如何使用c#中的XMLREADER从XML字符串中读取特定的元素

I have XML String:

我有XML字符串:

   <GroupBy Collapse=\"TRUE\" GroupLimit=\"30\">
      <FieldRef Name=\"Department\" />
   </GroupBy>
   <OrderBy>
      <FieldRef Name=\"Width\" />
   </OrderBy>
   <GroupBy Co