How to create a custom constraint in Symfony?

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namespace AppValidatorConstraints;

use SymfonyComponentValidatorConstraint;

/**
 * @Annotation
 */
class CustomConstraint extends Constraint
{
    public $message = 'My custom constraint message';

    public function validatedBy()
    {
        return get_class($this).'Validator';
    }
}

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namespace AppValidatorConstraints;

use SymfonyComponentValidatorConstraint;
use SymfonyComponentValidatorConstraintValidator;

class CustomConstraintValidator extends ConstraintValidator
{
    public function validate($value, Constraint $constraint)
    {
        if ($value < 0) {
            $this->context->buildViolation($constraint->message)
                ->addViolation();
        }
    }
}

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services:
    AppValidatorConstraintsCustomConstraint:
        tags:
            - { name: validator.constraint_validator, alias: custom }

    AppValidatorConstraintsCustomConstraintValidator: ~

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namespace AppEntity;

use AppValidatorConstraints as CustomAssert;
use SymfonyComponentValidatorConstraints as Assert;

class MyEntity
{
    /**
     * @AssertNotBlank
     * @CustomAssertCustomConstraint
     */
    private $myField;
}