Dependency Injection (DI) is a design pattern in object-oriented programming that minimizes the interactions between objects by having them receive their dependencies from an external source rather than creating them internally. This programming technique separates the concerns of constructing objects and using them, leading to loosely coupled programs. DI makes a class independent of its dependencies by decoupling the usage of an object from its creation.
Instead of hardcoding dependencies, they are injected through mechanisms like class constructors or public properties. In this scenario, a class gets passed into another class via a parameter, rather than the class creating the object itself. The result is code that is more readable, maintainable, testable, reusable, and flexible than tightly coupled code. DI also supports the dependency inversion principle by injecting dependencies into the class definitions instead of hardcoding them.
There are three main types of DI: Constructor Injection, Setter Injection, and Interface Injection. Application frameworks often combine dependency injection with inversion of control, where the framework first constructs an object and then passes control flow to it. DI helps to improve code reusability and reduces the frequency with which a class needs to be changed.
