[SOLVED] Are some MAPI named properties known and thus always return the same Tag when using GetIDsFromNames with MAPI

Are some MAPI named properties known and thus always return the same Tag when using GetIDsFromNames with MAPI_CREATE?

In my c# VSTO Outlook project I am using the GetNamesFromIDs to get a property name from one message and then adding it to a second message with GetIDsFromNames with the MAPI_CREATE flag set so that the property is added. I then add the value.

My understanding of GetIDsFromNames is that when it sets a "new" property then a new and unique tag is generated. This is done so that tags do not clash.

Is it possible on a newly created message that setting a named property with GetIDsFromNames that MAPI gives it the same Tag number it has had on previous messages?
Am I right that GetNamesFromIDs accepts, and GetIDsFromNames returns, only the the upper 2 byte property id and to use what I get from GetIDsFromNames I need to add back the type?

No.2 I cannot seem to read anywhere.

Solution

Yes, that is exactly how it works - once a mapping it created, is stays the same for the lifetime of the parent message store. You can see existing mapping in OutlookSpy (I am its author) - click IMsgStore button (or IMAPIFolder or IMessage) and then GetNamesFromIDs button.
Yes, you always need to set the type. Keep in mind that once a property is used (i.e. used to set a property) with a particular type (unless it is PT_STRING8 vs PT_UNICODE), you will not be able to use that property with a different type.