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:

YAML

# 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()"]

XML

<!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd">
    <services>
        <!-- ... -->
        <service id="App\Mail\MailerConfiguration"></service>
        <service id="App\Mailer">
            <argument type="expression">service('App\\Mail\\MailerConfiguration').getMailerMethod()</argument>
        </service>
    </services>
</container>

PHP

// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Mail\MailerConfiguration;
use App\Mailer;
return function(ContainerConfigurator $configurator) {
    // ...
    $services->set(MailerConfiguration::class);
    $services->set(Mailer::class)
        ->args([expr("service('App\\Mail\\MailerConfiguration').getMailerMethod()")]);
};

To learn more about the expression language syntax, see The Expression Syntax.

In this context, you have access to 2 functions:

service

Returns a given service (see the example above).

parameter

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

You also have access to the Symfony\Component\DependencyInjection\Container via a container variable. Here’s another example:

YAML

# 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'"]

XML

<!-- config/services.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<container xmlns="http://symfony.com/schema/dic/services"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="http://symfony.com/schema/dic/services
        https://symfony.com/schema/dic/services/services-1.0.xsd">
    <services>
        <service id="App\Mailer">
            <argument type="expression">container.hasParameter('some_param') ? parameter('some_param') : 'default_value'</argument>
        </service>
    </services>
</container>

PHP

// config/services.php
namespace Symfony\Component\DependencyInjection\Loader\Configurator;
use App\Mailer;
return function(ContainerConfigurator $configurator) {
    $services = $configurator->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 and as arguments to calls (method calls).

This work, including the code samples, is licensed under a Creative Commons BY-SA 3.0 license.