WM_USER is a constant used by applications to help define private messages.
The WM_USER constant is used to distinguish between message values that are reserved for use by Windows and values that can be used by an application to send messages within a private window class. There are four ranges of message numbers:
| Range | Meaning |
| 0 through WM_USER – 1 | Messages reserved for use by Windows. |
| WM_USER through 0x7FFF | Integer messages for use by private window classes. |
| 0x8000 through 0xBFFF | Messages reserved for use by Windows. |
| 0xC000 through 0xFFFF | String messages for use by applications. |
Message numbers in the first range (0 through WM_USER – 1) are defined by Windows. Values in this range that are not explicitly defined are reserved for future use by Windows. This chapter describes messages in this range.
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, such predefined control classes 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 reserved for future use by Windows.
Message numbers in the fourth range (0xC000 through 0xFFFF) are defined at run time when an application calls the RegisterWindowMessage function to obtain 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 in different Windows sessions.
RegisterWindowMessage