Base Classes and Inheritance

When to Use

When you have multiple classes of the same type (entities, forms, blocks, controllers) that share common patterns or need to override specific base functionality.

Core Base Classes

Base Class	Extends From	Use When
`ContentEntityBase`	`EntityBase`	Creating content entity types (nodes, users, custom entities)
`ConfigEntityBase`	`EntityBase`	Creating config entity types (views, field configs)
`FormBase`	- (implements `FormInterface`)	Creating forms (uses traits for common services)
`ConfigFormBase`	`FormBase`	Creating admin forms that save to config
`BlockBase`	`PluginBase`	Creating block plugins
`PluginBase`	-	Creating any plugin type
`ControllerBase`	-	Creating controllers (uses traits)

Decision

If you need...	Use...	Why
Custom content entity	Extend `ContentEntityBase`	Field API support, revisions, translations
Custom admin form	Extend `ConfigFormBase`	Built-in config save/load methods
Custom block	Extend `BlockBase`	Context, caching, plugin scaffolding
Multiple similar custom forms	Create shared base class extending `FormBase`	Reuse common methods, eliminate duplication
Override parent constructor	Call `parent::__construct()` AND override `create()`	Maintain proper DI chain

Pattern

// Extending core base class
namespace Drupal\mymodule\Entity;

use Drupal\Core\Entity\ContentEntityBase;
use Drupal\Core\Entity\EntityTypeInterface;
use Drupal\Core\Field\BaseFieldDefinition;

class MyEntity extends ContentEntityBase {

  public static function baseFieldDefinitions(EntityTypeInterface $entity_type) {
    $fields = parent::baseFieldDefinitions($entity_type);

    $fields['name'] = BaseFieldDefinition::create('string')
      ->setLabel(t('Name'))
      ->setRequired(TRUE);

    return $fields;
  }
}

// Creating project-specific base class
namespace Drupal\mymodule\Form;

use Drupal\Core\Form\FormBase;

abstract class MyProjectFormBase extends FormBase {

  // Shared validation logic
  protected function validateEmail(string $email): bool {
    return filter_var($email, FILTER_VALIDATE_EMAIL);
  }

  // Shared submission patterns
  protected function logFormSubmission(string $form_id): void {
    $this->getLogger('mymodule')->info('Form @id submitted', ['@id' => $form_id]);
  }
}

Reference: core/lib/Drupal/Core/Entity/ContentEntityBase.php, core/lib/Drupal/Core/Form/FormBase.php, core/lib/Drupal/Core/Block/BlockBase.php

Common Mistakes

Deep inheritance hierarchies — Keep inheritance shallow (2-3 levels max); prefer composition via traits or services
Overriding constructors without calling parent — Always call parent::__construct() and override create() method for DI
Creating base classes prematurely — Wait until you have 3+ similar classes before creating a shared base class
God base classes with too many methods — Keep base classes focused; use traits for orthogonal concerns
Forgetting to extend from correct core base — FormBase for forms, ContentEntityBase for content entities, BlockBase for blocks