title: CRM 2011 Ribbon DisplayRule ValueRule check for null
body: <p>Quick one, I don't see any direct answers out there so I thought I'd ask. (Here is the <a href="http://social.microsoft.com/Forums/eu/crmdevelopment/thread/44d1187d-547d-49c5-a734-d4ad347a3a40" rel="noreferrer">same question asked on the microsoft social site</a> and it is unanswered as well)</p>
  
  <p>If I have a DisplayRule and a ValueRule in that, how can I check for null? This suggested solution below apparently doesn't work (according to the linked question):</p>
  
  <pre><code>  &lt;DisplayRule Id="new.incident.form.IsNotRelatedToY.DisplayRule"&gt;
      &lt;ValueRule Field="YId" Value="null" InvertResult="true" /&gt;
    &lt;/DisplayRule&gt;
  </code></pre>
  
  <p>Does it depend on the type of Field? If so I need it to work in two seperate cases, when YId is an entity reference, and also when YId is a string. I figure "null" cannot work for a string, because "null" is a valid string.</p>
  
  <p>Anyone have a working example?</p>
  
  <p>EDIT: it looks like the <a href="http://msdn.microsoft.com/en-us/library/gg334317.aspx" rel="noreferrer">ValueRule documentation</a> has nothing on the subject.</p>