Drupal - How do I redirect users and show a message?

Set the repeat parameter in drupal_set_message() to TRUE and it will work:

$response = new RedirectResponse($url);
$response->send();
drupal_set_message(t('My message after redirect'), 'status', TRUE);

I had this same issue, but using an exit to end the script is not a good idea. Drupal has shutdown functions that it runs at the end of a page load which can be important. Instead I just had to return the redirect response directly:

public function buildForm(array $form, FormStateInterface $form_state) {
  $wizard_data = $form_state->getTemporaryValue('wizard');

  if (empty($wizard_data['csv_data'])) {
    drupal_set_message($this->t('Please upload a file first.'), 'error');
    return $this->redirect('my.route')->send();
  }

Redirect:

// Drupal\Core\Url; // Symfony\Component\HttpFoundation\RedirectResponse; $url = Url::fromUri('internal:/'); $response = new RedirectResponse($url->toString()); $response->send();

Messeges using static service:

\Drupal::messenger()->addStatus($message);

Other equivalents:

addMessage(), addError(), addWarning().

References:

RedirectResponse

Url

Drupal::messenger

MessengerInterface