c#里treeview控件的问题

using system;
using system.data;
using system.data.sqlclient;
using system.web;
using system.configuration;
using system.web.ui.webcontrols;

/// <summary>
/// class1 的摘要说明
/// </summary>
public partial class _default : system.web.ui.page
{
    public dataset getcategorys（)//获取category表里的数据
    {
        sqlconnection myconnection = new sqlconnection（configurationmanager.connectionstrings["sqlconnectionstring"].connectionstring);
        string cmdtext = "select * from category";
        sqldataadapter da = new sqldataadapter（cmdtext, myconnection);
        dataset ds = new dataset（);
        try
        {
            myconnection.open（);
            da.fill（ds);
        }
        catch （sqlexception sqlex)
        {
            throw new exception（sqlex.message);
        }
        finally
        {
            myconnection.close（);
        }
        return ds;
    }
    public void bindcategorytreeview（treeview treeview, bool isexpanded, string selecteddata)
    {
        datatable datatable = getcategorys（).tables[0];
        treeview.nodes.clear（);
        datarow[] rowlist = datatable.select（"parentid='-1'");
        if （rowlist.length <= 0) return;
        treenode rootnode = new treenode（);
        rootnode.text = rowlist[0]["desn"].tostring（);
        rootnode.value = rowlist[0]["categoryid"].tostring（);
        rootnode.expanded = isexpanded;
        rootnode.selected = true;
        treeview.nodes.add（rootnode);//添加根节点到treeview
        createchildnode（rootnode, datatable, isexpanded, selecteddata);//调用下面的成员函数，创建其他节点
    }
    private void createchildnode（treenode parentnode, datatable datatable, bool isexpanded, string selecteddata)//创建其他节点
    {
        datarow[] rowlist =  datatable.select（"parentid='" + parentnode.value + "'", "orderby");
        foreach （datarow row in rowlist)
        {
            treenode node = new treenode（);
            node.text = row["desn"].tostring（);
            node.value = row["categoryid"].tostring（);
            node.expanded = isexpanded;
            if （node.value == selecteddata)
            {
                node.selected = true;
            }
            parentnode.childnodes.add（node);//添加子节点到父节点
            createchildnode（node, datatable, isexpanded, selecteddata);//递归，创建子节点
        }
    }
    protected void page_load（object sender, eventargs e)
    {
        if （!page.ispostback)
        {
            bindcategorytreeview（mytreeview, true, ""); 
        }
    }
}