title: What do I use for ChangeKey in EWS UpdateItem?
body: <p>I'm trying to use Exchange Web Services to update a calendar item.  I'm creating an ItemChangeType, and then an ItemIdType.  I have a unique ID to use for ItemIdType.Id, but I have nothing to use for the ChangeKey.  When I leave it out, I get an ErrorChangeKeyRequiredForWriteOperations.  But when i try to just put something in there, I get an ErrorInvalidChangeKey.  </p>
  
  <p>What can I use for this to get it to work?</p>
  
  <p>I'm also trying to determine what is the best implementation of BaseItemIdType to use for ItemChangeType.Item.  So far, I'm using ItemIdType, and I'm guessing that's correct, but I haven't been able to find any particularly helpful documentation on this.</p>