diff --git a/modules/simple_fb_connect_rules/simple_fb_connect_rules.info.yml b/modules/simple_fb_connect_rules/simple_fb_connect_rules.info.yml
index ef68817..fb45d80 100755
--- a/modules/simple_fb_connect_rules/simple_fb_connect_rules.info.yml
+++ b/modules/simple_fb_connect_rules/simple_fb_connect_rules.info.yml
@@ -3,6 +3,7 @@ type: module
 description: 'Integrates Simple FB Connect with Rules.'
 package: Authentication
 core: 8.x
+core_version_requirement: ^8 || ^9
 dependencies:
   - simple_fb_connect
   - rules
diff --git a/simple_fb_connect.info.yml b/simple_fb_connect.info.yml
index f79fd72..5cc315a 100644
--- a/simple_fb_connect.info.yml
+++ b/simple_fb_connect.info.yml
@@ -3,4 +3,5 @@ type: module
 description: 'Allows users to authenticate with their Facebook accounts.'
 package: Authentication
 core: 8.x
+core_version_requirement: ^8 || ^9
 configure: simple_fb_connect.settings_form
diff --git a/simple_fb_connect.services.yml b/simple_fb_connect.services.yml
index 04ed4c7..0f4e28f 100644
--- a/simple_fb_connect.services.yml
+++ b/simple_fb_connect.services.yml
@@ -20,6 +20,7 @@ services:
       - '@token'
       - '@transliteration'
       - '@language_manager'
+      - '@file_system'
 
   simple_fb_connect.post_login_manager:
     class: Drupal\simple_fb_connect\SimpleFbConnectPostLoginManager
diff --git a/src/Controller/SimpleFbConnectController.php b/src/Controller/SimpleFbConnectController.php
index b3fead8..8998001 100644
--- a/src/Controller/SimpleFbConnectController.php
+++ b/src/Controller/SimpleFbConnectController.php
@@ -69,7 +69,7 @@ class SimpleFbConnectController extends ControllerBase {
   public function redirectToFb() {
     // Try to get an instance of Facebook service.
     if (!$facebook = $this->fbFactory->getFbService()) {
-      drupal_set_message($this->t('Simple FB Connect is not configured properly. Please contact site administrator.'), 'error');
+      $this->messenger()->addMessage($this->t('Simple FB Connect is not configured properly. Please contact site administrator.'), 'error');
       return $this->redirect('user.login');
     }
 
@@ -100,7 +100,7 @@ class SimpleFbConnectController extends ControllerBase {
   public function returnFromFb() {
     // Try to get an instance of Facebook service.
     if (!$facebook = $this->fbFactory->getFbService()) {
-      drupal_set_message($this->t('Simple FB Connect is not configured properly. Please contact site administrator.'), 'error');
+      $this->messenger()->addMessage($this->t('Simple FB Connect is not configured properly. Please contact site administrator.'), 'error');
       return $this->redirect('user.login');
     }
 
@@ -109,17 +109,17 @@ class SimpleFbConnectController extends ControllerBase {
 
     // Read user's access token from Facebook.
     if (!$access_token = $this->fbManager->getAccessTokenFromFb()) {
-      drupal_set_message($this->t('Facebook login failed.'), 'error');
+      $this->messenger()->addMessage($this->t('Facebook login failed.'), 'error');
       return $this->redirect('user.login');
     }
 
     // Check that user authorized our app to access user's email address.
     if (!$this->fbManager->checkPermission('email')) {
       if ($site_name = $this->config('system.site')->get('name')) {
-        drupal_set_message($this->t('Facebook login failed. @site_name requires permission to get your email address from Facebook. Please try again and give the permission.', ['@site_name' => $site_name]), 'error');
+        $this->messenger()->addMessage($this->t('Facebook login failed. @site_name requires permission to get your email address from Facebook. Please try again and give the permission.', ['@site_name' => $site_name]), 'error');
       }
       else {
-        drupal_set_message($this->t('Facebook login failed. This site requires permission to get your email address from Facebook. Please try again and give the permission.'), 'error');
+        $this->messenger()->addMessage($this->t('Facebook login failed. This site requires permission to get your email address from Facebook. Please try again and give the permission.'), 'error');
       }
       $this->persistentDataHandler->set('reprompt', TRUE);
       return $this->redirect('user.login');
@@ -127,13 +127,13 @@ class SimpleFbConnectController extends ControllerBase {
 
