C# Outlook Add-in: How can I delete a User-defined property programmatically?

Question

I have tried finding an answer to this question practically everywhere I could imagine, including here on StackOverflow. Unfortunately to no avail. So here it is.

I'm working on an Outlook Add-in (with Outlook 2021), and have developed some code that creates some ItemProperties specifically for use with that add-in. Now, when those properties are created, I can see them when I go to View->Settings->Advanced View Settings->Columns, as illustrated in the screenshot.

Screenshot of User-defined fields in Outlook

In some cases, though, I want to completely delete the properties. And as I know how to do that manually, as pointed out in the figure, I can't find out how to do that programmatically via C#. I have gone that far as to remove the properties from each mail containing that kind of property, like this:

IEnumerable<MailItem> listOfAssignedEmails = itemsToProcess.Where(
    t => t.ItemProperties[MailExpiration.ExpirationDatePropertyName] != null);
foreach (MailItem email in listOfAssignedEmails)
{
    // Note: The Delete() operation is deprecated. A more up-to-date method must be found.
    email.ItemProperties[MailExpiration.ExpirationDatePropertyName].Delete();
    email.Save();
}

... and yes, I know that the Delete() operation is deprecated; however, I couldn't find another method for removing the ItemProperty from the email (any suggestions are welcome).

Basically, the deletion of this Property is only going to be done very rarely (t. ex. if the user chooses to uninstall the Add-in. However, if there's any way to remove that property automatically, I would be happy to know.

Any suggestions will be greatly appreciated.

Answer 1

It is really a bad idea to remove a custom property from all emails that already have it: there is really no point since the user will never see them, but you will have to retouch (and thus change the last modified date) of a large number of emails.

Also note that named properties in MAPI are a finite resource - you can have at most 64k of them in a mailbox. Once a particular property mapping is used, you can never unmap it, even if there are no items that use that property.

Thirdly, doing anything Outlook related from an installer (rather than a VSTO addin) is a really bad idea - Windows installer runs in a service.

If you want to make sure the user no longer sees your custom fields as available properties in a view, you need to deal with the folder fields - they ar stored in a blob in a hidden (associated) message in that folder. OOM does not expose folder fields at all (if you don't count the AddToFolderFields parameter when calling UserProperties.Add). If using Redemption is an option (I am its author), it exposed RDOFolderFields object (accessible from RDOFolder2.FolderFields property) that allows to add or delete folder fields.