title: How to add to the Categories property of a (Calendar)Item in Exchange Web Services?
body: <p>The below SOAP call will update the <em>entire</em> list of categories (previous entries will be gone), but I want to add one or more.</p>
  
  <pre><code>&lt;soapenv:Envelope [snip]&gt;
     &lt;soapenv:Header&gt;
        &lt;typ:RequestServerVersion Version="Exchange2007_SP1"/&gt;
        &lt;typ:MailboxCulture&gt;en-US&lt;/typ:MailboxCulture&gt;
     &lt;/soapenv:Header&gt;
     &lt;soapenv:Body&gt;
        &lt;mes:UpdateItem ConflictResolution="AutoResolve" SendMeetingInvitationsOrCancellations="SendToNone"&gt;
           &lt;mes:ItemChanges&gt;
              &lt;typ:ItemChange&gt;
                 &lt;typ:ItemId Id="AQMk[snip]AAAA==" ChangeKey="DwA[snip]JPV57"/&gt;
                 &lt;typ:Updates&gt;
                    &lt;typ:SetItemField&gt;
                       &lt;typ:FieldURI FieldURI="item:Categories"/&gt;
                       &lt;typ:CalendarItem&gt;
                          &lt;typ:Categories&gt;
                            &lt;typ:String&gt;TIMETELL&lt;/typ:String&gt;
                          &lt;/typ:Categories&gt;
                       &lt;/typ:CalendarItem&gt;
                    &lt;/typ:SetItemField&gt;
                 &lt;/typ:Updates&gt;
              &lt;/typ:ItemChange&gt;
           &lt;/mes:ItemChanges&gt;
        &lt;/mes:UpdateItem&gt;
     &lt;/soapenv:Body&gt;
  &lt;/soapenv:Envelope&gt;
  </code></pre>
  
  <p>I tried AppendToItemField instead of SetItemField, but Categories is not a 'appendable' property (ErrorInvalidPropertyAppend - Only To, Cc, Bcc, Required, Optional, Resources, Body, and ReplyTo are).</p>
  
  <p>Is there a way to <em>add</em> Categories strings?</p>
  
  <p>Glen Scales' answer in <a href="http://social.msdn.microsoft.com/Forums/en-US/exchangesvrdevelopment/thread/3f57c2ae-fb97-4fca-997e-4150c0208b9b" rel="nofollow">this post</a> suggests that I would have to refresh the entire categories list but he's not sure</p>