//设定可用性（当你打开下拉菜单时便会激发UPDATE _COMMAND_UI消息，映射如下函数：
void C***Dlg::OnUpdateMenuLogin(CCmdUI* pCmdUI) 
{
    // TODO: Add your command update UI handler code here
    pCmdUI->Enable(mMenuActive & 0x01);
}
//手动添加下面函数到你的主DIALOG里
void C***Dlg::OnInitMenuPopup(CMenu* pPopupMenu, UINT nIndex, BOOL bSysMenu) 
{
    if (!bSysMenu)
    {
        ASSERT(pPopupMenu != NULL);
        
        // check the enabled state of various menu items
        CCmdUI state;        
        state.m_pMenu = pPopupMenu;
        ASSERT(state.m_pOther == NULL);
        
        state.m_nIndexMax = pPopupMenu->GetMenuItemCount();
        for (state.m_nIndex = 0; state.m_nIndex < state.m_nIndexMax;
             state.m_nIndex++)
        {
            state.m_nID = pPopupMenu->GetMenuItemID(state.m_nIndex);
            if (state.m_nID == 0)
                continue; // menu separator or invalid cmd - ignore it
                
            ASSERT(state.m_pOther == NULL);
            ASSERT(state.m_pMenu != NULL);
            if (state.m_nID == (UINT)-1)
            {
                // possibly a popup menu, route to first item of that popup
                state.m_pSubMenu = pPopupMenu->GetSubMenu(state.m_nIndex);
                if (state.m_pSubMenu == NULL ||
                    (state.m_nID = state.m_pSubMenu->GetMenuItemID(0)) == 0 ||
                    state.m_nID == (UINT)-1)
                {                                 
                    continue; // first item of popup can't be routed to
                }
                state.DoUpdate(this, FALSE);  // popups are never auto disabled
            }
            else
            {
                // normal menu item
                // Auto enable/disable if command is _not_ a system command
                state.m_pSubMenu = NULL;
                state.DoUpdate(this, state.m_nID < 0xF000);
            }
        }
    }
 }
//还得添加消息映射：
BEGIN_MESSAGE_MAP(CcontrolDlg, CDialog)
ON_WM_INITMENUPOPUP()
END_MESSAGE_MAP()//现在便会按照你要打开菜单时它的可用性而显示是否变灰：）