What is dependency injection, and what benefits does it provide?

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Accepted Answer

Dependency injection is a design pattern where objects receive their dependencies from external sources rather than creating them internally.

Constructor injection is the most common approach. The dependency is required at object creation, making dependencies explicit. This is preferred for required dependencies.

Setter injection provides dependencies through setter methods after construction. Useful for optional dependencies but can leave objects partially initialized.

Method injection passes dependencies as method parameters when the dependency varies between calls.

Benefits include improved testability since you can inject mocks during testing. Loose coupling results from depending on abstractions rather than implementations. Flexibility increases because configurations can change without modifying code. Explicit dependencies make code easier to understand.

Dependency injection containers like Spring, Guice, or .NET's built-in container automate wiring of dependencies. However, you can implement dependency injection manually without any framework.