     // Get user's FB profile from Facebook API.
     if (!$fb_profile = $this->fbManager->getFbProfile()) {
-      drupal_set_message($this->t('Facebook login failed, Facebook profile could not be loaded. Please contact site administrator.'), 'error');
+      $this->messenger()->addMessage($this->t('Facebook login failed, Facebook profile could not be loaded. Please contact site administrator.'), 'error');
       return $this->redirect('user.login');
     }
 
     // Get user's email from the FB profile.
     if (!$email = $this->fbManager->getEmail($fb_profile)) {
-      drupal_set_message($this->t('Facebook login failed. This site requires an email address. Please add one in your Facebook profile and try again.'), 'error');
+      $this->messenger()->addMessage($this->t('Facebook login failed. This site requires an email address. Please add one in your Facebook profile and try again.'), 'error');
       return $this->redirect('user.login');
     }
 
@@ -162,7 +162,7 @@ class SimpleFbConnectController extends ControllerBase {
       if ($this->userManager->loginUser($drupal_user)) {
         // Check if new users should be redirected to Drupal user form.
         if ($this->postLoginManager->getRedirectNewUsersToUserFormSetting()) {
-          drupal_set_message($this->t("Please take a moment to confirm your account details. Since you logged in with Facebook, you don't need to update your password."));
+          $this->messenger()->addMessage($this->t("Please take a moment to confirm your account details. Since you logged in with Facebook, you don't need to update your password."));
           return new RedirectResponse($this->postLoginManager->getPathToUserForm($drupal_user));
         }
 
@@ -174,7 +174,7 @@ class SimpleFbConnectController extends ControllerBase {
         // New user was created but the account is pending approval.
         // Unset access token from session.
         $this->persistentDataHandler->set('access_token', NULL);
-        drupal_set_message($this->t('You will receive an email when a site administrator activates your account.'), 'warning');
+        $this->messenger()->addMessage($this->t('You will receive an email when a site administrator activates your account.'), 'warning');
         return $this->redirect('user.login');
       }
     }
diff --git a/src/SimpleFbConnectUserManager.php b/src/SimpleFbConnectUserManager.php
index e0973cb..6dbae86 100644
--- a/src/SimpleFbConnectUserManager.php
+++ b/src/SimpleFbConnectUserManager.php
@@ -17,13 +17,15 @@ use Drupal\Core\Transliteration\PhpTransliteration;
 use Symfony\Component\EventDispatcher\EventDispatcherInterface;
 use Symfony\Component\EventDispatcher\GenericEvent;
 use Drupal\Core\Entity\EntityStorageException;
-use Drupal\Component\Utility\Unicode;
+use Drupal\Core\File\FileSystemInterface;
+use Drupal\Core\Messenger\MessengerTrait;
 
 /**
  * Contains all logic that is related to Drupal user management.
  */
 class SimpleFbConnectUserManager {
   use StringTranslationTrait;
+  use MessengerTrait;
 
   protected $configFactory;
   protected $loggerFactory;
@@ -33,6 +35,7 @@ class SimpleFbConnectUserManager {
   protected $token;
   protected $transliteration;
   protected $languageManager;
+  protected $fileSystem;
 
   /**
    * Constructor.
@@ -55,8 +58,10 @@ class SimpleFbConnectUserManager {
    *   Used for user picture directory and file transiliteration.
    * @param \Drupal\Core\Language\LanguageManagerInterface $language_manager
    *   Used for detecting the current UI language.
+   * @param \Drupal\Core\File\FileSystemInterface $file_system
+   *   Provides helpers to operate on files and stream wrappers.
    */
-  public function __construct(ConfigFactoryInterface $config_factory, LoggerChannelFactoryInterface $logger_factory, TranslationInterface $string_translation, EventDispatcherInterface $event_dispatcher, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager, Token $token, PhpTransliteration $transliteration, LanguageManagerInterface $language_manager) {
+  public function __construct(ConfigFactoryInterface $config_factory, LoggerChannelFactoryInterface $logger_factory, TranslationInterface $string_translation, EventDispatcherInterface $event_dispatcher, EntityTypeManagerInterface $entity_type_manager, EntityFieldManagerInterface $entity_field_manager, Token $token, PhpTransliteration $transliteration, LanguageManagerInterface $language_manager, FileSystemInterface $file_system) {
     $this->configFactory      = $config_factory;
     $this->loggerFactory      = $logger_factory;
     $this->stringTranslation  = $string_translation;
@@ -66,6 +71,7 @@ class SimpleFbConnectUserManager {
     $this->token              = $token;
     $this->transliteration    = $transliteration;
     $this->languageManager    = $language_manager;
+    $this->fileSystem         = $file_system;
   }
 
