Dependency Injection - Basics

Basics

Without dependency injection, a consumer component that needs a particular service in order to accomplish a task must create an instance of a class that concretely implements the dependency interface.

When using dependency injection, a consumer component specifies the service contract by interface, and the injector component selects an implementation on behalf of the dependent component.

In its simplest implementation, code that creates a dependent object supplies dependencies to that object via constructor arguments or by setting properties on the object.

More complicated implementations, such as Spring, Google Guice, and Microsoft Managed Extensibility Framework (MEF), automate this procedure. These frameworks identify constructor arguments or properties on the objects being created as requests for dependent objects, and automatically inject constructor arguments or set properties with pre-constructed instances of dependencies as part of the process of creating the dependent object. The client makes a request to the dependency injection system for an implementation of a particular interface; the dependency injection system creates the object, automatically filling in dependencies as required.