How do I inject services in a class that implements an interface.

class RoleNegotiator implements ThemeNegotiatorInterface {

  public static function create(ContainerInterface $interface) {
    return new static(
  public function __construct(RouteMatchInterface $route) {
    $this->route = $route;

The above code throws an error.

  • 2
    What's the error? You should always include that. – Jaypan Jan 21 '17 at 10:33

You define the necessary services as arguments of the service you implement.

Since you are showing a class that implements ThemeNegotiatorInterface, see the theme.negotiator.default service and the class implementing it, Drupal\Core\Theme\DefaultNegotiator.

Service definition

    class: Drupal\Core\Theme\DefaultNegotiator
    arguments: ['@config.factory']
      - { name: theme_negotiator, priority: -100 }

Class constructor

public function __construct(ConfigFactoryInterface $config_factory) {
  $this->configFactory = $config_factory;

There isn't any static create() method as in your code.

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  • 1
    Thanks for this answer. I'm used to implementing ContainerInjectionInterface in my classes whenever I need to inject some services. In what cases would I want to define the services in my *.services.yml versus implementing ContainerInjectionInterface? Thank you. – edwardchiapet May 31 '17 at 12:42
  • As far as I can recall, if you aren't replacing a service implemented by another module, you should define your service in the .services.yml file. – kiamlaluno May 31 '17 at 15:37

You need to implement ContainerInjectionInterface as well to inject the dependency in your class (if its not a service). But if you are creating a new service you can inject it in module.services.yml itself as said by @kiamlaluno in his answer.

class RoleNegotiator implements ThemeNegotiatorInterface, ContainerInjectionInterface {

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