ListBox selectItem事件触发不正确(Atlas)

[soddyque]

public void listbox1Evet(Object sender, EventArgs e)
{
ListBox senders = (ListBox)sender;
Response.Write(senders.ID);
XmlNodeList oList = dataXml.FirstChild.SelectNodes("type");
XmlNodeList tList = oList[senders.SelectedIndex].SelectNodes("node");
ListBox2.Items.Clear();
for (int tn = 0, tl = tList.Count; tn < tl; tn++)
{
ListBox2.Items.Add(tList[tn].Attributes[2].Value + " " + tList[tn].Attributes[0].Value);
}
}

Response.Write(senders.ID);输出时是正确的"ListBox1"
可到XmlNodeList tList = oList[senders.SelectedIndex].SelectNodes("node");行就提示未将对象引用实例之类的....但明明可以访问了...跟了一下.发现senders.SelectedIndex还是-1.可明明已经选择了..