Dependency Injection

In C# is a design pattern that allows you to achieve Inversion of Control (IoC) by injecting dependencies (objects or services that a class needs) into a class from the outside, rather than letting the class create them itself.

Key Benefits of Dependency Injection:

1. Loose Coupling: Classes depend on abstractions (interfaces) instead of concrete implementations, making the code more flexible and testable.

2. Easier Testing: Dependencies can be replaced with mock or fake objects for unit testing.

3. Simplified Code Maintenance: Changing a dependency doesn't require altering the dependent class.

Example Without Dependency Injection:

public class Service {

    public string GetData() => "Service Data";

}

public class Client {

    private Service _service = new Service(); // Tight coupling

    public string UseService() => _service.GetData();

}

Here, `Client` directly creates an instance of `Service`, which makes it hard to replace or test `Service`.

Example With Dependency Injection:

public class Service : IService {

    public string GetData() => "Service Data";

}

public interface IService {

    string GetData();

}

public class Client {

    private readonly IService _service;

    public Client(IService service) { // Dependency is injected

        _service = service;

    }

    public string UseService() => _service.GetData();

}

Here, `Client` depends on the abstraction (`IService`), and the concrete implementation (`Service`) is injected into it. 

Implementing Dependency Injection in ASP.NET Core:

ASP.NET Core has built-in support for DI through its service container. 

Example:

// Registering services in the DI container

builder.Services.AddScoped<IService, Service>();

// Injecting the service into a controller

public class ClientController : ControllerBase {

    private readonly IService _service;

    public ClientController(IService service) {

        _service = service;

    }

    [HttpGet]

    public IActionResult Get() {

        return Ok(_service.GetData());

    }

}

In this approach, ASP.NET Core automatically resolves the `IService` dependency when creating `ClientController`.