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Home Interviews What is dependency injection and how is it implemented in .NET? – MindStick
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1 Answers

ICSM Computer
ICSM Computer 16 Jun 2025
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Dependency Injection (DI) is a design pattern used to achieve loose coupling between classes by injecting dependencies from the outside, rather than a class creating its own dependencies.

Why Use Dependency Injection?

  • Improves testability
  • Encourages modular design
  • Enables inversion of control (IoC) — classes depend on abstractions, not implementations

Real-World Analogy

Instead of a class baking its own cake, it asks for a cake to be delivered.

Without Dependency Injection (Tightly Coupled)

public class OrderService
{
    private EmailService _emailService = new EmailService(); // tightly coupled

    public void ProcessOrder()
    {
        // business logic
        _emailService.SendEmail();
    }
}

With Dependency Injection (Loosely Coupled)

public interface IEmailService
{
    void SendEmail();
}

public class EmailService : IEmailService
{
    public void SendEmail() { /* send logic */ }
}

public class OrderService
{
    private readonly IEmailService _emailService;

    public OrderService(IEmailService emailService) // dependency injected
    {
        _emailService = emailService;
    }

    public void ProcessOrder()
    {
        _emailService.SendEmail();
    }
}

How It's Implemented in .NET (ASP.NET Core)

1. Register services in Startup.cs (or Program.cs in .NET 6+)

services.AddScoped<IEmailService, EmailService>();
services.AddScoped<OrderService>();

2. Constructor injection (automatically handled by framework)

public class HomeController : Controller
{
    private readonly OrderService _orderService;

    public HomeController(OrderService orderService)
    {
        _orderService = orderService;
    }

    public IActionResult Index()
    {
        _orderService.ProcessOrder();
        return View();
    }
}

Types of DI in .NET

Type Description
Constructor Injection Most common, safe for required dependencies
Property Injection Optional dependencies
Method Injection Dependencies passed to methods

Lifetime Scopes

Method Lifetime
AddSingleton<T>() One instance for the whole app
AddScoped<T>() One per request
AddTransient<T>() New instance every time it's requested

Benefits

  • Makes unit testing easier (e.g., mock services)
  • Reduces tight coupling
  • Promotes separation of concerns
  • Enables inversion of control
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