Dependency Injection in .NET

What is Dependency Injection?

Dependency Injection (DI) means a class receives its dependencies from outside rather than creating them. .NET has DI built in, making code testable and loosely coupled.

The Problem DI Solves

// ❌ Tight coupling — hard to test
public class OrderService {
    private EmailSender _email = new EmailSender();
}

// ✅ Injected — swap implementations easily
public class OrderService {
    private readonly IEmailSender _email;
    public OrderService(IEmailSender email) { _email = email; }
}

Registering Services

builder.Services.AddScoped<IEmailSender, EmailSender>();
builder.Services.AddSingleton<IConfig, Config>();
builder.Services.AddTransient<IHelper, Helper>();

Service Lifetimes

Lifetime	Instance
Singleton	One for the whole app
Scoped	One per request
Transient	New every time

FAQs

Why use interfaces with DI?

So you can swap real implementations for mocks in tests. More in our .NET guides.

Which lifetime should I default to?

Scoped for most services in web apps.