What is the mysterious fourth message box button?

When you call the MessageBox function, you pass flags specifying which of a fixed set of button patterns you want (for example, Yes/No and OK/Cancel) and which button you want to be the default (MB_DEFBUTTON1 through MB_DEFBUTTON4.)

Wait a second. What’s with this MB_DEFBUTTON4? None of the button patterns are four-button patterns. The highest number of buttons you can specify is three: Abort/Retry/Ignore. How can you set a nonexistent button to be the default?

Let’s do some header file spelunking. The flag for this magical fourth button is defined here:

#define MB_DEFBUTTON1               0x00000000L
#define MB_DEFBUTTON2               0x00000100L
#define MB_DEFBUTTON3               0x00000200L
#if(WINVER >= 0x0400)
#define MB_DEFBUTTON4               0x00000300L
#endif /* WINVER >= 0x0400 */

Aha, the magic fourth button was added in WINVER 4.0. Therefore, whatever the fourth button is, it was introduced when WINVER == 0x0400. Let’s see what other message box flags were introduced then:

#define MB_OK                       0x00000000L
#define MB_OKCANCEL                 0x00000001L
#define MB_ABORTRETRYIGNORE         0x00000002L
#define MB_YESNOCANCEL              0x00000003L
#define MB_YESNO                    0x00000004L
#define MB_RETRYCANCEL              0x00000005L
#if(WINVER >= 0x0500)
#define MB_CANCELTRYCONTINUE        0x00000006L
#endif /* WINVER >= 0x0500 */

#define MB_ICONHAND                 0x00000010L
#define MB_ICONQUESTION             0x00000020L
#define MB_ICONEXCLAMATION          0x00000030L
#define MB_ICONASTERISK             0x00000040L
#if(WINVER >= 0x0400)
#define MB_USERICON                 0x00000080L
#define MB_ICONWARNING              MB_ICONEXCLAMATION
#define MB_ICONERROR                MB_ICONHAND
#endif /* WINVER >= 0x0400 */
#define MB_ICONINFORMATION          MB_ICONASTERISK
#define MB_ICONSTOP                 MB_ICONHAND
#define MB_DEFBUTTON1               0x00000000L
#define MB_DEFBUTTON2               0x00000100L
#define MB_DEFBUTTON3               0x00000200L
#if(WINVER >= 0x0400)
#define MB_DEFBUTTON4               0x00000300L
#endif /* WINVER >= 0x0400 */
#define MB_APPLMODAL                0x00000000L
#define MB_SYSTEMMODAL              0x00001000L
#define MB_TASKMODAL                0x00002000L
#if(WINVER >= 0x0400)
#define MB_HELP                     0x00004000L // Help Button
#endif /* WINVER >= 0x0400 */
#define MB_NOFOCUS                  0x00008000L
#define MB_SETFOREGROUND            0x00010000L
#define MB_DEFAULT_DESKTOP_ONLY     0x00020000L
#if(WINVER >= 0x0400)
#define MB_TOPMOST                  0x00040000L
#define MB_RIGHT                    0x00080000L
#define MB_RTLREADING               0x00100000L
#endif /* WINVER >= 0x0400 */
#ifdef _WIN32_WINNT
#if (_WIN32_WINNT >= 0x0400)
#define MB_SERVICE_NOTIFICATION          0x00200000L
#else
#define MB_SERVICE_NOTIFICATION          0x00040000L
#endif
#define MB_SERVICE_NOTIFICATION_NT3X     0x00040000L
#endif

We can discount the flags like MB_ICONWARNING which are just alternate names for existing flags, as well as MB_SERVICE_NOTIFICATION which already existed but with a different value. This leaves the following:

#define MB_USERICON                 0x00000080L
#define MB_HELP                     0x00004000L // Help Button
#define MB_TOPMOST                  0x00040000L
#define MB_RIGHT                    0x00080000L
#define MB_RTLREADING               0x00100000L

Of these flags, MB_USERICON affects the icon, and MB_TOPMOST, MB_RIGHT and MB_RTLREADING affect the dialog box’s position and layout; none of them affect the buttons. But wait, there’s MB_HELP. Ah, that flag “adds a Help button to the message box.” That’s our magical fourth button! Let’s celebrate by showing a four-button message box with the default set to the fourth button:

#include <windows.h>

int WINAPI WinMain(HINSTANCE hinst, HINSTANCE hinstPrev,
                   LPSTR lpCmdLine, int nShowCmd)
{
  return MessageBox(NULL, TEXT(“Four buttons!”), TEXT(“Title”),
            MB_ABORTRETRYIGNORE | MB_HELP | MB_DEFBUTTON4);
}