I have read that when dealing with meetings in Exchange Server, the ItemID property can change because Exchange server recreates meeting appointments behind the scenes (I can not find that page again).

It is advized to use the UID (which since after Exchange 2007 SP1 contains the PidLidCleanGlobalObjectID) to identify the appointment.

I want to know in which circumstances (how often) Exchange server recreates meeting appointments - in order to estimate if it is worth rewriting our synchronization tool (or just accepting the occasional duplicate that will show up in our DB if we keep synchronising on ItemID).

Can anyone shed some light on this?

Notes:

- There may be ambiguity here because ItemID contains both ID and ChangeKey. Because I cannot find the article where this 'change' was mentioned I am unsure which of the two I'm "quoting". I have no issues with the ItemID.ChangeKey changing, I'm concerned about the ItemID.ID.

- I talk about 'normal' operations, not about data integrity issues that are corrected by the Exchange Calendar Repair Assistant or by The Outlook Calendar Checking Tool CalCheck

- Our code synchronizes only the default calendar folder for a user, so events moving between folders are not an issue.

• Edited by JanDoggen Friday, August 14, 2015 2:06 PM

As a general rule the advice has always been to not rely on the EWSId or the MAPI equivalent PR_EntryId see https://msdn.microsoft.com/en-us/library/office/dn605828(v=exchg.150).aspx (Identifiers in Exchange are opaque)

One definite situation that this will occur is documented in remarks section of https://msdn.microsoft.com/en-us/library/office/ff867625.aspx . So how often this happens depends on user behaviour and how often they update appointments.

Cheers
Glen