.net递归生成TreeView

protected void Page_Load(object sender, EventArgs e)
    {
        if (!Page.IsPostBack)
        {
            TreeTn("0", (TreeNode)null);
        }
        
    }
    public void TreeTn(string pid, TreeNode tn)
    {

        DataTable dt = mmsdb.dt("select * from [SJ_MMSProgressSort] where parent=" + pid + "");
        for (int i = 0; i < dt.Rows.Count; i++)
        {
            TreeNode tmptn = new TreeNode();
            tmptn.Value = dt.Rows[i]["ID"].ToString();
            tmptn.Text = dt.Rows[i]["sortname"].ToString();
            tmptn.NavigateUrl = "MMS_SortEdit.aspx?id=" + dt.Rows[i][0].ToString() + "";
            tmptn.Target = "frmright";
            
            if (tn == null)
            {
                this.TreeView1.Nodes.Add(tmptn);
                TreeTn(dt.Rows[i]["id"].ToString(), tmptn);
            }
            else
            {
                tn.ChildNodes.Add(tmptn);
                TreeTn(dt.Rows[i]["id"].ToString(),tmptn);
            }
        }

        
    }