public static bool IsCheckMessage(this QueuedNotification queuedNotification, string identifier)
        {
            if (!string.IsNullOrEmpty(identifier))
            {
                return(queuedNotification.ListOfObserverIdentifier.Any(e => e == identifier));
            }

            throw new ArgumentNullException(identifier);
        }
 public static void AddObserver(this QueuedNotification queuedNotification, string identifier)
 {
     if (!string.IsNullOrEmpty(identifier))
     {
         if (!queuedNotification.ListOfObserverIdentifier.Any(e => e == identifier))
         {
             List <string> identifiers = queuedNotification.ListOfObserverIdentifier.ToList();
             identifiers.Add(identifier);
             queuedNotification.ObserverIdentifier = string.Join(",", identifiers);
         }
     }
 }