sebastianbergmann/phpunit · error · PropertyCannotBeDoubledException

Trying to double property "%s" of class "%s" with doubleProp

Error message

Trying to double property "%s" of class "%s" with doubleProperties(), but it is readonly

What it means

Thrown by PHPUnit's TestDoubleBuilder::doubleProperties() when you ask it to generate property hooks (get/set) for a property that is declared readonly. The test double is generated as a subclass of the mocked type, and PHP forbids redeclaring a readonly property with hooks in a child class, so PHPUnit rejects the request up front via ReflectionProperty::isReadOnly().

Source

Thrown at src/Framework/MockObject/TestDoubleBuilder.php:139

        }

        foreach ($properties as $propertyName) {
            if (!$reflector->hasProperty($propertyName)) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it does not exist');
            }

            $property = $reflector->getProperty($propertyName);

            if (!$property->isPublic()) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it is not public');
            }

            if ($property->isStatic()) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it is static');
            }

            if ($property->isReadOnly()) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it is readonly');
            }

            if ($property->isFinal()) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it is final');
            }

            if (!$property->hasType()) {
                throw new PropertyCannotBeDoubledException($this->type, $propertyName, 'it does not declare a type');
            }
        }

        $this->doubledProperties = array_merge($this->doubledProperties, $properties);

        return $this;
    }

    /**
     * Specifies the arguments for the constructor.

View on GitHub (pinned to f123cdb2a2)

Solutions

Remove the readonly property from the list passed to doubleProperties()
If you only need a fixed value, keep the property untouched and configure it through the constructor (setConstructorArgs()) instead of doubling it
Restructure the test to assert the real value instead of configuring get/set hook expectations for that property
If the type genuinely needs hook doubling in tests, drop 'readonly' from the property declaration in the production code

Example fix

// before
$double = (new TestDoubleBuilder(Invoice::class))
    ->doubleProperties(['lines', 'total']) // 'total' is readonly
    ->generate();

// after
$double = (new TestDoubleBuilder(Invoice::class))
    ->doubleProperties(['lines']) // only double non-readonly properties
    ->setConstructorArgs([['line 1'], 100])
    ->generate();

Defensive patterns

Strategy: type-guard

Validate before calling

$reflector = new ReflectionClass(Invoice::class);
$doublable = array_filter(
    ['lines', 'total'],
    static fn (string $name) => $reflector->hasProperty($name)
        && $reflector->getProperty($name)->isPublic()
        && !$reflector->getProperty($name)->isStatic()
        && !$reflector->getProperty($name)->isReadOnly(),
);

Type guard

/** @param class-string $class @param list<string> $properties @return list<string> */
function doublableProperties(string $class, array $properties): array
{
    $reflector = new ReflectionClass($class);

    return array_values(array_filter(
        $properties,
        static fn (string $name): bool => $reflector->hasProperty($name)
            && $reflector->getProperty($name)->isPublic()
            && !$reflector->getProperty($name)->isStatic()
            && !$reflector->getProperty($name)->isReadOnly(),
    ));
}

Prevention

Run doublableProperties() over the list before calling doubleProperties() in shared test helpers
Prefer configuring readonly properties through setConstructorArgs() with real values
Review the class's property modifiers before adding new names to a doubling list

When it happens

Trigger: Calling doubleProperties(['someProperty']) (or a builder/wrapper that forwards to it) where someProperty is declared 'readonly' or is a readonly promoted constructor property of the doubled class.

Common situations: Trying to double properties on PHP 8.1+ value objects / DTOs that use readonly promoted constructor properties; upgrading a codebase to readonly and re-running an older test suite that doubled those properties.

Related errors

AI-assisted analysis of sebastianbergmann/phpunit@f123cdb2a2 (2026-08-23). Data as JSON: /api/errors/29a926addaf452b7. Report an issue: GitHub.