# DI / IoC

## IoC

**inversion of control** (**IoC**) is a programming principle. IoC inverts the [flow of control](https://en.wikipedia.org/wiki/Control_flow) as compared to traditional control flow. In IoC, custom-written portions of a [computer program](https://en.wikipedia.org/wiki/Computer_program) receive the flow of control from a generic [framework](https://en.wikipedia.org/wiki/Software_framework). A [software architecture](https://en.wikipedia.org/wiki/Software_architecture) with this design inverts control as compared to traditional [procedural programming](https://en.wikipedia.org/wiki/Procedural_programming): in traditional programming, the custom code that expresses the purpose of the program [calls](https://en.wikipedia.org/wiki/Function_call#Main_concepts) into reusable libraries to take care of generic tasks, but with inversion of control, it is the framework that calls into the custom, or task-specific, code.

**dependency injection** is a [design pattern](https://en.wikipedia.org/wiki/Software_design_pattern) in which an [object](https://en.wikipedia.org/wiki/Object_\(computer_science\)) or [function](https://en.wikipedia.org/wiki/Subroutine) receives other objects or functions that it depends on. A form of [inversion of control](https://en.wikipedia.org/wiki/Inversion_of_control), dependency injection aims to [separate the concerns](https://en.wikipedia.org/wiki/Separation_of_concerns) of constructing objects and using them, leading to [loosely](https://en.wikipedia.org/wiki/Loose_coupling) [coupled](https://en.wikipedia.org/wiki/Coupling_\(computer_programming\)) programs.[\[1\]](https://en.wikipedia.org/wiki/Dependency_injection#cite_note-1)[\[2\]](https://en.wikipedia.org/wiki/Dependency_injection#cite_note-MarkSeeman2011P4-2)[\[3\]](https://en.wikipedia.org/wiki/Dependency_injection#cite_note-3) The pattern ensures that an object or function which wants to use a given [service](https://en.wikipedia.org/wiki/Service_\(systems_architecture\)) should not have to know how to construct those services. Instead, the receiving '[client](https://en.wikipedia.org/wiki/Client_\(computing\))' (object or function) is provided with its dependencies by external code (an 'injector'), which it is not aware of.[\[4\]](https://en.wikipedia.org/wiki/Dependency_injection#cite_note-HollywoodPrinciple.c2-4) Dependency injection helps by making implicit dependencies explicit and helps solve the following problems:[\[5\]](https://en.wikipedia.org/wiki/Dependency_injection#cite_note-5)

* How can a [class](https://en.wikipedia.org/wiki/Class_\(computer_programming\)) be independent from the creation of the objects it depends on?
* How can an application, and the objects it uses support different configurations?
* How can the behavior of a piece of code be changed without editing it directly?


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