How to Inject Instances into the Container

When using the container in your application, you sometimes need to inject an instance instead of configuring the container to create a new instance.

For instance, if you're using the HttpKernel component with the DependencyInjection component, then the kernel service is injected into the container from within the Kernel class:

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// ...
abstract class Kernel implements KernelInterface, TerminableInterface
{
    // ...
    protected function initializeContainer()
    {
        // ...
        $this->container->set('kernel', $this);

        // ...
    }
}

The kernel service is called a synthetic service. This service has to be configured in the container, so the container knows the service does exist during compilation (otherwise, services depending on this kernel service will get a "service does not exist" error).

In order to do so, you have to use Definition::setSynthetic():

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use Symfony\Component\DependencyInjection\Definition;

// synthetic services don't specify a class
$kernelDefinition = new Definition();
$kernelDefinition->setSynthetic(true);

$container->setDefinition('your_service', $kernelDefinition);

Now, you can inject the instance in the container using Container::set():

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$yourService = new YourObject();
$container->set('your_service', $yourService);

$container->get('your_service') will now return the same instance as $yourService.