The most easy to understand dependency injection and control inversion

This is one about ASP.NET 6 Dependency injection series .

In this series , We'll learn what dependency injection is 、 Inversion of control , What it can do , And why we use it .

after , We will learn more about ASP.NET 6 The life cycle of dependency injection 、 Important concepts such as service container .

Last , Let's further understand the service scope and more service registration and injection methods .

summary

Dependency injection is ASP.NET The core of the framework .

It mainly provides two core functions ：

1. 「 Registration type 」, These types registered to the dependency injection system , stay .NET There is a tall name in the —— service .

   Because these types are designed to provide some kind of service to the system , So type registration is also called service registration .

2. 「 Resolving instances 」, service / After the type is registered , It can also be injected into other services in the dependency injection system / Type used .

   Such as type A Dependency type B, type A You can get the type through dependency injection B Example .

Want a thorough understanding of dependency injection , We need to know something about inversion of control .

Inversion of control

There is something called a container in dependency injection , Also known as IoC（Inversion of Control - Inversion of control ） Containers .

This container holds all instances created by the dependency injection system , So why is it called IoC And the container ？

「 Inversion of control （IoC） It's a reverse flow 、 How dependencies and interfaces work , It gives the calling right of the object that is traditionally directly controlled by the program code to the third party , The assembly and management of object components are realized through the third party .」

Suppose we have such a scenario , A four 、 Five year old children , Want to eat .

If he finds something to eat from the fridge , May cause some bad results , For example, you may get food that you can't eat directly , Or forget to turn off the refrigerator door and other lights .

You can think about it , A four 、 A five-year-old , Go to the fridge and get your own food , Even to process their own ingredients , Do you think this is not normal ？

So what is the normal situation ？

As a four 、 A five-year-old , Just ask your parents to state your needs , such as " i am hungry 、 I would like to eat XX", This is the time , Parents will prepare the food they need , Then give it to him .

Take the initiative to get it from the fridge （ Positive rotation ） food , Passively get what you want to eat （ reverse ）, This is actually a process of transferring object call rights .

The former is to take the initiative to obtain food , The latter is by parents , That is, the third party gives him the food , And he doesn't care where the food comes from , And how to get .

So-called “ Inversion of control ” The concept of 「 Transfer of control over component objects , From the program code itself to a third party outside the code .」

In layman's terms , That is, the user code of the application is not responsible for the creation and maintenance of dependent objects , Instead, it is left to a third party , In this way, control is transferred from the application to a third party , That is, control realizes the so-called reversal .

Such as type A Need to use type B Example , type B Instance creation is not controlled by the type A Take care of yourself , But through a third party .

So what does it have to do with dependency injection ？

Dependency injection

「 Dependency injection is IoC An implementation of , Used to reverse dependencies .」

In the previous scene , A four 、 A five-year-old , Want to eat .

He just needs to state his needs , Parents will prepare the food they need （ rely on ）, Then give it to him （ Inject ）.

Dependency injection is not the purpose , It's just a way to reverse control .

The purpose of using dependency injection , To manage complex dependencies in applications , It can help us develop loose coupling 、 Maintainable 、 Testable code and programs .

Now? , Let's review the explanation of inversion of control .

A reverse flow 、 How dependencies and interfaces work , It gives the calling right of the object that is traditionally directly controlled by the program code to the third party , The assembly and management of object components are realized through the third party .

The third party in this description is the control system in dependency injection .

IoC Containers

In the scene of children eating , One side plays IoC The role of the container , You analyze who it is ？

I think the first instinct of many people is “ Parents ”？

Actually , Loosely speaking , The refrigerator is playing IoC The role of the container , Parents play the role of control system in dependency injection .

IoC A container is something in dependency injection that holds the created instance , If the instance has been created , And has an effective life cycle , Then the control system will directly from IoC Container get instance , Otherwise, the instance will be created .

What's in the fridge , They were bought and put in by their parents , They also need to take it out of the fridge .

「 The so-called dependency injection system , In essence, it is an implementation of type registration 、 Components of instance resolution and instance lifecycle management .」

About the life cycle , We will discuss in detail in the next article in this series .

If you know the factory model , Then you can simply think of the dependency injection system as a super factory .

stay .NET Dependency injection system , All registered types , Are called services .

ASP.NET Components in , Most of them need to obtain the service instances it depends on through dependency injection .

So the whole ASP.NET frame , Are built on the dependency injection system , It's also development ASP.NET The most basic programming mode of application .

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