C# - Treeview in XML schreiben

 public static void exportToXml(TreeView tv, string filename) 
        {
            sr = new StreamWriter(filename, false, System.Text.Encoding.UTF8);
            sr.WriteLine("<?xml version=\"1.0\" encoding=\"utf-8\" ?>");

            IEnumerator ie = tv.Nodes.GetEnumerator();
            if (ie.MoveNext()) 
            {
                TreeNode tn = (TreeNode) ie.Current;
                sr.WriteLine("<" + tn.Text + ">");
                parseNode(tn);
            }

            sr.Close();
        }

private static void parseNode(TreeNode tn) 
{
  IEnumerator ie = tn.Nodes.GetEnumerator();

  string parentnode = "";

  parentnode = tn.Text;

  while (ie.MoveNext()) 
  {
    TreeNode ctn = (TreeNode) ie.Current;

    if (ctn.GetNodeCount(true) == 0) 
    {
      sr.Write(ctn.Text);
    } 
    else 
    {
      sr.Write("<" + ctn.Text + ">");
    }
    if (ctn.GetNodeCount(true) > 0) 
    {
      parseNode(ctn);
    }
  }

  sr.Write("</" + parentnode + ">");
  sr.WriteLine("");

}