about treeview

using ..;

namespace WebApp
{
    public partial class _Default : System.Web.UI.Page
    {
        TreeNode node;

        protected void Page_Load(object sender, EventArgs e)
        {
            
            BindTree();
        }       

       

        private void BindTree()
        {
            string cmdText = "SELECT AreaID,Name FROM Area WHERE substring(AreaID,3,4)='0000'";

            using (SqlDataReader dr = SqlHelper.ExcuteReader(ConfigurationManager.ConnectionStrings["SqlConn"].ConnectionString, cmdText, null))
            {
                while (dr.Read())
                {
                    node = new TreeNode();

                    node.Text = dr.GetString(1);
                    node.Value = dr.GetString(0);
                    node.NavigateUrl = "page.html?id=" + dr.GetString(0);
                    node.Target = "_blank";

                    TreeView1.Nodes.Add(node);

                    GetChilds(dr.GetString(0));
                }
            }
           
        }

        private void GetChilds(string parentid)
        {
            string cmdText = "SELECT AreaID,Name FROM Area WHERE substring(AreaID,1,2)='" + parentid.Substring(0, 2) + "' AND substring(AreaID,5,2)='00'";

            using (SqlDataReader dr = SqlHelper.ExcuteReader(ConfigurationManager.ConnectionStrings["SqlConn"].ConnectionString, cmdText, null))
            {
                while (dr.Read())
                {
                    TreeNode cnode = new TreeNode();

                    cnode.Text = dr.GetString(1);
                    cnode.NavigateUrl = "subpage.html?id="+dr.GetString(0);
                    cnode.Target = "_blank";

                    node.Expanded = false;                    
                    node.ChildNodes.Add(cnode);
                }
            }
        }

        protected void TreeView1_SelectedNodeChanged(object sender, EventArgs e)
        {
            TreeView1.SelectedNode.Expanded = true;
        }
    }
}