MessageBox 函式用於創建、顯示並操作一個訊息對話框。該對話框包含由調用程式定義的信息和標題，以及預先定義的圖示和按鈕。
定義,參數說明,
定義
（適用於Visual Basic 6.0）：
Declare Function MessageBox Lib "user32" Alias "MessageBoxA" (ByVal hwnd As Long, ByVal lpText As String, ByVal lpCaption As String, ByVal wType As Long) As Long
參數說明
hWnd：
指定該對話框的所有者視窗。如果該參數為空(0)，則該對話框不屬於任何視窗（？）。
lpText:
顯示在對話框中的訊息。
lpCaption：
wType：
指定顯示按鈕的數目及形式，使用的圖示樣式，預設按鈕是什麼以及訊息框的強制回應等。可以為下列常數之一或某幾個數值相加的和。
指定按鈕樣式：
MB_ABORTRETRYIGNORE
顯示 Abort、Retry 及 Ignore 按鈕。
MB_OK
只顯示 OK 按鈕。（默認）
MB_OKCANCEL
顯示 OK 及 Cancel 按鈕。
MB_RETRYCANCEL
顯示 Retry 及 Cancel 按鈕。
MB_YESNO
顯示 Yes 及 No 按鈕。
MB_YESNOCANCEL
顯示 Yes、No 及 Cancel 按鈕。
指定圖示樣式：
MB_ICONEXCLAMATION, MB_ICONWARNING
顯示 Warning Message 圖示。
MB_ICONINFORMATION, MB_ICONASTERISK
顯示 Information Message 圖示。
MB_ICONQUESTION
顯示 Warning Query 圖示。
MB_ICONSTOP, MB_ICONERROR, MB_ICONHAND
顯示 Critical Message 圖示。
指定默認按鈕：
MB_DEFBUTTON1
第一個按鈕是預設值。（默認）
MB_DEFBUTTON2
第二個按鈕是預設值。
MB_DEFBUTTON3
第三個按鈕是預設值。
MB_DEFBUTTON4
第四個按鈕是預設值。
指定對話框模式等：
MB_APPLMODAL
應用程式強制返回；應用程式一直被掛起，直到用戶對訊息框作出回響才繼續工作。
MB_SYSTEMMODAL
系統強制返回；全部應用程式都被掛起，直到用戶對訊息框作出回響才繼續工作。
MB_TASKMODAL
MB_DEFAULT_DESKTOP_ONLY
MB_HELP
將Help按鈕添加到訊息框。
MB_RIGHT
文本為右對齊。
MB_RTLREADING
指定文本應為在希伯來和阿拉伯語系統中的從右到左顯示。
MB_SETFOREGROUND
指定訊息框視窗作為前景視窗。
MB_TOPMOST
MB_SERVICE_NOTIFICATION
MB_SERVICE_NOTIFICATION_NT3X
MB_SERVICE_NOTIFICATION
返回值：
如果沒有足夠記憶體創建訊息對話框，則返回0。
如果函式成功運行，則返回下列數值之一：
IDABORT
用戶點擊了 Abort 按鈕。
IDCANCEL
用戶點擊了 Cancel 按鈕。
IDIGNORE
用戶點擊了 Ignore 按鈕。
IDNO
用戶點擊了 No 按鈕。
IDOK
用戶點擊了 OK 按鈕。
IDRETRY
用戶點擊了 Retry 按鈕。
IDYES
用戶點擊了 Yes 按鈕。
如果對話框顯示 Cancel 按鈕，則按下 ESC 鍵與單擊 Cancel 按鈕的效果相同。
原文：
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.
Declaration:
Declare Function MessageBox Lib "user32" Alias "MessageBoxA" (ByVal hwnd As Long, ByVal lpText As String, ByVal lpCaption As String, ByVal wType As Long) As Long
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
Points to a null-terminated string containing the message to be displayed.
?lpCaption
Points 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:
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:
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:
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:
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 applications 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 application. 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 task 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 current application without suspending other applications.
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, Windows 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 only: 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. 1. 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 only: 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:
IDABORT
Abort button was selected.
IDCANCEL
Cancel button was selected.
IDIGNORE
Ignore button was selected.
IDNO
No button was selected.
IDOK
OK button was selected.
IDRETRY
Retry button was selected.
IDYES
Yes button was selected.
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.
