microsoft treeview vs asp.net treeview

Question

My treeview looks like below
ItemGrpA
  item1
  item2
  item3
ItemGrpB
  item18
  item20
Everything working fine with Microsoft.Web.UI.WebControls treeview control.
Now we want to use the  System.Web.UI.WebControls (asp:TreeView) insteadof microsoft control.
But here its not allowing me to add the node to treenode control. ( showing syntax error here in my code ti.Nodes.Add(tn) )
So how to add a node to a treenode?
 
My code
 
 private void expandtree(DataSet ds)
        {
            XmlDocument objDoc = new XmlDocument();
          
            try
            {
               
                for (int iRow = 0; iRow < ds.Tables[0].Rows.Count; iRow++)
                {
                    DataRow dr = ds.Tables[0].Rows[iRow];
                    if (dr["i_level"].ToString() == "0")
                    {
                        Microsoft.Web.UI.WebControls.TreeNode tn = new Microsoft.Web.UI.WebControls.TreeNode();
                        tn.Text = dr["c_item"].ToString();
                        tn.NodeData = dr["id_item"].ToString();
                        GetChildreninbound(int.Parse(dr["i_level"].ToString()), ds, iRow + 1, tn);
                        TreeView11.Nodes.Add(tn);
                    }
                }
                TreeView11.ExpandLevel = 2;

            }
            catch (Exception ex)
            {
                throw ex;
            }
        }
        public void GetChildreninbound(int id, DataSet ds, int iRowStart, Microsoft.Web.UI.WebControls.TreeNode ti)
        {
            for (int iRow = iRowStart; iRow < ds.Tables[0].Rows.Count; iRow++)
            {
                DataRow dr = ds.Tables[0].Rows[iRow];
                if (ti.NodeData == dr["id_parent"].ToString())
                {
                    Microsoft.Web.UI.WebControls.TreeNode tn = new Microsoft.Web.UI.WebControls.TreeNode();
                   
                    tn.NodeData = dr["id_item"].ToString();
                  
                     tn.Text = dr["c_item"].ToString();
                  
                    GetChildreninbound(int.Parse(dr["i_level"].ToString()), ds, iRow + 1, tn);
                     
                   

ti.Nodes.Add(tn);//showing syntax error here
                }
                if (int.Parse(dr["i_level"].ToString()) < id)
                {
                    return;
                }
            }
        }