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October 30, 2020

What is Dependency Injection?

Definition, principles, and uses of this programming technique
dependencyinjectionsoftwaredevelopmenttestingcode

Written by:

Tiago Fernandes

Dependency Injection was introduced by Martin Fowler in the blog post Inversion of Control Containers and the Dependency Injection Pattern. The principal objective is to separate the responsibility of resolving object dependency from its behavior.

Definition

Dependency Injection (DI) is a programming technique that makes a class independent of its dependencies.

“In software engineering, dependency injection is a technique whereby one object supplies the dependencies of another object. A ‘dependency’ is an object that can be used, for example as a service. Instead of a client specifying which service it will use, something tells the client what service to use. The ‘injection’ refers to the passing of a dependency (a service) into the object (a client) that would use it. The service is made part of the client’s state. Passing the service to the client, rather than allowing a client to build or find the service, is the fundamental requirement of the pattern.” Source: Wikipedia

Creating objects directly within the class is inflexible because it commits the class to particular objects and makes it impossible to change the instantiation later independently from the class. It stops the class from being reusable if other objects are required and it makes the class hard to test because real objects can not be replaced with mock objects.

Depending on an interface is more flexible than depending on concrete classes. Object-oriented languages provide ways in which you can replace those abstractions with concrete implementations at runtime. You want to do this as much as possible since it is the best way to make your codebase flexible and reusable.
To learn more about the benefits of using interfaces, check out this article on “Why composition is superior to inheritance as a way of sharing code.”

Separation of Concerns

  • DI assumes that the tight coupling of object implementations is bad.
  • DI assumes that the dependent class is never a good place to create a dependency.
  • DI by its nature centralizes knowledge of all dependencies and their implementations.

Tight Coupling and Loose Coupling

Tight coupling is when a group of classes are highly dependent on one another. This scenario arises when a class assumes too many responsibilities, or when one concern is spread over many classes rather than having its own class.
Loose coupling is achieved by means of a design that promotes single-responsibility and separation of concerns.
A loosely-coupled class can be consumed and tested independently of other (concrete) classes.
Interfaces are a powerful tool to use for decoupling. Classes can communicate through interfaces rather than other concrete classes, and any class can be on the other end of that communication simply by implementing the interface.

Inversion of Control (IoC), Dependency Inversion

Principle (DIP) and Dependency Injection (DI)

Inversion of Control (IoC) is a design principle used to invert different kinds of controls in object-oriented design to achieve loose coupling.

Dependency Inversion Principle (DIP) says that high-level modules should not depend on low-level modules. Both should depend on the abstraction.
Abstractions should not depend on details should depend on abstractions.

Dependency Injection (DI) is a design pattern used to implement IoC.

IoC Container

When a new object is required, its dependencies need to be assigned to concrete classes. This task can be delegated to a container. When an instance of a particular type is requested to a container, it will inject the implementations required by that type. Those implementations are defined in a set of mappings that can easily be changed.

The Container creates an object of the specified class and also injects all the dependency objects through a constructor, a property, or a method at run time and disposes it at the appropriate time. This is done so that we don’t have to create and manage objects manually.
Containers provide support for the following DI life cycle.

  • Register: Determines which dependency to instantiate when it encounters a particular type.
  • Resolve: We don’t need to create objects manually. The container does it for us. The container includes some methods to resolve the specified type and creates an object of the specified type, injects the required dependencies, and returns the object.
  • Dispose: Manages the lifetime of the dependent objects.

Service lifetimes

The service life-time means how long the service will live before it’s disposed of. There are currently three different lifetimes:

  • Transient: The service is created each time it is requested
  • Singleton: Created once for the lifetime of the application
  • Scoped: Created once per request

Examples of Containers:
StructureMap
Unity
Ninject
Autofact
.Net Core Container

Implementation Example with .NET Core

This is a simple example to show how you register and resolve dependencies using the Dependency Injection technique with .NET Core.

The objective is to provide a web API that gets the list of albums consuming a Service.

Create a new class library named Services and a new Class to represent an Album

Create class Album:

namespace Services.Entities
{
8 references
public class Album
{
3 references
public string Name { get; set; }
3 references
public string ArtistName { get; set; }
}
}

Define the operations to be implemented:

namespace Services.Interfaces
{
4 references
public interface IAlbumService
{
2 references
IEnumerable GetAlbums ();
}
}

Create a class to implement IAlbumService:

namespace Services
{
1 reference
public class AlbumService : IAlbumService
{
private readonly List _albums = new List()
{
new Album { Name= "Battle of Los Angeles", ArtistName="Rage Against the Machine" },
new Album { Name= "Cowboys from Hell", ArtistName="Pantera" },
new Album { Name= "Black Album", ArtistName="Metalica" },
};
2 references
public IEnumerable GetAlbums()
{
return _albuns.AsEnumerable();
}
}
}

Create a new WebAPI project and Register Dependencies

First, we must install the Microsoft.Extensions.DependencyInjection package using Nuget.

The entry point of a Web API is the Startup.cs. We will register a dependency on the ConfigureService:

public void ConfigureServices(IServiceCollection services)
{
services.AddTransient();
}

Add a new Controller Album with a method Get to return the list of albums. The dependency will be injected by the constructor.

namespace Api.Controllers
{
[Route(template:"api/[controller]"]
[APIController]
1 reference
public class AlbumController: ControllerBase
{
private readonly IAlbumsService _albumsService;
0 references
public AlbumController(IAlbumsService albumsService)
{
_albumsService = albumsService;
}
[HttpGet]
0 references
public IEnumerable GetAlbums()
}
return _albumsService.GetAlbums();
}
}
}

Conclusion

Dependency Injection allows a client to remove all knowledge of a concrete implementation. It allows us to create more flexible code and switch out one implementation for another without changing the layer.
We can more easily test our code without worrying about side-effects with dependency injection, as it Promotes reusability, testability, and maintainability.

References

About the Author

Tiago Fernandes

I’m a 34 years old Software Developer with 8 years of experience. I have a degree in Management of Information Systems and Technologies and experience with small businesses, service companies, and end customers as well as various programming languages and business areas. My interests include music, travel, and reading. Currently, I’m learning to play the guitar. I’m interested in new technologies, frameworks, and best practices.

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