IsNull

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验证一个值是否完全等于 null。要强制属性为空白（空字符串或 null），请参阅 Blank 约束。要确保属性不为 null，请参阅 NotNull。

另请参阅 NotNull。

适用于	属性或方法
类	IsNull
验证器	IsNullValidator

基本用法

如果出于某种原因，您想确保 Author 类的 firstName 属性完全等于 null，您可以执行以下操作

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        // src/Entity/Author.php
namespace App\Entity;

use Symfony\Component\Validator\Constraints as Assert;

class Author
{
    #[Assert\IsNull]
    protected ?string $firstName = null;
}
    

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        # config/validator/validation.yaml
App\Entity\Author:
    properties:
        firstName:
            - 'IsNull': ~
    

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        <!-- config/validator/validation.xml -->
<?xml version="1.0" encoding="UTF-8" ?>
<constraint-mapping xmlns="https://symfony.ac.cn/schema/dic/constraint-mapping"
    xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
    xsi:schemaLocation="https://symfony.ac.cn/schema/dic/constraint-mapping https://symfony.ac.cn/schema/dic/constraint-mapping/constraint-mapping-1.0.xsd">

    <class name="App\Entity\Author">
        <property name="firstName">
            <constraint name="IsNull"/>
        </property>
    </class>
</constraint-mapping>
    

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        // src/Entity/Author.php
namespace App\Entity;

use Symfony\Component\Validator\Constraints as Assert;
use Symfony\Component\Validator\Mapping\ClassMetadata;

class Author
{
    // ...

    public static function loadValidatorMetadata(ClassMetadata $metadata): void
    {
        $metadata->addPropertyConstraint('firstName', Assert\IsNull());
    }
}
    