Outlook VSTO AddIn How to avoid RaceOnRCWCleanup

Question

What I am trying to achieve is to also handle drag and drop events on the calendar properly using the AppProperty Change event on the Inspector:

I update the currentAppointmentItem whenever the user interacts with the interface (Explorer.SelectionChange, NewInspector, CloseInspector, etc.) I update the currentInspector whenever the user interacts with the interface (SelectionChange, NewInspector, CloseInspector) Update means that I try to set/unset the event handlers appropriately and to Marshal.ReleaseComObject accordingly. Finally to null the reference.

But when the user just clicks on an AppointmentItem in the calendar no Inspector window is created. Thus I wouldn't be able to catch AppPropertyChange Events. So I decided to call GetInspector on the selected AppointmentItem in case it is not null. I try to use this to receive changes on the AppProperty Event so I can handle drag and drop events on the calendar properly

The problem: From the Microsoft documentation I understand whenever you lose a reference to currentAppointmentItem you should also use Marshal.ReleaseComObject otherwise you risk other problems. Now I experience exceptions which I cannot catch: RaceOnRCWCleanup ... it seems that I try to release a COM object which is still in use (probably by Outlook). How can I avoid that? Is it correct to Marshal.ReleaseComObject(currentAppointmentItem)

I registered on the SelectionChange Event on the Outlook.Explorer. In there I try to register the currentAppointment with:

[...]
            log.Info("Selection_Change");
            if (currentExplorer == null)
            {
                return;
            }

            try
            {
                log.Info("Selection_Change: " + currentExplorer.Caption);
                Outlook.MAPIFolder selectedFolder = currentExplorer.CurrentFolder;
                if (currentExplorer.Selection.Count > 0)
                {
                    Object selObject = currentExplorer.Selection[1];
                    if (selObject is Outlook.AppointmentItem)
                    {
                        currentAppointmentItem = (Outlook.AppointmentItem)selObject;
                        Inspectors_NewInspector(currentAppointmentItem.GetInspector);                     
                    }

[...]

Please Note: INspectors_NewInspector is also called on the Inspectors Collection.

The code of NewInspector is like

    void Inspectors_NewInspector(Microsoft.Office.Interop.Outlook.Inspector Inspector)
            {
                try
                {
                    log.Info("Inspectors_NewInspector");
                    //  This function (apparently) gets kicked off whenever a user opens a new or existing item
                    //  in Outlook (Calendar appointment, Email, etc).  
                    //  We can intercept it, modify it's properties, before letting our Ribbon know about its existance.
                    //            
                    if (Inspector != null)
                    {
                        log.Info("Inspectors_NewInspector: " + Inspector.Caption);
                        unregisterCurrentInspector();
                        currentInspector = Inspector;

                        object item = Inspector.CurrentItem;
                        if (item == null)
                            return;

                        if (!(item is Outlook.AppointmentItem))
                            return;
                        unregisterCurrentAppointmentItem();
                        currentAppointmentItem = (Outlook.AppointmentItem)item;
                        currentAppointmentItem.PropertyChange += AppPropertyChanged; // Handle situations where the 
                                                                                     // user tries to convert an appointment w/ an agreedo protocol to a recurring appointment. 
                                                                                     // This needs to be avoided .                
                        currentAppointmentItem.CustomPropertyChange += AppPropertyChanged;
                    }
                    ((Microsoft.Office.Interop.Outlook.InspectorEvents_10_Event)Inspector).Close += Inspector_Close;

                } catch (Exception ex)
                {
                    log.Error(ex.Message);
                }
            }

the unregisterCurrentApppointmentItem :

private void unregisterCurrentAppointmentItem()
    {
        try
        {
            log.Info("unregisterCurrentAppointmentItem");
            if (currentAppointmentItem != null)
            {
                currentAppointmentItem.PropertyChange -= AppPropertyChanged; // Handle situations where the 
                currentAppointmentItem.CustomPropertyChange -= AppPropertyChanged;

                Marshal.ReleaseComObject(currentAppointmentItem);
                currentAppointmentItem = null;
            }
        } catch (Exception ex)
        {
            log.Error(ex.Message);
        }
    }

the unregisterCurrentInspector:

    private void unregisterCurrentInspector()
            {
                log.Info("unregisterCurrentInspector");
                if (currentInspector != null)
                {
                    ((Microsoft.Office.Interop.Outlook.InspectorEvents_10_Event)currentInspector).Close -= Inspector_Close;
                    Marshal.ReleaseComObject(currentInspector);
                    currentInspector = null;
                }
            }

Any advice on this?

What I already tried / taken into account:

Eugene Astafiev Eugene Astafiev · Accepted Answer · 2020-05-17T10:44:01

First of all, there is no need to simulate the NewInspector event. Instead, you need to set up event handlers correctly. It seems you just need to implement an inspector or explorer wrappers. See Implement a wrapper for inspectors and track item-level events in each inspector for more information.

it seems that I try to release a COM object which is still in use (probably by Outlook). How can I avoid that? Is it correct to Marshal.ReleaseComObject(currentAppointmentItem)

Yes, it is. But you should really use this method against objects retrieved in your code by calling properties and methods. You SHOULD NOT release objects passed as parameters by the Office applications. Take a look a the When to release COM objects in Office add-ins developed in .NET article which explains possible pitfalls and give answers to the most widely spread questions.

Outlook VSTO AddIn How to avoid RaceOnRCWCleanup

2 Answers