Dependency Injection is a software design pattern in which one or more dependencies (or services) are injected, or passed by reference, into a dependent object (or client) and are made part of the client's state. The pattern separates the creation of a client's dependencies from its own behavior, which allows program designs to be loosely coupled and to follow the inversion of control and single responsibility principles.

Use the Dependency Injection pattern when

• when you need to remove knowledge of concrete implementation from object
• to enable unit testing of classes in isolation using mock objects or stubs