如何通过asp.net实现递归方法取出菜单并显示在DropDownList中？

protected void Page_Load(object sender, EventArgs e)
{
  if (!IsPostBack)
  {
   DataTable table = bllSite.GetChannelNames(CurrentUser.ParkID);
   BindChannelType(table, 0, 0);
  }
}
//递归方法取
private void BindChannelType(DataTable dt, int categoryid, int layer)
{
  DataView dv = new DataView(dt);
  dv.RowFilter = " Pid= " + categoryid.ToString(); //过滤 
  if (categoryid != 0)
  {
   layer++; //默认为第一层
  }
  foreach (DataRowView drv in dv)
  {
   string span = "";
   if (categoryid != 0)
   {
    for (int i = 0; i < layer; i++)//父级菜单不缩进
    {
     span += "";
    }
    span += "└";
   }
   ListItem li = new ListItem();
   li.Text = span + drv["ChannelName"].ToString();
   li.Value = drv["Id"].ToString();
   this.ddlChannel.Items.Add(li);
   BindChannelType(dt, Convert.ToInt32(drv["Id"]), layer);
  }
}