Depenency Injection, or DI, is a Software Architecture Design Pattern. DI is something that comes up during discussions on SOLID, IoC (Inversion of Control), testing, and refactoring. I want to speak on each of these briefly because DI touches all of these. But before I really dive into things, I want to define what a dependency is. A dependency is any object that another object requires. So all of those classes, services, and libraries that we use to build our applications are dependencies.