messagebox()地返回值可以判断
IDOK 1 The user chose the OK button.
IDCANCEL 2 The user chose the Cancel button.
IDABORT 3 The user chose the Abort button.
IDRETRY 4 The user chose the Retry button.
IDIGNORE 5 The user chose the Ignore button.
IDYES 6 The user chose the Yes button.
IDNO 7 The user chose the No button.