public async Task <IActionResult> AddNotificationList(IEnumerable <UserNotificationDTO> userNotifications)
        {
            IEnumerable <UserNotificationDTO> AddedUserNotifications;

            try
            {
                AddedUserNotifications = await _notificationService.AddListUserNotificationAsync(userNotifications);
            }
            catch (InvalidOperationException)
            {
                return(BadRequest());
            }

            var tasks = GetOnlineUserFromList(AddedUserNotifications).Select(un => SendPrivateNotification(un));
            await Task.WhenAll(tasks);

            return(NoContent());
        }