TreeView 异步逐层加载

当Tree需要加载的节点数很多时，我们通常会想到逐层加载！好的，VS2005的TreeView控件已经可以满足我们的需求了！

处理TreeView实体的 OnTreeNodePopulate事件就可以了

部分例子

代码如下：
    private void BuildSubNodes(TreeNode parentNode)
    {
        long OID = long.Parse(parentNode.Value);
        IList<Function> list = (new FunctionBLL()).GetSubFunction(OID);

        foreach (Function subFunction in list)
        {
            TreeNode subNode = new TreeNode();           
            subNode.Value = subFunction.OID.ToString();
            subNode.Text = subFunction.Name;
            subNode.PopulateOnDemand = true;
            subNode.SelectAction = TreeNodeSelectAction.Expand;

            parentNode.ChildNodes.Add(subNode);
        }
    }

    protected void functionTree_TreeNodePopulate(object sender, TreeNodeEventArgs e)