title: XMLDocument GetElementByID returning null
body: <p>I'm trying to get an element of a xml by using the function GetElementById, but the function is always returning null.</p>
  
  <pre><code>//get xml text from a web service
  string xml = aS.createTree();
  XmlDocument tree = new XmlDocument();
  tree.LoadXml(xml);
  
  //get all nodes with the tag name "item"
  XmlNodeList node = tree.GetElementsByTagName("item");
  //just for test to see if i could get the attribute value which returns the expected
  string idTemp = node[0].Attributes["ID"].Value;
  XmlElement elem = tree.GetElementById("1");
  </code></pre>
  
  <p>elem is always returning null. Can you guys help me out?</p>
  
  <p>By the way this is the xml that i'm trying to parse:</p>
  
  <pre class="lang-xml prettyprint-override"><code>&lt;?xml version="1.0" encoding="utf-8" ?&gt;
  &lt;!DOCTYPE tree [
     &lt;!ELEMENT tree ANY&gt;
     &lt;!ELEMENT item ANY&gt;
     &lt;!ATTLIST item id ID #REQUIRED&gt;
  ]&gt;
  &lt;tree&gt;
     &lt;item id="1"&gt;
         &lt;item id="2"&gt;&lt;/item&gt;
     &lt;/item&gt;
     &lt;item id="5"&gt;
         &lt;item id="6"&gt;&lt;/item&gt;
         &lt;item id="7"&gt;
             &lt;item id="8"&gt;
                 &lt;item id="10"&gt;
                     &lt;item id="11"&gt;&lt;/item&gt;
                 &lt;/item&gt;
             &lt;/item&gt;
             &lt;item id="9"&gt;&lt;/item&gt;
         &lt;/item&gt;
     &lt;/item&gt;
  &lt;/tree&gt;
  </code></pre>