重复表中连级下拉框采用数据源时，子级下拉框列表数据无法持久的问题

谁能解决这个问题？谢谢了

//一级下拉框改变事件
public void field1_Changed(object sender, XmlEventArgs e)
        {
            XPathNavigator me = (XPathNavigator)sender;
            if (me.Value == "")
                return;
            // 主数据源
            XPathNavigator MainXpn = MainDataSource.CreateNavigator();
           //辅助数据源
            XPathNavigator RequestXpn = this.DataSources["getCodeTable"].CreateNavigator();
            RequestXpn.SelectSingleNode("/dfs:myFields/dfs:queryFields/tns:getCodeTable/tns:codeTabelSetType", NamespaceManager).SetValue("codetable.xml");
           
           
            //开始取参数
            RequestXpn.SelectSingleNode("/dfs:myFields/dfs:queryFields/tns:getCodeTable/tns:codeTabelType", NamespaceManager).SetValue(me.Value);
            //开始更新辅助数据源，问题出在这里：所有的二级下拉框菜单均访问这个数据源！
            //所以二级下拉框菜单将同时更新！
            this.DataSources["getCodeTable"].QueryConnection.Execute();
 
   //尝试遍历？
            XPathNodeIterator NodeIter = this.MainDataSource.CreateNavigator().Select("/my:myFields/my:group1/my:group2", this.NamespaceManager);
            string str1="";
            foreach (XPathNavigator item in NodeIter)
            {
                XPathNavigator n1 = item.SelectSingleNode("my:field1", NamespaceManager);
                str1 += n1.Value+",";
                //遍历毫无意义！因为所有的二级下拉框菜单还是访问最后一次遍历得到的数据源
    //只要下拉框使用数据源，下拉框的列表数据都将无法持久，选用其他方式的数据源还是会出现同样问题
                RequestXpn.SelectSingleNode("/dfs:myFields/dfs:queryFields/tns:getCodeTable/tns:codeTabelType", NamespaceManager).SetValue(n1.Value);
                this.DataSources["getCodeTable"].QueryConnection.Execute();

            }