Q: Find the
Registered Message Name, Given Its Number
I'm a Delphi developer, and am quite horrifically stuck on a program I'm
putting together that involves subclassing areas of Windows and dealing
with certain messages. My problem is that Windows is sending a seemingly
unidentifiable message that falls into the registered messages range. I
know that if I could send this message, my program would go from a state
of "Nice idea, shame about the implementation," to "Woohoo—it
works!" What I can't figure out is how to identify the string that
has been registered for this message.
A: Some background for those not
familiar with registered messages—Windows provides a mechanism by
which applications can generate a unique message number for their own
use. An app can call the RegisterWindowMessage API, passing an
identification string such as "MyAppsSpecialMessage", and
Windows registers this string with a value that any other app can then
retrieve by attempting to register the same string. Registered messages
fall into this range: &hC000& (49152) to &hFFFF&
(65535). The trouble is, there is no obvious method by which an
application can do the reverse—retrieve the string given a message
number.
The clue needed here is the explicit range used by registered
messages. Searching MSDN turns up a variety of other things that also
fall into this exact range, among them global atom values, registered
clipboard formats, and system-wide hotkeys. Two of these provide reverse
lookups that offer potential means of locating what you need. Trying
GlobalFindAtom reveals that Windows uses a different table to store
these string values. But a quick call to GetClipboardFormatName shows
you can retrieve a registered message name as if it were a clipboard
format name.
In fact, Windows seems to cache a variety of different strings within
this single table. To get an idea of how many different uses this core
algorithm addresses, you can iterate the entire registered message
range, calling GetClipboardFormatName against each possible value and
adding the returned strings to a listbox (see
Listing 1). You'll see quite a range of items, many unreadable, but
many instantly recognizable. —K.E.P.
About the Author
Karl E. Peterson is a GIS analyst with a regional
transportation-planning agency and serves as a member of the VSM
Technical Review and Editorial Advisory boards. Online, he's a Microsoft
MVP and a section leader on several VSM forums. Find more of
Karl's VB samples at www.mvps.org/vb.
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