   /**
@@ -188,7 +194,7 @@ class SimpleFbConnectUserManager {
 
     // Validate the new user.
     $violations = $new_user->validate();
-    if (count($violations) > 0) {
+    if (!is_null($violations) && count($violations) > 0) {
       $property = $violations[0]->getPropertyPath();
       $msg      = $violations[0]->getMessage();
       $this->drupalSetMessage($this->t('Error while creating user account. Please contact site administrator.'), 'error');
@@ -312,7 +318,7 @@ class SimpleFbConnectUserManager {
     // Truncate to max length. We use hard coded length because using
     // USERNAME_MAX_LENGTH cause unit tests to fail.
     $max_length = 60;
-    $fb_name = Unicode::substr($fb_name, 0, $max_length);
+    $fb_name = mb_substr($fb_name, 0, $max_length);
 
     // Add a trailing number if needed to make username unique.
     $base = $fb_name;
@@ -322,7 +328,7 @@ class SimpleFbConnectUserManager {
       $i++;
       // Calculate max length for $base and truncate if needed.
       $max_length_base = $max_length - strlen((string) $i) - 1;
-      $base = Unicode::substr($base, 0, $max_length_base);
+      $base = mb_substr($base, 0, $max_length_base);
       $candidate = $base . " " . $i;
     }
 
@@ -565,7 +571,7 @@ class SimpleFbConnectUserManager {
    * @see file_prepare_directory
    */
   protected function filePrepareDirectory(&$directory, $options) {
-    return file_prepare_directory($directory, $options);
+    return $this->fileSystem->prepareDirectory($directory, $options);
   }
 
   /**
@@ -591,7 +597,7 @@ class SimpleFbConnectUserManager {
    * @see drupal_set_message
    */
   protected function drupalSetMessage($message = NULL, $type = 'status', $repeat = FALSE) {
-    return drupal_set_message($message, $type, $repeat);
+    return $this->messenger()->addMessage($message, $type, $repeat);
   }
 
   /**
diff --git a/tests/src/Unit/SimpleFbConnectFbFactoryTest.php b/tests/src/Unit/SimpleFbConnectFbFactoryTest.php
index d2f48e6..dfd929f 100644
--- a/tests/src/Unit/SimpleFbConnectFbFactoryTest.php
+++ b/tests/src/Unit/SimpleFbConnectFbFactoryTest.php
@@ -19,10 +19,10 @@ class SimpleFbConnectFbFactoryTest extends UnitTestCase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
-    $this->loggerFactory = $this->getMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
+    $this->loggerFactory = $this->createMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
 
     $this->persistentDataHandler = $this->getMockBuilder('Drupal\simple_fb_connect\SimpleFbConnectPersistentDataHandler')
       ->disableOriginalConstructor()
diff --git a/tests/src/Unit/SimpleFbConnectFbManagerTest.php b/tests/src/Unit/SimpleFbConnectFbManagerTest.php
index b81630b..9a146e9 100644
--- a/tests/src/Unit/SimpleFbConnectFbManagerTest.php
+++ b/tests/src/Unit/SimpleFbConnectFbManagerTest.php
@@ -22,16 +22,16 @@ class SimpleFbConnectFbManagerTest extends UnitTestCase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
-    $this->loggerFactory = $this->getMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
+    $this->loggerFactory = $this->createMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
 
-    $this->eventDispatcher = $this->getMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
+    $this->eventDispatcher = $this->createMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
 
-    $this->entityFieldManager = $this->getMock('Drupal\Core\Entity\EntityFieldManagerInterface');
+    $this->entityFieldManager = $this->createMock('Drupal\Core\Entity\EntityFieldManagerInterface');
 
-    $this->urlGenerator = $this->getMock('Drupal\Core\Routing\UrlGeneratorInterface');
+    $this->urlGenerator = $this->createMock('Drupal\Core\Routing\UrlGeneratorInterface');
 
     $this->persistentDataHandler = $this->getMockBuilder('Drupal\simple_fb_connect\SimpleFbConnectPersistentDataHandler')
       ->disableOriginalConstructor()
diff --git a/tests/src/Unit/SimpleFbConnectPersistentDataHandlerTest.php b/tests/src/Unit/SimpleFbConnectPersistentDataHandlerTest.php
index ada0a31..ea4425b 100644
--- a/tests/src/Unit/SimpleFbConnectPersistentDataHandlerTest.php
+++ b/tests/src/Unit/SimpleFbConnectPersistentDataHandlerTest.php
@@ -17,10 +17,10 @@ class SimpleFbConnectPersistentDataHandlerTest extends UnitTestCase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
-    $this->session = $this->getMock('Symfony\Component\HttpFoundation\Session\SessionInterface');
+    $this->session = $this->createMock('Symfony\Component\HttpFoundation\Session\SessionInterface');
 
