namespace Drupalmodule_namePluginBlock;

    use DrupalblockBlockBase;
    use DrupalCoreSessionAccountInterface;

    /**
     * Provides a 'Demo' block.
     *
     * @Block(
     *   id = "demo_block",
     *   admin_label = @Translation("Demo block"),
     * )
     */

    class MyCustomblockName extends BlockBase {

      /**
       * {@inheritdoc}
       */
      public function build() {
        return array(
          '#markup' => $this->t('Hello World!'),
        );
      }

      /**
       * {@inheritdoc}
       */

  public function access(AccountInterface $account) {
    return $account->hasPermission('access content');
  }

}