Windows custom message ID

Wm_user to 0x7fff are reserved for private window classes. You can think of this range as meaningful private messages in a specific form class. Applications that want to define their own messages ProgramWm_app should be used.

Wm_app ensures that it does not conflict with system (wm_create, etc.) or class/specific control messages such as dm_getdefid

Message ranges in Windows

0-(WM_USER-1)Reserved messages used by the system, such as wm_create and wm_scroll.

Wm_user-0x7fffInteger messages used by the private window class, such as dm_getdefid (dialog box), tb_enablebutton (toolbar), and pbm_setrange (progress bar ).

Wm_app-0 xbfffThe message used by the application.

0xc000-0 xFFFFThe string message used by the application (registerwindowmessage).> 0 xFFFF windows is retained for future use.