     $this->persistentDataHandler = new SimpleFbConnectPersistentDataHandler(
       $this->session
diff --git a/tests/src/Unit/SimpleFbConnectPostLoginManagerTest.php b/tests/src/Unit/SimpleFbConnectPostLoginManagerTest.php
index 8e844de..6b97b05 100644
--- a/tests/src/Unit/SimpleFbConnectPostLoginManagerTest.php
+++ b/tests/src/Unit/SimpleFbConnectPostLoginManagerTest.php
@@ -20,7 +20,7 @@ class SimpleFbConnectPostLoginManagerTest extends UnitTestCase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
     $this->configFactory = $this->getConfigFactoryStub(
@@ -31,7 +31,7 @@ class SimpleFbConnectPostLoginManagerTest extends UnitTestCase {
       ]
     );
 
-    $this->requestContext = $this->getMock('Drupal\Core\Routing\RequestContext');
+    $this->requestContext = $this->createMock('Drupal\Core\Routing\RequestContext');
 
     $this->pathValidator = $this->getMockBuilder('Drupal\Core\Path\PathValidatorInterface')
       ->disableOriginalConstructor()
diff --git a/tests/src/Unit/SimpleFbConnectUserManagerTest.php b/tests/src/Unit/SimpleFbConnectUserManagerTest.php
index 6823537..c2f7877 100644
--- a/tests/src/Unit/SimpleFbConnectUserManagerTest.php
+++ b/tests/src/Unit/SimpleFbConnectUserManagerTest.php
@@ -24,7 +24,7 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
   /**
    * {@inheritdoc}
    */
-  protected function setUp() {
+  protected function setUp(): void {
     parent::setUp();
 
     $this->configFactory = $this->getConfigFactoryStub(
@@ -42,11 +42,11 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
       ]
     );
 
-    $this->loggerFactory = $this->getMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
+    $this->loggerFactory = $this->createMock('Drupal\Core\Logger\LoggerChannelFactoryInterface');
 
-    $this->stringTranslation = $this->getMock('Drupal\Core\StringTranslation\TranslationInterface');
+    $this->stringTranslation = $this->createMock('Drupal\Core\StringTranslation\TranslationInterface');
 
-    $this->eventDispatcher = $this->getMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
+    $this->eventDispatcher = $this->createMock('Symfony\Component\EventDispatcher\EventDispatcherInterface');
 
     $this->entityTypeManager = $this->getMockBuilder('Drupal\Core\Entity\EntityTypeManagerInterface')
       ->setMethods(['load'])
@@ -66,6 +66,9 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
     $this->languageManager = $this->getMockBuilder('Drupal\Core\Language\LanguageManagerInterface')
       ->getMock();
 
+    $this->fileSystem = $this->getMockBuilder('Drupal\Core\File\FileSystemInterface')
+      ->getMock();
+
     // Note that we are creating an instance of TestSimpleFbConnectUserManager
     // instead of SimpleFbConnectUserManager. The test class overriders the
     // wrapper methods that call procedural Drupal functions.
@@ -78,7 +81,8 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
       $this->entityFieldManager,
       $this->token,
       $this->transliteration,
-      $this->languageManager
+      $this->languageManager,
+      $this->fileSystem
     );
   }
 
@@ -402,7 +406,7 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
   public function testDownloadProfilePicWhenDirectoryNotWriteable() {
     $picture_directory = 'not/writeable/directory';
 
-    $field_definition = $this->getMock('Drupal\Core\Field\FieldDefinitionInterface');
+    $field_definition = $this->createMock('Drupal\Core\Field\FieldDefinitionInterface');
 
     $field_definition
       ->expects($this->once())
@@ -450,7 +454,7 @@ class SimpleFbConnectUserManagerTest extends UnitTestCase {
   public function testSetProfilePic() {
     $picture_directory = 'writeable/directory';
 
-    $field_definition = $this->getMock('Drupal\Core\Field\FieldDefinitionInterface');
+    $field_definition = $this->createMock('Drupal\Core\Field\FieldDefinitionInterface');
 
     $field_definition
       ->expects($this->once())