invoke on UI thread

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Paul - 09 Sep 2003 15:43 GMT

I understand the notion that a UI thread should be
responsible for managing the UI elements; therefore,
worker threads need to call invoke. I don't understand
the technical reason for this. What is it about the
Win32 message loop that doesn't like worker threads to
update the UI and why does it work anyway (at least most
of the time)?

Thanks for shedding some light,
Paul

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Thomas Tomicek [MVP] - 09 Sep 2003 16:09 GMT

Read up the erquirements for ActiveX Controls and the meaning of Single
Threaded Appartment (in the COM documentation, not in the .NET
documentation).

The reason is indeed in the Win32 message loop and legacy.

It works often because not all controls are native- if basically blows the
moment you touch a critical component that is managed by the OS and actually
does use the environment it has a right to be there.

The documentation on whow COM works and what ActiveX controls are is pretty
enlighting for this.

Signature

Regards

Thomas Tomiczek
THONA Software & Consulting Ltd.
(Microsoft MVP C#/.NET)

> I understand the notion that a UI thread should be
> responsible for managing the UI elements; therefore,
[quoted text clipped - 6 lines]
> Thanks for shedding some light,
> Paul

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Michael Bray - 10 Sep 2003 23:28 GMT

On a similar subject, I've had problems with Invoke() locking up -
I've verified (with Console.WriteLine's) that the function that I'm
requestion with Invoke never executes... so I've resorted to NOT
using Invoke, even though I know I should. Any ideas?

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Thomas Tomicek [MVP] - 11 Sep 2003 07:39 GMT

> On a similar subject, I've had problems with Invoke() locking up -
> I've verified (with Console.WriteLine's) that the function that I'm
> requestion with Invoke never executes... so I've resorted to NOT
> using Invoke, even though I know I should. Any ideas?

Change the (bad) decision to not use invoke where you should.

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Paul - 11 Sep 2003 14:34 GMT

In COM, when calling an object in STA from another
apartment, COM will marshall the call across apartments.
Tom, are you saying that .NET uses COM underneath to
marshall calls from free-threaded apartment to STA where
the UI elements reside? Essentially, calling Invoke(),
makes sure that the correct thread manages the UI element
even though a worker thread also has the UI element's
handle?

>-----Original Message-----
>> On a similar subject, I've had problems with Invoke() locking up -
[quoted text clipped - 5 lines]
>
>.

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Thomas Tomicek [MVP] - 11 Sep 2003 14:45 GMT

> In COM, when calling an object in STA from another
> apartment, COM will marshall the call across apartments.
[quoted text clipped - 4 lines]
> even though a worker thread also has the UI element's
> handle?

Naturally there sis a COM threading model below. How else could it accept
ActiveX controls on a form without problems?

The documentation is clear: you are supposed to invoke and thus marshal back
to the UI thread.

This, though, has nothing to do with COM, btw- the threading model was IMHO
there before COM came around.

Signature

Regards

Thomas Tomiczek
THONA Software & Consulting Ltd.
(Microsoft MVP C#/.NET)

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invoke on UI thread

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