Auto-wiring

An id with no factory and no binding that names an existing class is built from the class constructor, and cached under the class name like any other service:

final class ReportService
{
    public function __construct(
        private Database $database,      // get(Database::class)
        private LoggerInterface $logger, // get(LoggerInterface::class) — its binding
        private int $pageSize = 50,      // the default
    ) {
    }
}

$reports = $container->get(ReportService::class);

Resolving Parameters

Each constructor parameter is resolved in turn:

1. With #[Inject('id')], from that service id — see Injection by Id.
2. A class or interface type is fetched from the container: its factory, its binding, or auto-wiring again. With #[Lazy], a lazy proxy is injected instead — see Lazy Services.
3. Any other parameter takes its default value.
4. Without one, the class cannot be built: ContainerException.

A class without a constructor is created with new. A parameter typed self or parent resolves to the declaring class or its parent.

Optional Dependencies

A dependency the container does not know — an interface that is neither bound nor registered, an id that is not registered — leaves a parameter with a default value at that default:

final class Checkout
{
    public function __construct(
        private ?PaymentGateway $gateway = null,
        private Clock $clock = new SystemClock(),
    ) {
    }
}

$checkout = $container->get(Checkout::class); // $gateway is null, $clock a new SystemClock

Once PaymentGateway is bound or registered, it is injected. A dependency the container knows but cannot build — its own dependency is missing, its factory returns the wrong type — still throws: only an unknown dependency falls back.

Other Parameters

Built-in types (string, int, array, …), union and intersection types and untyped parameters are never looked up. They need a default value or #[Inject]:

use Solo\Container\Attribute\Inject;

final class Mailer
{
    public function __construct(
        private Transport $transport,
        #[Inject('mail.from')] private string $from,
        private int $retries = 3,
    ) {
    }
}

Failures

Situation	Exception
A required class or interface parameter the container does not know	ContainerException: Cannot resolve Mailer: Service 'Transport' not found in container.
get() of an abstract class, an enum, or a class with a non-public constructor	ContainerException: Class 'X' is not instantiable.
get() of an interface that is neither bound nor registered	NotFoundException
A parameter with no class type, no default and no #[Inject]	ContainerException: Cannot resolve parameter '$x' in class 'Y'.
A class that depends on itself, directly or through others	ContainerException: Circular dependency detected: … — see Circular Dependencies

A missing dependency deeper in the graph names the whole chain — Cannot resolve A -> B: Service 'C' not found in container. — and keeps the NotFoundException as its previous exception.