$bid = 'myblock';
$block = \Drupal\block_content\Entity\BlockContent::load($bid);
$render = \Drupal::entityTypeManager()->getViewBuilder('block_content')->view($block);

$block_manager = \Drupal::service('plugin.manager.block');
$config = [];
$plugin_block = $block_manager->createInstance('system_breadcrumb_block', $config);
$access_result = $plugin_block->access(\Drupal::currentUser());
if (is_object($access_result) && $access_result->isForbidden() || is_bool($access_result) && !$access_result) {
  return [];
}
$render = $plugin_block->build();

\Drupal::service('config.factory')->getEditable('system.performance');

\Drupal::service('config.factory')->getEditable('system.performance')->set('cache.page.enabled', 1)->save();

\Drupal::service('state')->set('du_admission_steps.importer.last_run', NULL);

$results = \Drupal::database()->query('select * from purge_queue')->fetchAll();

$entity_query->addTag('debug')->execute();

$result = \Drupal::entityQuery('node')
  ->condition('field_number', [1, 3], 'BETWEEN')
  ->execute();

$result = \Drupal::entityQuery('node')
  ->condition('field_banner_code', NULL, 'IS NULL')
  ->execute();

$result = \Drupal::entityQuery('node')
  ->condition('type', 'event')
  ->execute();
entity_delete_multiple('node', $result);
// Add ->range(0, 10) to delete a range

$query = \Drupal::database()->insert('purge_queue');
$query->fields(['data', 'created']);
$query->values(['a:4:{i:0;s:3:"url";i:1;a:4:{s:10:"66849f6f11";i:3;s:10:"c990b129a0";i:3;s:10:"c618828456";i:3;s:10:"453d844ea2";i:3;}i:2;s:66:"http://www.example.com";i:3;a:0:{}}', time()]);
$query->execute();

$query = \Drupal::database()->update('mcpl_events_feeds_item');
$query->fields(['hash' => 'update']);
$query->condition('nid', 1);
$query->execute();

$query = \Drupal::database()->delete('purge_queue');
$query->condition('data', '%url%', 'LIKE');
$query->execute();

ini_set('display_errors', 1);
ini_set('display_startup_errors', 1);
error_reporting(E_ALL);

If kint is taking forever to load or crashing the page, try reducing the max level.

// Change kint maxLevels setting.
include_once(DRUPAL_ROOT . '/modules/contrib/devel/kint/kint/Kint.class.php');
if(class_exists('Kint')){
  Kint::$maxLevels = 5;
}

You will need to copy sites/example.settings.local.php to sites/default/settings.local.php (and ensure settings.php includes settings.local.php) or put this in your settings.php file: $settings['container_yamls'][] = DRUPAL_ROOT . '/sites/development.services.yml';.

Update /sites/development.services.yml:

parameters:
  http.respone.debug_cacheability_headers: true
  twig.config:
    debug: true
    auto_reload: true
    cache: false

\Drupal::logger('my_module')->debug(kpr($var, TRUE));

// This function exists in core/includes/bootstrap.inc.
// Just need to add lines 6-8 to it.
function _drupal_error_handler($error_level, $message, $filename, $line, $context) {
  require_once DRUPAL_ROOT . '/includes/errors.inc';
  require_once DRUPAL_ROOT . '/modules/contrib/devel/kint/kint.module';
  $d = debug_backtrace(DEBUG_BACKTRACE_IGNORE_ARGS);
  ksm($message, $d);
  _drupal_error_handler_real($error_level, $message, $filename, $line, $context);
}

// You can output the Request object using kint/kpm, but it can be hard
// to figure out where to set the debugging code. The best place is in
// the executeRequest function in the following file:
// search_api_solr/src/SolrConnector/SolrConnectorPluginBase.php

// src/ElasticSearch/Parameters/Builder/SearchBuilder.php:
// Add ksm at the end of build() and getSearchQueryOptions()

services:
  du_user_management:
    class: Drupal\du_user_management\UserManagementService
    arguments: ['@cache.default', '@logger.channel.du_user_management']
  logger.channel.du_user_management:
    class: Drupal\Core\Logger\LoggerChannel
    factory: logger.factory:get
    arguments: ['du_user_management']

