title: Outlook AddIn: How to change the date of an AppointmentItem in Outlook with an Email (ICS) sent to Outlook? Possible at all?
body: <p>I work on an Outlook Addin in C# which interacts with a service also accessible via a web browser.
  Now to create an appointment the user may either choose to create it in Outlook or on the web.
  Now I ran into the following problem:</p>
  
  <ol>
  <li>The user creates an appointment in Outlook and adds it to the web using the Outlook plugin</li>
  <li>Now the user logs into the web interface and changes the information
  stored there. For example, he reschedules it. After that, he uses the
  functionality to send out new invitations (using mail + ICS
  attachments) </li>
  <li>In the end, the same appointment now appears twice in the calendar.</li>
  </ol>
  
  <p>How can I prepare an invitation mail so Outlook recognizes this as an UPDATE to the existing appointment rather than as a new appointment?</p>
  
  <p>My guess is:</p>
  
  <ol>
  <li><p>Outlook looks at the UID in the ICS attachment </p></li>
  <li><p>Outlook checks who is sending the invitation. If the sender != the
  owner of the current appointment, Outlook will reject any change to
  the existing appointment</p></li>
  </ol>
  
  <p>Is this correct?</p>
  
  <p>So I would end up in analyzing any incoming MailItems and thus extracting the ICS attachment, then parsing it, and then find the correct AppointentItem in Outlook using the UID (=GlobalAppointmentID) and then change it programmatically. As I understand this is not so easy as you cannot find an AppointmentItem using an GlobalAppointmentID unless you use Redemption or other APIs (ExtendedMAPI?) </p>