The WM_USER constant is used by applications to help define private messages, usually of the form WM_USER+X, where X is an integer value.
#define WM_USER 0x0400RemarksThere are five ranges of message numbers:
| Range | Meaning |
| 0 through WM_USER – 1 | Messages reserved for use by the system. |
| WM_USER through 0x7FFF | Integer messages for use by private window classes. |
| WM_APP through 0xBFFF | Messages available for use by applications. |
| 0xC000 through 0xFFFF | String messages for use by applications. |
| Greater than 0xFFFF | Reserved by the system for future use. |
Message numbers in the first range (0 through WM_USER – 1) are defined by the system. Values in this range that are not explicitly defined are reserved for future use by the system.
Message numbers in the second range (WM_USER through 0x7FFF) can be defined and used by an application to send messages within a private window class. These values cannot be used to define messages that are meaningful throughout an application, because some predefined window classes already define values in this range. For example, predefined control classes such as BUTTON, EDIT, LISTBOX, and COMBOBOX may use these values. Messages in this range should not be sent to other applications unless the applications have been designed to exchange messages and to attach the same meaning to the message numbers.
Message numbers in the third range (0x8000 through 0xBFFF) are available for application to use as private messages. Message in this range do not conflict with system messages.
Message numbers in the fourth range (0xC000 through 0xFFFF) are defined at run time when an application calls the RegisterWindowMessage function to retrieve a message number for a string. All applications that register the same string can use the associated message number for exchanging messages. The actual message number, however, is not a constant and cannot be assumed to be the same between different sessions.
Message numbers in the fifth range (greater than 0xFFFF) are reserved for future use by the system.
QuickInfoWindows 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.所以自定義訊息的步驟:
（一）先在頭檔案中定義自己訊息的ID，如 #define WM_MY_FIRST_MESSAGE (WM_USER + 0x001)
（二）再聲明自己的訊息處理函式：afx_msg void OnMyFirstMessage();
（三）在CPP檔案中添加訊息映射宏：
在BEGIN_MESSAGE_MAP 和END_MESSAGE_MAP代碼塊中添加訊息映射宏ON_MESSAGE，（建議添加到//}}後面）如下：
BEGIN_MESSAGE_MAP(CAboutDlg, CDialog)
//{{afx_msg_map(CAboutDlg)
// No message handlers
//}}AFX_MSG_MAP
ON_MESSAGE(WM_MY_FIRST_MESSAGE, OnMyFirstMessage)
END_MESSAGE_MAP()
（四）最後定義訊息處理函式：
OnMyFirstMessage（）
{
// TODO: 在此添加控制項通知處理程式代碼
}
