抓不上TVN_SELCHANGED

我在对话框中有一个CTreeCtrl并捕获TVN_SELCHANGED消息，但执行不会进入处理程序。可能出了什么问题？我的代码：

BEGIN_MESSAGE_MAP(CMaterParamExtDlg, CDialog)
   ON_NOTIFY(TVN_ITEMCHANGING, IDC_MATERIAL_PROP_TREE, OnSelChangingTreeCtrl)
   ON_NOTIFY(TVN_ITEMCHANGED, IDC_MATERIAL_PROP_TREE, OnSelChangedTreeCtrl)
END_MESSAGE_MAP()
void CMaterParamExtDlg :: OnSelChangedTreeCtrl (NMHDR* pNMHDR, LRESULT* pResult) 
{
    if (m_TreeCtrl != 0)
    {
        HTREEITEM treeitem = m_TreeCtrl->GetSelectedItem();
        CString treeitemtext = m_TreeCtrl->GetItemText(treeitem);
        MessageBox(treeitemtext);
    }
    *pResult = 0;
}
void CMaterParamExtDlg :: OnSelChangingTreeCtrl (NMHDR* pNMHDR, LRESULT* pResult)
{
    if (m_TreeCtrl != 0)
    {
        HTREEITEM treeitem = m_TreeCtrl->GetSelectedItem();
        CString treeitemtext = m_TreeCtrl->GetItemText(treeitem);
        MessageBox(treeitemtext);
    }
    *pResult = 0;
}

我尝试使用断点来查看执行是否进入处理程序，但没有任何反应。

树控件属性：

CONTROL         "",IDC_MATERIAL_PROP_TREE,"SysTreeView32",TVS_HASBUTTONS 
| TVS_HASLINES | TVS_LINESATROOT | TVS_DISABLEDRAGDROP 
| TVS_TRACKSELECT | WS_BORDER | WS_HSCROLL | WS_TABSTOP,4,4,115,218