延迟加载的Treeview选中时背景不变色

Question

    public void BuildOuTree(string sPath)
    {        DirectoryEntry RootOu = ADMan.GetDirectoryObject(sPath);        TreeNode RootNode = new TreeNode();
        RootNode.Text = "&nbsp" + RootOu.Name;
        RootNode.Value = RootOu.NativeGuid;
        RootNode.Target = "mmc";
        RootNode.NavigateUrl = "mmc2.aspx?guid=" + RootNode.Value;
        RootNode.PopulateOnDemand = true;
        RootNode.Expanded = false;
        
        if (sPath.ToUpper() == "LDAP://DC=CONTOSO,DC=COM")
        {
            RootNode.ImageUrl = @"~\images\dc.png";
        }
        else
        {
            RootNode.ImageUrl = @"~\images\ou.png";
        }
        OuTree.Nodes.Add(RootNode);        //GetChildNode(RootNode);
    }   protected void OuTree_TreeNodePopulate(object sender, TreeNodeEventArgs e)
    {
        TreeNode ParentNode = e.Node;
        
        string sPath = ParentNode.Value;
        sPath = "LDAP://<GUID=" + sPath + ">";
        //注意path的处理，必须带上ldap
        ArrayList ChildOus = ADMan.GetChildOus(sPath);
        if (ChildOus.Count > 0)
        {
            foreach (string[] ChildOu in ChildOus)
            {
                TreeNode ChildNode = new TreeNode();
                ChildNode.Text = "&nbsp" + ChildOu[1];
                ChildNode.Value = ChildOu[0];
                ChildNode.Target = "mmc";
                ChildNode.NavigateUrl = "mmc2.aspx?guid=" + ChildNode.Value;
                ChildNode.ImageUrl = @"~\images\ou.png";
                ChildNode.PopulateOnDemand = true;
                ChildNode.Expanded = false;                ParentNode.ChildNodes.Add(ChildNode);
                //OuTree.Nodes.Add(ChildNode);                //GetChildNode(ChildNode);
            }
        }
    }<asp:TreeView ID="OuTree" runat="server" ExpandDepth="0" ShowLines="True" 
                        LineImagesFolder="~/TreeLineImages" 
                        ontreenodepopulate="OuTree_TreeNodePopulate">
                        <SelectedNodeStyle BackColor="Yellow" BorderColor="#003300" BorderStyle="Solid" 
                            BorderWidth="1px" Font-Italic="True" />
                    </asp:TreeView>只有根节点点击有改变了样式，其他的节点点击都没有样式变化，求指点