title: Why OrderByDescending not recognized as build method
body: <pre><code>private static void findNewestFile(List&lt;string&gt; list)
  {
      foreach (string dir in list)
      {
          var directory = new DirectoryInfo(dir);
          var file = directory.GetFiles("*.doc").OrderByDescending(f =&gt; f.LastWriteTime).First();
      }
  }
  </code></pre>
  
  <p>This function received list of folders I try to find the newest file from each directory but the method <code>OrderByDescending</code> not recognized and the compiler error is: </p>
  
  <blockquote>
    <p><em>Error    1   'System.Array' does not contain a definition for 'OrderByDescending' and no extension method 'OrderByDescending'
    accepting a first argument of type 'System.Array' could be found (are
    you missing a using directive or an assembly reference?)</em></p>
  </blockquote>