Dependency injection intro

ASP.NET Core includes a built-in DI container. Register services in Program.cs; the framework injects them into controllers, Razor Pages, and middleware via constructors.

Registration lifetimes

• Transient — new instance every time requested
• Scoped — one per HTTP request (typical for DbContext)
• Singleton — one for app lifetime (caches, config readers)

Example

builder.Services.AddScoped<IEmailSender, SmtpEmailSender>();

public class AccountController : Controller {
    private readonly IEmailSender _email;
    public AccountController(IEmailSender email) => _email = email;
}

Important interview questions and answers

1. Q: Why Scoped for DbContext?
   A: One context per request aligns with unit of work; sharing across requests causes threading and stale data bugs.
2. Q: Captive dependency?
   A: Singleton holding Scoped service—lifetime mismatch; fix by adjusting registrations or using factories.

Self-check

1. Which lifetime fits IEmailSender per request?
2. Why inject interfaces instead of concrete classes?

Pitfall: Registering DbContext as Singleton causes threading bugs—always Scoped. Never inject Scoped services into Singleton without a factory.