A class that depends from an other object, receive the reference of the dependent object from the outside world instead of create the instance itself.

In the example above, the engine implementation is manually injected into the class car by means of the constructor.

The dependency injection is tipically accomplished using ad-och framework. For Microsoft .Net you can use Microsoft's Unity Application Block that is part of the Enterprise Library 4.0.