Dependency Injection

Symfony resolves and injects services into your code via the container, instead of you newing things up.

Foundational

A "service" is just a class managed by the container — typically with logic you want to share.
Constructor injection is the default. Type-hint a service in your constructor and Symfony provides it.
Autowiring uses the type-hint to figure out which concrete class to inject, no config required.
Autoconfiguration applies tags automatically based on interfaces (e.g. EventSubscriberInterface).
When more than one class implements an interface, you must alias or bind the right one explicitly.
Services are singletons by default — one instance per request. Mark them shared: false to change that.

// src/Service/Mailer.php
class Mailer
{
    public function __construct(private LoggerInterface $logger) {}
}

// Anywhere else
class NotifyController
{
    public function __construct(private Mailer $mailer) {}
}

Common gotchas

Don't confuse autowiring (resolving constructor args by type) with autoconfiguration (applying tags by interface). They run together but do different things.
Public vs private services: in Symfony 4+ services are private by default. You inject them — you do not pull them from `\$container->get()`.
Circular dependencies fail at compile time, not runtime. Read the error — it usually points at the cycle.