If a message box has a Cancel button, the function returns the IDCANCEL value if either the ESC key is pressed or the Cancel button is selected. If the message box has no Cancel button, pressing ESC has no effect.
If the function fails, the return value is zero. To get extended error information, call GetLastError.
If the function succeeds, the return value is one of the following menu-item values.
IDABORT Abort button was selected.
IDCANCEL Cancel button was selected.
IDCONTINUE Continue button was selected.
IDIGNORE Ignore button was selected.
IDNO No button was selected.
IDOK OK button was selected.
IDRETRY Retry button was selected.
IDTRYAGAIN Try Again button was selected.
IDYES Yes button was selected.