  /**
   * Constructor.
   *
   * @param \Drupal\Core\Cache\CacheBackendInterface $cache
   *   The cache.
   * @param \Drupal\Core\Logger\LoggerChannelInterface $logger
   *   The logger channel.
   */
  public function __construct(CacheBackendInterface $cache, LoggerChannelInterface $logger) {
    $this->cache = $cache;
    $this->logger = $logger;
  }

  /**
   * {@inheritdoc}
   */
  public function __construct(UserManagementService $user_management) {
    $this->userManagement = $user_management;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container) {
    return new static(
      $container->get('du_user_management')
    );
  }

  /**
   * {@inheritdoc}
   */
  public function __construct(array $configuration, $plugin_id, $plugin_definition, UserManagementService $client, ConfigFactoryInterface $config_factory) {
    parent::__construct($configuration, $plugin_id, $plugin_definition);
    $this->client = $client;
    $this->configFactory = $config_factory;
  }

  /**
   * {@inheritdoc}
   */
  public static function create(ContainerInterface $container, array $configuration, $plugin_id, $plugin_definition) {
    return new static(
      $configuration,
      $plugin_id,
      $plugin_definition,
      $container->get('du_user_management'),
      $container->get('config.factory')
    );
  }

https://www.drupal.org/docs/8/api/routing-system/altering-existing-routes-and-adding-new-routes-based-on-dynamic-ones

https://docs.acquia.com/tutorials/fast-track-drupal-8-coding/add-custom-variable-drupalsettings/

https://www.axelerant.com/resources/team-blog/drupal-8-service-decorators

$node = \Drupal::entityTypeManager()->getStorage('node')->load(23);
$search_api_index = \Drupal::entityTypeManager()->getStorage('search_api')->load('title_records');

$node = \Drupal::entityTypeManager()->getStorage('node')->loadMultiple($entity_ids);

$result = \Drupal::entityQuery('taxonomy_term')
      ->condition('vid', 'libraries')
      ->execute();
entity_delete_multiple('taxonomy_term', $result);

$new_field = BaseFieldDefinition::create('string')
  ->setLabel(new TranslatableMarkup('New Field'))
  ->setDescription(new TranslatableMarkup('New field description.'));
\Drupal::entityDefinitionUpdateManager()->installFieldStorageDefinition('<field_name>', '<entity_type_id>', '<provider>', $new_field);

\Drupal::entityDefinitionUpdateManager()->applyUpdates();

Whether a multifield or single field, you can use ->first(), but if you leave it off, it'll automatically get the first value.

$node->field_ref->target_id; // Entity ID.
$node->field_ref->entity; // Full entity.

$node->field_url->uri;
$node->field_url->title;
$node->field_url->options; // Array of options.

$file = \Drupal\file\Entity\File::load(1007);
OR
$file = \Drupal::entityTypeManager()->getStorage('file')->load(1007);

// Get the URI (including wrapper, such as public://).
$uri = $file->getFileUri();

// Get the full URL path.
$url = file_create_url($file->getFileUri());

// Get relative path of the URL (w/o domain).
$path = file_url_transform_relative($url);

$render = [
  '#theme' => 'image_style',
  '#style_name' => 'thumbnail',
  '#uri' => 'public://my-image.png',
];

$style = \Drupal::entityTypeManager()->getStorage('image_style')->load('thumbnail');
$image_url = $style->buildUrl('public://du_content_gallery-article.jpg');

$style = ImageStyle::load('thumbnail');
$image_url = $style->buildUri('public://du_content_gallery-article.jpg');

$variables['#attached']['library'][] = 'lotus/lotus-js';

{{ attach_library('hcpl_zen/title-record') }}

// Libraries is an array of the library data.
// Extension is 'core' or the module/theme that defined the libraries.
function hook_library_info_alter(&$libraries, $extension)

// Remove the prepareUpdate() section if you just want a normal import.
// To rollback just change 'import' to 'rollback'.
$migration = \Drupal::service('plugin.manager.migration')->createInstance('machine_name');
$migration->getIdMap()->prepareUpdate();
$executable = new \Drupal\migrate_tools\MigrateExecutable($migration, new \Drupal\migrate\MigrateMessage());
$executable->import();

// MigrateExecutable takes an optional third argument where you can provide options including
// limit to limit the number of migrations to perform, and idlist to only migrate certain source
// IDs.
// Example: ['limit' => 10, 'idlist' => '1,2,3']

