Dependency Injection (DI) is a design pattern used in software development to establish loosely linked components by allowing dependencies to be injected into a class rather than created within it. This improves code reuse, testability, and maintainability. An Inversion of Control (IoC) container is a tool that manages dependency injection in the context of Dependency Injection.

Step 1: Begin by creating an ASP.NET Core Web API Project.
Launch Visual Studio.
Make a new project in ASP.NET Core Web Application.
Select the API template and make sure ASP.NET Core 3.1 or later is chosen.
Step 2: Establish Dependencies
Assume you wish to build a basic service to manage articles. Here's an example of how you might define your dependencies:
Create the service's interface.

Author: Peter
public interface IArticleService
{
    List<Article> GetAllArticles();
    Article GetArticleById(int id);
}

Implement the service
Author: Peter

public class ArticleService: IArticleService
{
    private List<Article> _articles = new List<Article>
    {
        new Article { Id = 1, Title = "Introduction to Dependency Injection By Peter
        Khan", Content = "..." },
        new Article { Id = 2, Title = "ASP.NET Core Web API Basics", Content = "..." }
    };

    public List<Article> GetAllArticles()
    {
        return _articles;
    }

    public Article GetArticleById(int id)
    {
        return _articles.FirstOrDefault(article => article.Id == id);
    }
}

Step 2. Configure Dependency Injection
In your Startup.cs file, configure the dependency injection container:
public void ConfigureServices(IServiceCollection services)
{
    services.AddControllers();

    // Register the ArticleService
    services.AddScoped<IArticleService, ArticleService>();
}

Step 3. Create Controller
Create a controller that uses the IArticleService:
[Route("api/[controller]")]
[ApiController]
public class ArticlesController : ControllerBase
{
    private readonly IArticleService _articleService;

    public ArticlesController(IArticleService articleService)
    {
        _articleService = articleService;
    }

    [HttpGet]
    public ActionResult<IEnumerable<Article>> Get()
    {
        var articles = _articleService.GetAllArticles();
        return Ok(articles);
    }

    [HttpGet("{id}")]
    public ActionResult<Article> Get(int id)
    {
        var article = _articleService.GetArticleById(id);
        if (article == null)
            return NotFound();
        return Ok(article);
    }
}

Step 4: Test the API
Run the application and navigate to the appropriate API endpoints, for example:
    GET /api/articles: Retrieve all articles.
    GET /api/articles/{id}: Retrieve an article by its ID.

Remember, this example focuses on setting up a simple ASP.NET Core Web API project with Dependency Injection. For a complete production-ready solution, you'd need to consider error handling, validation, authentication, and other aspects.
Conclusion

We explored the concepts of Dependency Injection (DI) and demonstrated how to integrate DI into an ASP.NET Core Web API project. Dependency Injection is a powerful design pattern that promotes loosely coupled components, better testability, and maintainability. Here's a recap of what we covered:

1. Dependency Injection (DI)
DI is a design pattern that focuses on providing the dependencies a class needs from the outside, rather than creating them internally. This promotes modularity, reusability, and easier testing.

2. Advantages of DI

• Loose Coupling: Components are decoupled, making it easier to replace or update individual parts without affecting the whole system.
• Testability Dependencies can be easily mocked or replaced during testing, leading to more effective unit testing.
• Maintainability: Changes to dependencies can be managed more centrally, making maintenance and updates simpler.

3. Integration with ASP.NET Core Web API

• We created a simple ASP.NET Core Web API project.
• We defined a service interface (IArticleService) and an implementation (ArticleService) to manage articles.
• We configured the dependency injection container in the Startup.cs file using the AddScoped method.
• We created an API controller (ArticlesController) that uses the IArticleService through constructor injection.

4. Testing the API
We ran the application and tested the endpoints using tools like Postman or a web browser.
We observed how the API endpoints interact with the injected service to provide data.

Dependency Injection is a fundamental concept in modern software development, and integrating it into your projects can lead to more maintainable, testable, and scalable applications. As you continue your journey in software development, these principles will prove to be valuable tools in your toolkit.