public static XmlDocument Struttura = new XmlDocument();

    protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack) {
            
            Struttura.LoadXml("<?xml version='1.0' encoding='iso-8859-1' ?><course><lesson id='1'><title>prova</title><unit id='2' idRef='1' ><title>unit</title></unit></lesson></course>");
        }
        BindTreeView();
        
    }

    private void BindTreeView() {
        TreeViewStruttura.Nodes.Clear();
        TreeNode root = new TreeNodeStruttura("Corso","");
        root.SelectAction = TreeNodeSelectAction.Select;
        TreeViewStruttura.Nodes.Add(root);
        foreach (XmlNode nodo in Struttura.DocumentElement.SelectNodes("lesson")) {
            TreeNode lezione = new TreeNode(nodo.SelectSingleNode("title").Value, nodo.Attributes["id"].Value);
            lezione.SelectAction = TreeNodeSelectAction.Select;    
            foreach (XmlNode nodounit in nodo.SelectNodes("unit")) {
                TreeNode unita = new TreeNode(nodounit.SelectSingleNode("title").Value, nodounit.Attributes["id"].value);
                unita.SelectAction = TreeNodeSelectAction.Select;
                lezione.ChildNodes.Add(unita);
            }
            root.ChildNodes.Add(lezione);
        }
        TreeViewStruttura.ExpandAll();
    }

   protected void TreeViewStruttura_SelectedNodeChanged(object sender, EventArgs e) {
        Response.Write (TreeViewStruttura.SelectedNode.Text);
    }