Which message numbers belong to whom?
Valid window messages break down into four categories.
-
0 .. 0x3FF (WM_USER-1): System-defined messages.
-
The meanings of these messages are defined by the operating
system and cannot be changed. Do not invent new messages
here. Since the meanings are defined by Windows,
the operating system understands how to parse the WPARAM
and LPARAM parameters and can marshal the messages between
processes (or knows to refuse to do so).
-
0x400 .. 0x7FFF (WM_USER .. WM_APP-1): Class-defined messages.
-
The meanings of these messages is determined by the
implementor of the window class.
(Informally: By the person who calls RegisterClass for that
window class.)
For example, the WM_USER+1
message means TB_ENABLEBUTTON if the window is a toolbar control,
but it means TTM_ACTIVATE if it is a tooltip control, and it
means DM_SETDEFID if it is a dialog box.
If you created your own control, it would mean something else
completely different.
Since anybody can create a message in this range,
the operating system does not know what the parameters
mean and cannot perform automatic marshalling.
-
0x8000 .. 0xBFFF (WM_APP ... MAXINTATOM-1): Application-defined
messages.
-
The meanings of these messages is determined by
the application that created the window.
(Informally: By the person who calls CreateWindow.)
This message region was created in Windows 95 to ensure that
applications which subclass a window and generate custom
messages will not interfere with new messages created by the
window class in future versions.
Again, since anybody can create a message in this range,
the operating system does not know what the parameters
mean and cannot perform automatic marshalling.
-
0xC000 .. 0xFFFF (MAXINTATOM .. MAXWORD): Registered messages.
-
The meanings of these messages is determined by the
caller of RegisterWindowMessage. Note that the numerical
value of registered messages can change from run to run,
so you must use RegisterWindowMessage to obtain the message
number. Once again,
since anybody can create a message in this range,
the operating system does not know what the parameters
mean and cannot perform automatic marshalling.
