MessageBox

The MessageBox function creates, displays, and operates a message box. The message box contains an application-defined message and title, plus any combination of predefined icons and push buttons.

int MessageBox(
  HWND hWnd,          // handle of owner window
  LPCTSTR lpText,     // address of text in message box
  LPCTSTR lpCaption,  // address of title of message box
  UINT uType          // style of message box
);
 

Parameters

hWnd

Identifies the owner window of the message box to be created. If this parameter is NULL, the message box has no owner window.

lpText

Pointer to a null-terminated string containing the message to be displayed.

lpCaption

Pointer to a null-terminated string used for the dialog box title. If this parameter is NULL, the default title Error is used.

uType

Specifies a set of bit flags that determine the contents and behavior of the dialog box. This parameter can be a combination of flags from the following groups of flags.

Specify one of the following flags to indicate the buttons contained in the message box:

Flag	Meaning
MB_ABORTRETRYIGNORE	The message box contains three push buttons: Abort, Retry, and Ignore.
MB_OK	The message box contains one push button: OK. This is the default.
MB_OKCANCEL	The message box contains two push buttons: OK and Cancel.
MB_RETRYCANCEL	The message box contains two push buttons: Retry and Cancel.
MB_YESNO	The message box contains two push buttons: Yes and No.
MB_YESNOCANCEL	The message box contains three push buttons: Yes, No, and Cancel.

Specify one of the following flags to display an icon in the message box:

Flag	Meaning
MB_ICONEXCLAMATION, MB_ICONWARNING	An exclamation-point icon appears in the message box.
MB_ICONINFORMATION, MB_ICONASTERISK	An icon consisting of a lowercase letter i in a circle appears in the message box.
MB_ICONQUESTION	A question-mark icon appears in the message box.
MB_ICONSTOP, MB_ICONERROR, MB_ICONHAND	A stop-sign icon appears in the message box.

Specify one of the following flags to indicate the default button:

Flag	Meaning
MB_DEFBUTTON1	The first button is the default button. MB_DEFBUTTON1 is the default unless MB_DEFBUTTON2, MB_DEFBUTTON3, or MB_DEFBUTTON4 is specified.
MB_DEFBUTTON2	The second button is the default button.
MB_DEFBUTTON3	The third button is the default button.
MB_DEFBUTTON4	The fourth button is the default button.

Specify one of the following flags to indicate the modality of the dialog box:

Flag	Meaning
MB_APPLMODAL	The user must respond to the message box before continuing work in the window identified by the hWnd parameter. However, the user can move to the windows of other threads and work in those windows. Depending on the hierarchy of windows in the application, the user may be able to move to other windows within the thread. All child windows of the parent of the message box are automatically disabled, but popup windows are not. MB_APPLMODAL is the default if neither MB_SYSTEMMODAL nor MB_TASKMODAL is specified.
MB_SYSTEMMODAL	Same as MB_APPLMODAL except that the message box has the WS_EX_TOPMOST style. Use system-modal message boxes to notify the user of serious, potentially damaging errors that require immediate attention (for example, running out of memory). This flag has no effect on the user's ability to interact with windows other than those associated with hWnd.
MB_TASKMODAL	Same as MB_APPLMODAL except that all the top-level windows belonging to the current thread are disabled if the hWnd parameter is NULL. Use this flag when the calling application or library does not have a window handle available but still needs to prevent input to other windows in the calling thread without suspending other threads.

In addition, you can specify the following flags:

MB_DEFAULT_DESKTOP_ONLY

The desktop currently receiving input must be a default desktop; otherwise, the function fails. A default desktop is one an application runs on after the user has logged on.

MB_HELP

Adds a Help button to the message box. Choosing the Help button or pressing F1 generates a Help event.

MB_RIGHT

The text is right-justified.

MB_RTLREADING

Displays message and caption text using right-to-left reading order on Hebrew and Arabic systems.

MB_SETFOREGROUND

The message box becomes the foreground window. Internally, the system calls the SetForegroundWindow function for the message box.

MB_TOPMOST

The message box is created with the WS_EX_TOPMOST window style.

MB_SERVICE_NOTIFICATION

Windows NT: The caller is a service notifying the user of an event. The function displays a message box on the current active desktop, even if there is no user logged on to the computer.

If this flag is set, the hWnd parameter must be NULL. This is so the message box can appear on a desktop other than the desktop corresponding to the hWnd.

For Windows NT version 4.0, the value of MB_SERVICE_NOTIFICATION has changed. See WINUSER.H for the old and new values. Windows NT 4.0 provides backward compatibility for pre-existing services by mapping the old value to the new value in the implementation of MessageBox and MessageBoxEx. This mapping is only done for executables that have a version number, as set by the linker, less than 4.0.

To build a service that uses MB_SERVICE_NOTIFICATION, and can run on both Windows NT 3.x and Windows NT 4.0, you have two choices.

At link-time, specify a version number less than 4.0; or

2.    At link-time, specify version 4.0. At run-time, use the GetVersionEx function to check the system version. Then when running on Windows NT 3.x, use MB_SERVICE_NOTIFICATION_NT3X; and on Windows NT 4.0, use MB_SERVICE_NOTIFICATION.

MB_SERVICE_NOTIFICATION_NT3X

Windows NT: This value corresponds to the value defined for MB_SERVICE_NOTIFICATION for Windows NT version 3.51.

Return Values

The return value is zero if there is not enough memory to create the message box.

If the function succeeds, the return value is one of the following menu-item values returned by the dialog box:

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.

Remarks

When you use a system-modal message box to indicate that the system is low on memory, the strings pointed to by the lpText and lpCaption parameters should not be taken from a resource file, because an attempt to load the resource may fail.

When an application calls MessageBox and specifies the MB_ICONHAND and MB_SYSTEMMODAL flags for the uType parameter, the system displays the resulting message box regardless of available memory. When these flags are specified, the system limits the length of the message box text to three lines. The system does not automatically break the lines to fit in the message box, however, so the message string must contain carriage returns to break the lines at the appropriate places.

If you create a message box while a dialog box is present, use the handle of the dialog box as the hWnd parameter. The hWnd parameter should not identify a child window, such as a control in a dialog box.

Windows 95: The system can support a maximum of 16,364 window handles.

Windows CE: Windows CE does not support the following styles for the uType parameter:

MB_SYSTEMMODAL

MB_TASKMODAL

MB_HELP

MB_RIGHT

MB_RTLREADING

MB_DEFAULT_DESKTOP_ONLY

MB_SERVICE_NOTIFICATION

MB_USERICON.

The following return values are not supported:

IDCLOSE

IDHELP

QuickInfo

  Windows NT: Requires version 3.1 or later.
  Windows: Requires Windows 95 or later.
  Windows CE: Requires version 1.0 or later.
  Header: Declared in winuser.h.
  Import Library: Use user32.lib.
  Unicode: Implemented as Unicode and ANSI versions on Windows and Windows NT.

See Also

Dialog Boxes Overview, Dialog Box Functions, FlashWindow, MessageBeep, MessageBoxEx, MessageBoxIndirect, SetForegroundWindow