MessageBox 函数用于创建、显示并操作一个消息对话框。该对话框包含由调用程序定义的信息和标题，以及预先定义的图标和按钮。
定义（适用于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
参数说明：
hWnd：
指定该对话框的所有者窗口。如果该参数为空(0)，则该对话框不属于任何窗口（？）。
lpText:
显示在对话框中的消息。
lpCaption：
在对话框标题栏中显示的字符串表达式。如果该参数为空（vbNullString），则使用默认的“错误”作为对话框的标题。
wType：
指定显示按钮的数目及形式，使用的图标样式，缺省按钮是什么以及消息框的强制回应等。可以为下列常数之一或某几个数值相加的和。
指定按钮样式：
MB_ABORTRETRYIGNORE
显示 Abort、Retry 及 Ignore 按钮。
MB_OK
只显示 OK 按钮。（默认）
MB_OKCANCEL
显示 OK 及 Cancel 按钮。
MB_RETRYCANCEL
显示 Retry 及 Cancel 按钮。
MB_YESNO
显示 Yes 及 No 按钮。
MB_YESNOCANCEL
显示 Yes、No 及 Cancel 按钮。
指定图标样式：
MB_ICONEXCLAMATION, MB_ICONWARNING
显示 Warning Message 图标。
MB_ICONINFORMATION, MB_ICONASTERISK
显示 Information Message 图标。
MB_ICONQUESTION
显示 Warning Query 图标。
MB_ICONSTOP, MB_ICONERROR, MB_ICONHAND
显示 Critical Message 图标。
指定默认按钮：
MB_DEFBUTTON1
第一个按钮是缺省值。（默认）
MB_DEFBUTTON2
第二个按钮是缺省值。
MB_DEFBUTTON3
第三个按钮是缺省值。
MB_DEFBUTTON4
第四个按钮是缺省值。
指定对话框模式等：
MB_APPLMODAL
应用程序强制返回；应用程序一直被挂起，直到用户对消息框作出响应才继续工作。
MB_SYSTEMMODAL
系统强制返回；全部应用程序都被挂起，直到用户对消息框作出响应才继续工作。
MB_TASKMODAL
MB_DEFAULT_DESKTOP_ONLY
MB_HELP
将Help按钮添加到消息框。
MB_RIGHT
文本为右对齐。
MB_RTLREADING
指定文本应为在希伯来和阿拉伯语系统中的从右到左显示。
MB_SETFOREGROUND
指定消息框窗口作为前景窗口。
MB_TOPMOST
MB_SERVICE_NOTIFICATION
MB_SERVICE_NOTIFICATION_NT3X
MB_SERVICE_NOTIFICATION
返回值：
如果没有足够内存创建消息对话框，则返回0。
如果函数成功运行，则返回下列数值之一：
IDABORT
用户点击了 Abort 按钮。
IDCANCEL
用户点击了 Cancel 按钮。
IDIGNORE
用户点击了 Ignore 按钮。
IDNO
用户点击了 No 按钮。
IDOK
用户点击了 OK 按钮。
IDRETRY
用户点击了 Retry 按钮。
IDYES
用户点击了 Yes 按钮。
如果对话框显示 Cancel 按钮，则按下 ESC 键与单击 Cancel 按钮的效果相同。
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.
