title: Null value when trying to retrieve a node from XML
body: <p>I need to read and update the value of CollectionPlan -> CheckpointOptions -> ThreadCheckpoints -> Mode from xml below</p>
  
  <pre><code>&lt;?xml version="1.0" encoding="utf-8"?&gt;
  &lt;CollectionPlan xmlns="urn:schemas-microsoft-com:visualstudio:tracelog"&gt;
    &lt;StartupInfo&gt;
      &lt;LogFileName&gt;
      &lt;/LogFileName&gt;
      &lt;LogFileDirectory&gt;C:\LogFiles&lt;/LogFileDirectory&gt;
      &lt;MaximumLogFileSize&gt;16384&lt;/MaximumLogFileSize&gt;
    &lt;/StartupInfo&gt;
    &lt;CheckpointOptions&gt;
      &lt;ThreadCheckpoints&gt;
        &lt;Mode&gt;FixedEventRate&lt;/Mode&gt;
        &lt;StartRate&gt;30000&lt;/StartRate&gt;
      &lt;/ThreadCheckpoints&gt;
      &lt;NotifyPointCheckpoints&gt;
        &lt;Mode&gt;FixedEventRate&lt;/Mode&gt;
        &lt;StartRate&gt;1000&lt;/StartRate&gt;
      &lt;/NotifyPointCheckpoints&gt;
    &lt;/CheckpointOptions&gt;
    &lt;!-- many other nodes --&gt;
   &lt;/CollectionPlan&gt;
  </code></pre>
  
  <p>For this, I have  following code.</p>
  
  <pre><code>XmlDocument doc = new XmlDocument();
  XmlNamespaceManager manager = new XmlNamespaceManager(doc.NameTable);
  manager.AddNamespace("urn:", "schemas-microsoft-com:visualstudio:tracelog");
  doc.Load(@"c:\myXml.xml");
  var aNodes = doc.SelectSingleNode("CollectionPlan/CheckpointOptions/ThreadCheckpoints/Mode", manager);
  </code></pre>
  
  <p>However aNodes is coming as NULL. What I have incorrect here?</p>