void MonitoredItemChangedEvent(object self, EasyUAMonitoredItemChangedEventArgs e) { 
      if ((e.Succeeded && e.Exception == null)) {
           // store the VQT
      } else {
           long handle = (long)e.Arguments.State;
           lock (this)
           {
               if (!failedSubscriptionHandles.Contains(handle))
               {
                   failedSubscriptionHandles.Add(handle); // these items will be unsubscribed
               }
           }
      }

void RefreshFailedSubscriptionsCallback(object sender, ElapsedEventArgs eargs)
        {
            lock (this)
            {
                try
                {
                    this.retryUnsubscribedItems.Enabled = false;
 
                    // Stop failed subscriptions
                    if (failedSubscriptionHandles.Count > 0)
                    {
                        StopMonitoringItems(failedSubscriptionHandles.ToList<long>());
                    }
 
                    // Re-subscribe items that are not subscribed yet (because of a subscription failure)
                    if (IsMonitoring)
                    {
                        IList<long> handlesToSubscribe = new List<long>();
                        foreach (KeyValuePair<long, DataItem.DataItem> dataItem in dataItems)
                        {
                            if (!dataItemHandleMappings.ContainsKey(dataItem.Key))
                            {
                                handlesToSubscribe.Add(dataItem.Key);
                            }
                        }
 
                        if (handlesToSubscribe.Count > 0)
                        {
                            StartMonitoringItems(handlesToSubscribe); // <<<<<<<<<<<< 
                        }
                    }
 
                }
                catch (Exception e)
                {
                    logger.Log("[OpcDataCollector] Exception: " + e.Message, Log.LogType.Error);
                }
 
                this.retryUnsubscribedItems.Enabled = true;
            }
        }

// Select data items to start monitoring (all items must be added to the dataItems list)
        private void StartMonitoringItems(IEnumerable<long> dataItemHandles)
        {
            IList<EasyUAMonitoredItemArguments> easyUAMonitoredItemArguments = new List<EasyUAMonitoredItemArguments>();
            foreach (long dataItemHandle in dataItemHandles)
            {
                DataItem.DataItem dataItem = dataItems[dataItemHandle];
                dataItem.SubscriptionParametersChangedEvent += DataItemSubscriptionParametersChanged;
                UANodeDescriptor nodeDescriptor = new UANodeDescriptor(dataItem.ToNodeID());
                easyUAMonitoredItemArguments.Add(
                    new EasyUAMonitoredItemArguments(MonitoredItemChangedEvent,
                        dataItemHandle,
                        endPointDescriptor,
                        nodeDescriptor,
                        new UAMonitoringParameters(dataItem.SamplingIntervalMs,
                                               new UADataChangeFilter(dataItem.DeadBandType, dataItem.DeadBandValue, UADataChangeTrigger.StatusValue)
                                               , 100 // queue size 
                                               )
                ));
            }
 
            int[] monitoredItemHandles = easyUAClient.SubscribeMultipleMonitoredItems(easyUAMonitoredItemArguments.ToArray()); // <<<<<< problem is here
            // ....
        }