Path: utzoo!utgpu!jarvis.csri.toronto.edu!clyde.concordia.ca!uunet!microsoft!michaelt
From: michaelt@microsoft.UUCP (Michael THURLKILL)
Newsgroups: comp.windows.ms
Subject: Re: RegisterClass in DLL
Message-ID: <10126@microsoft.UUCP>
Date: 3 Jan 90 15:06:50 GMT
References: <6970014@hpmcaa.mcm.hp.com> <34458@mirror.UUCP>
Reply-To: michaelt@microsoft.UUCP (Michael THURLKILL)
Organization: Microsoft Corp., Redmond WA
Lines: 34

In article <34458@mirror.UUCP> bobg@mirror.TMC.COM (Bob Goodearl) writes:
>In article <6970014@hpmcaa.mcm.hp.com> marco@hpmcaa.mcm.hp.com
> (Marco Dalla-Gasperina) writes:
>=>I'm having a problem registering a window class inside a DLL...  It seems
>=>to work for a "few" times (usually 3) but then it fails?
>=>
>
>When writing DLL code, I found that a window class registered using the
>DLL instance handle caused problems.  If the application calling the
>DLL was closed and then started a second time without exiting windows
>first, the call to RegisterClass in the DLL would fail the second time.
>The solution I found was to use the parent application's instance
>handle when calling RegisterClass in the DLL.  This works for the code
>I'm writing because only 1 application can use the DLL and only 1
>instance of the application can exist at any one time.
>
The problem is that the class isn't automatically de-registered when
the DLL  exits, and it is not possible to request that the class be
de-registered. Things will work ok in the case of using hInst of the app
because the class then "belongs" to the task and goes away when the task
terminates. However, if more than one app uses the class, and the first
app terminates, the class is destroyed, and the other apps will shortly
crash. A class can be registered, and shared by applications, if it is
registered in the DLL, uses the DLL's hInst, AND the DLL stays around 
forever. To keep the DLL around, it must be loaded independantly, ie.
file-run from the MSDOS Executive or placed on the load= line in win.ini.

This is one of those things that "is under consideration and will be
addressed in a future release." But seriously, I think it will be fixed.

Mike Thurlkill

Disclaimer: These thoughts and opinions are mine. They should not be
mis-construed as being correct or in any way related to my employer.