Outlook VSTO add-in provokes "Would you like to save changes" dialog

Question

I have a bare bone VSTO add-in. This is the entire code:

public partial class ThisAddIn
    {
        private Outlook.Explorer _currentExplorer;
        private void ThisAddIn_Startup(object sender, System.EventArgs e)
        {
        }

        private void ThisAddIn_Shutdown(object sender, System.EventArgs e)
        {

        }

        #region VSTO generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InternalStartup()
        {
            this.Startup += new System.EventHandler(ThisAddIn_Startup);
            this.Shutdown += new System.EventHandler(ThisAddIn_Shutdown);
            _currentExplorer = this.Application.ActiveExplorer();
            _currentExplorer.SelectionChange += CurrentExplorer_SelectionChange;
        }

        private void CurrentExplorer_SelectionChange()
        {
            Outlook.MAPIFolder selectedFolder = _currentExplorer.CurrentFolder;
            if (_currentExplorer.Selection.Count > 0)
            {
                var selObject = _currentExplorer.Selection[1];
                if (selObject is Outlook.MailItem mailItem)
                {
                    var property = mailItem.Sender;
                }

            }

        }

        #endregion
    }

The following bug occurs with this add-in installed:

1. select an email in the explorer
2. right click it and do "junk mail" --> "block user"
3. close Outlook

On closing Outlook a dialog "Would you like to save changes" pops up. (What changes ??)
If I comment out the line var property = mailItem.Sender; above, this does not occur anymore.
Also it does not seem to matter what property of the mailItem I read. It happens with pretty much every property.

Any suggestions?

Answer 1

This doesn't happens if you release all underlying COM objects timely. So, I'd recommend starting from releasing underlying COM objects instantly using the Marshal.ReleaseComObject method. Then set a variable to Nothing in Visual Basic (null in C#) to release the reference to the object. Read more about that in the Systematically Releasing Objects article.

Answer 2

Hi Eugene Astafiev

Thank you for your reply.

private void CurrentExplorer_SelectionChange()
        {
            Outlook.MAPIFolder selectedFolder = _currentExplorer.CurrentFolder;
            if (_currentExplorer.Selection.Count > 0)
            {
                var selObject = _currentExplorer.Selection[1];
                if (selObject is Outlook.MailItem mailItem)
                {
                    var property = mailItem.Sender;
                    Marshal.ReleaseComObject(property );
                    property = null;
                }

            }

        }

unfortunately does not do the trick.

Answer 3

This works !

private void OnSelectionChange()
        {
            Selection selection = null;
            MailItem mailItem = null;
            AddressEntry adr = null;
            try
            {
                selection = _explorer.Selection;
                if (selection.Count == 1)
                {
                    mailItem = selection[1] as MailItem;
                    if (mailItem != null)
                    {
                        adr = mailItem.Sender;
                    }
                }
            }
            finally
            {
                if (selection != null) Marshal.ReleaseComObject(selection.UnderlyingObject);
                if (mailItem != null) Marshal.ReleaseComObject(mailItem.UnderlyingObject);
                if (adr != null) Marshal.ReleaseComObject(adr.UnderlyingObject);
            }
        }