VSTO - Outlook 2007 How do I show a user property even when it's not present?

I have added user property in a mail folder, let's call it UserProperty01. Some of the folder items have this property, some don't. I need to show in the folder's view if that property is set or not, so I added another property called UserProperty01Present, and I set it to true when I set the original property and false when I deleted it; then I added it to the folder's view.

The problem is that the little checkbox appears checked in those mails where the property is set, blank in those where the property was deleted... and nowhere at all in those where the property was never set in the first place.

My question is, is there开发者_StackOverflow社区 any way to show the blank checkbox when the property is not set? The only alternative I found so far is looking in every mail item and setting it to false when it's not set every time the folder is opened; needless to say, I'm not very happy with this solution (there may be more than 800 mails in there). Is there any other way?

Thank you for your time.

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EDIT There is no way to do it via the OOM.

I think a way to do it would be to add a foumula colunm, and in the formula do a

IIf([UserProperty01],"YES","NO")

Marcus