子类化控件怎么收不到消息

class CMyComboBox : public CComboBox
{
public:
afx_msg void OnKeyDown(UINT nChar,UINT nRepcnt,UINT nFlags);
DECLARE_MESSAGE_MAP()
};
BEGIN_MESSAGE_MAP(CMyComboBox,CComboBox)
ON_CBN_EDITCHANGE(IDC_COMBO1,Change)
ON_WM_KEYDOWN()
END_MESSAGE_MAP()void CMyComboBox::OnKeyDown(UINT nChar, UINT nRepCnt, UINT nFlags)
{
CComboBox::OnKeyDown (nChar,nRepCnt,nFlags);
::MessageBox (0,0,0,0);//没反应
}