$migration = \Drupal::service('plugin.manager.migration')->createInstance('machine_name');
$migration->interruptMigration(\Drupal\migrate\Plugin\MigrationInterface::RESULT_STOPPED);

$migration = \Drupal::service('plugin.manager.migration')->createInstance('machine_name');
$migration->setStatus(\Drupal\migrate\Plugin\MigrationInterface::STATUS_IDLE);

use Drupal\migrate\MigrateExecutable;
use Drupal\migrate\MigrateMessage;

/**
 * Implements hook_preprocess_page().
 */
function example_module_preprocess_page(&$vars) {
  if ($qs = \Drupal::requestStack()->getCurrentRequest()->getQueryString()) {
   if (strpos($qs, 'start-migration') !== FALSE) {
      $migration_id = 'your_migration_id';
      $migration = \Drupal::service('plugin.manager.migration')->createInstance($migration_id);
      $executable = new MigrateExecutable($migration, new MigrateMessage());
      $executable->import();
    }
  }
}

$node = \Drupal::routeMatch()->getParameter('node');

$path = \Drupal::service('path.current')->getPath();

$path_args = explode('/', $path);

$route_name = \Drupal::service('current_route_match')->getRouteName();

$name = \Drupal::request()->query->get('name');

$name = \Drupal::request()->request->get('name');

$host = \Drupal::request()->getHost();

use Symfony\Component\HttpFoundation\RedirectResponse;
new RedirectResponse(\Drupal::url($route_name));

use Drupal\Core\StringTranslation\StringTranslationTrait;
class MyClass {
  use StringTranslationTrait;
}

\Drupal::service('module_installer')->install(['media']);
\Drupal::service('module_installer')->uninstall(['media']);

$nid = 1;
$entity_type = 'node';
$view_mode = 'teaser';
$view_builder = \Drupal::entityTypeManager()->getViewBuilder($entity_type);
$storage = \Drupal::entityTypeManager()->getStorage($entity_type);
$node = $storage->load($nid);
$build = $view_builder->view($node, $view_mode);
$output = render($build);

$view_builder = \Drupal::entityTypeManager()->getViewBuilder('node');
$storage = \Drupal::entityTypeManager()->getStorage('node');
$nid = 1;
$node = $storage->load($nid);
$view = $view_builder->viewField($node->get('body'), [
  'type' => 'string', // string, entity_reference_label
  'label' => 'hidden',
  'settings' => ['link' => FALSE],
]);
$output = render($view);

When you can't use render() then you need to do this:

\Drupal::service('renderer')->renderRoot($render_array);

// This is for Title Record entities, but any entity will do.
use Drupal\search_api\Plugin\search_api\datasource\ContentEntity;
use Drupal\omega_hub\Entity\TitleRecord;
$entity_ids = [507863, 509240, 513703, 515100, 536124, 537058, 541569];
$combine_id = function ($entity_id) {
  return $entity_id . ':und';
};
$update_ids = array_map($combine_id, $entity_ids);
$entity = TitleRecord::load(507863);
$indexes = ContentEntity::getIndexesForEntity($entity);
foreach ($indexes as $index) {
  $index->trackItemsUpdated('entity:title_record', $update_ids);
}

use Drupal\Core\Url;
$url = Url::fromRoute($route_name, $params);

use Drupal\Core\Url;
$url = Url::fromUri('internal:/mypath/to/style.css');

$url->setOptions([
  'attributes' => [
    'target' => '_blank',
  ],
]);

$my_link = \Drupal::service('link_generator')->generate($text, Url $url);

OR

use Drupal\Core\Link;
$renderable_link = Link::fromTextAndUrl($text, Url $url);

Link::createFromRoute($text, $route_name, ['arg1' => 'value'], ['attributes' => ['class' => 'use-ajax']]);

$link_render_array = $renderable_link->toRenderable();
$link_string = $renderable_link->toString();

$node = \Drupal::entityTypeManager()->getStorage('user')->load(23);

$account = \Drupal::currentUser();

$account = \Drupal::currentUser()->id();

use Drupal\views\Views;
$view = Views::getView('my_view');
$view->setDisplay('my_display');
$view->preExecute();
$view->execute();
$view->render();

// Get query object (only after executing the view).
$query = $view->query;

// Get view ID.
$view->id();

// Get current display.
$view->current_display();

// Get results (after execution).
$view->result;