Parsing/Looping through Treeview using Interface

private static void parseNode(TreeNode tn)
{

IEnumerator ie = tn.ChildNodes.GetEnumerator();

string parentnode = "";

parentnode = tn.Text;

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

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

sr.Write("");
sr.WriteLine("");

}

Saving the treeview to XML

public static void exportToXml(TreeView tv, string filename)
{
sr = new StreamWriter(filename, false, System.Text.Encoding.UTF8);
sr.WriteLine("");
if (tv.Nodes.Count > 0)
{
IEnumerator ie = tv.Nodes.GetEnumerator();
ie.Reset();
if (ie.MoveNext())
{
TreeNode tn = (TreeNode)ie.Current;
sr.WriteLine("<" + tn.Text + ">");
parseNode(tn);
}
}

sr.Close();
}