How to Inject Values Based on Complex Expressions

The service container also supports an "expression" that allows you to inject very specific values into a service.

For example, suppose you have a service (not shown here), called App\Mail\MailerConfiguration, which has a getMailerMethod() method on it. This returns a string - like sendmail based on some configuration.

Suppose that you want to pass the result of this method as a constructor argument to another service: App\Mailer. One way to do this is with an expression:

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# config/services.yaml services: # ... App\Mail\MailerConfiguration: ~ App\Mailer: # the '@=' prefix is required when using expressions for arguments in YAML files arguments: ['@=service("App\\Mail\\MailerConfiguration").getMailerMethod()'] # when using double-quoted strings, the backslash needs to be escaped twice (see https://yaml.org/spec/1.2/spec.html#id2787109) # arguments: ["@=service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()"]

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// config/services.php namespace Symfony\Component\DependencyInjection\Loader\Configurator; use App\Mail\MailerConfiguration; use App\Mailer; return function(ContainerConfigurator $container): void { // ... $services->set(MailerConfiguration::class); $services->set(Mailer::class) // because of the escaping applied by PHP, you must add 4 backslashes for each original backslash ->args([expr("service('App\\\\Mail\\\\MailerConfiguration').getMailerMethod()")]); };

Learn more about the expression language syntax.

In this context, you have access to 3 functions:

service

Returns a given service (see the example above).

parameter

Returns a specific parameter value (syntax is like service).

env

Returns the value of an env variable.

You also have access to the Container via a container variable. Here's another example:

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# config/services.yaml services: App\Mailer: # the '@=' prefix is required when using expressions for arguments in YAML files arguments: ["@=container.hasParameter('some_param') ? parameter('some_param') : 'default_value'"]

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// config/services.php namespace Symfony\Component\DependencyInjection\Loader\Configurator; use App\Mailer; return function(ContainerConfigurator $container): void { $services = $container->services(); $services->set(Mailer::class) ->args([expr("container.hasParameter('some_param') ? parameter('some_param') : 'default_value'")]); };

Expressions can be used in arguments, properties, as arguments with configurator, as arguments to calls (method calls) and in factories (service factories).