Question

I have to watch five calendars parallel if any notification appears. It works fine but anytime after a few notifications or a period of time I got this Exception “The Watermark is invalid”.
I have a list of mailboxes which I want to watch ( only the Calendar Folder). The trigger starts every few seconds and the method look whether there is a notification or not.
If I create a few appointments and anytime the exception “The Watermark is invalid” was thrown. It appears in the Line where i get the Events.

 public Notification(ExchangeService _server1,  string[] _mailboxName1)
        {
            _server = _server1;
            _listOfList = _listOfList1;
            _mailboxName = _mailboxName1;
            foreach (Mailbox m in _mailboxName)
            {
                FolderId _Id = new FolderId(WellKnownFolderName.Calendar, m);
                PullSubscription pullSub = _server.SubscribeToPullNotifications(new FolderId[] { _Id }, 5, null, EventType.Copied, EventType.Created, EventType.Deleted, EventType.Modified, EventType.Moved);
                _subList.Add(pullSub);
            }
            System.Timers.Timer aTimer = new System.Timers.Timer();
            aTimer.Elapsed += new ElapsedEventHandler(OnTimedEvent);
            AppSettingsReader config = new AppSettingsReader();
            int time = Convert.ToInt16(config.GetValue("TimerInterval", typeof(string)));
            aTimer.Interval = time;
            aTimer.Enabled = false;
            aTimer.Start();

        }
     private static void OnTimedEvent(object source, ElapsedEventArgs e)
    {
        Console.WriteLine("Timer");
        bool noteWatch = false;
        foreach (PullSubscription p in _subList)
        {
            string type = null;
            ItemId eventId = null;
            //Exception the watermark is invalid!! 
            GetEventsResults events = p.GetEvents();

            foreach (ItemEvent itemEvent in events.ItemEvents)
            {

                switch (itemEvent.EventType)
                {
                    case EventType.Created:
                        noteWatch = true;
                        eventId = itemEvent.ItemId;
                        type = "Created";
                        break;
                }
            }
        }
     }

Answer 1

I found the reason:
My List of subscription was not thread safe.
I added this to my code and it works fine:

lock (_subList)
                {
                     events = p.GetEvents();
                }