title: TFS Work Item type - Access to field based on rights
body: <p><br>
   I have work item type template where i want to assign right to change drop down list to
  default value as "Requested" for new item and every one can see and read it.
  If user is in group "[TEAM FOUNDATION]\Approvers" or "[TEAM FOUNDATION]\Developers" they are able to change this item.</p>
  
  <p>I have a problem if user is not in either of the groups it will fail and blank display disabled field.
  How do i define these permissions?</p>
  
  <p>My definition of field is below.</p>
  
  <pre><code>&lt;FIELD name="Approval" refname="Approval" type="String" reportable="dimension"&gt;
      &lt;REQUIRED /&gt;
      &lt;DEFAULT from="value" value="Requested" /&gt;
      &lt;ALLOWEDVALUES&gt;
        &lt;LISTITEM value="Approved" /&gt;
        &lt;LISTITEM value="Requested" /&gt;
        &lt;LISTITEM value="Rejected" /&gt;
      &lt;/ALLOWEDVALUES&gt;        
      &lt;DEFAULT from="value" value="Requested" /&gt;
      &lt;READONLY not="[TEAM FOUNDATION]\Approvers" /&gt;
      &lt;READONLY not="[TEAM FOUNDATION]\Developers" /&gt;
      &lt;HELPTEXT&gt;Shows whether the task has been approved by management.&lt;/HELPTEXT&gt;
    &lt;/FIELD&gt;
  </code></pre>