delete a config.php file located at bootstrap->cache->config.php [It is working for me.]

use Modules\User\Contracts\Authentication;
class DashboardController extends AdminBaseController
{
/**
* @var Authentication
*/
private $auth;

public function __construct( Authentication $auth)
{        
    $this->auth = $auth;
}

public function index()
{
$logged_in_user= $this->auth->user();
return $logged_in_user;
}
}