asked on

Intercepting a WM_ShutDown

I need to know how at the system level I can intercept a WM_Shutdown (Windows Shutdown message) and absorb it so that it doesnt occur.

The application I need to create needs to be able to tie into the system message loop and intercept any shutdown message and based on a registry flag either allow the shutdown or present a dialog that informs the user of the diversion.

Thanks

Q2

robert_marquardt

Catch the WM_QUERYENDSESSION message and give the correct answer.
The help on "WM_QUERYENDSESSION" and "message" should give enough info. The VCL source is full of message handler methods.

maverick65

add a function to your Form

TForm1 = class(TForm)
private
procedure onClose(var Msg:TMessage) message WM_Close;
procedure onShutdown(var Msg:TMessage) message WM_Shutdown;
end;

You can handle these messages now yourself.

maverick65

Sorry wrong syntax:

Take this one:
TForm1 = class(TForm)
private
procedure onClose(var Msg:TMessage); message WM_Close;
procedure onShutdown(var Msg:TMessage); message WM_Shutdown;
end;

quantum2

ASKER

Maverick,
is this to handle the shutdown at my individual application or at the OS. Basically, if someone slects Start/Shutdown I want to be able to intercept that message before it is broadcast across all open process and divert it.

-Q2

Robert, I will look into your suggestion.

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maverick65

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asymmetric

The good news :

You can intercept and halt shutdowns via the WM_QUERYENDSESSION message. Look in the Windows Platform SDK for details; basically it's a callback message. If you don't want windows to shutdown, just return false, and windows will stop sending these messages to applications and abort the shutdown.

The bad news :

You have absolutely, 100%, for sure, no way to ensure that you're the first window that receives this message. This means that when you get it, there's a good chance some others got it before you and already responded one way or the other. While this won't prevent you from preventing the shutdown, other applications that got the message before yours might have already shutdown.

In other words : You can prevent the system from shutting down, but you can't prevent any application in particular from closing due to this notification.. for all you know, your application will be the only one running once this is done.

quantum2

ASKER

Thanks for the links and the information. I really apprecaite it.

Q2