PostMessage unsafe?

[Sergei]

Can anyone explain why PostMessage to a Windows Form
is considered unsafe and raises InvalidOperationException?
I can get rid of that setting CheckForIllegalCrossThreadCalls
to false and everything works just fine. Still I feel somewhat
uncomfortable.

Sergei

[Stoitcho Goutsev \(100\)]

Sergei,

What is this method? AFAIK Widnows Forms doesn't provide such a method. Are
you talking about the API method that can be used through PInvoke? If you
are then this is the answer of your question why it is unsafe.

Brief cosulting with the MSDN tells me that there is PostMessage for
WindowsCE. Is it the one that you are asking for?
--

Stoitcho Goutsev (100)

"Sergei" <se****@nospam.summertime.mtu-net.ruwrote in message
news:%2****************@TK2MSFTNGP02.phx.gbl...

Can anyone explain why PostMessage to a Windows Form
is considered unsafe and raises InvalidOperationException?
I can get rid of that setting CheckForIllegalCrossThreadCalls
to false and everything works just fine. Still I feel somewhat
uncomfortable.

Sergei

[Willy Denoyette [MVP]]

"Sergei" <se****@nospam.summertime.mtu-net.ruwrote in message
news:%2****************@TK2MSFTNGP02.phx.gbl...
| Can anyone explain why PostMessage to a Windows Form
| is considered unsafe and raises InvalidOperationException?
| I can get rid of that setting CheckForIllegalCrossThreadCalls
| to false and everything works just fine. Still I feel somewhat
| uncomfortable.
|
| Sergei

IMO this is not caused by the PostMessage API call, but what I know is that
you are somehow "touching" the UI from a thread other than the UI thread.
This is something you should not do, when you need to access UI elements
from another thread, you have to marshal the call's to the control's owning
thread using one of the Control.Invoke or BeginInvoke methods.

Willy.

[sergei]

Stoitcho Goutsev (100) wrote:

Sergei,

What is this method? AFAIK Widnows Forms doesn't provide such a method. Are
you talking about the API method that can be used through PInvoke? If you
are then this is the answer of your question why it is unsafe.

Brief cosulting with the MSDN tells me that there is PostMessage for
WindowsCE. Is it the one that you are asking for?

No, it's API and I call it from another the UI thread.
Still I don't understand why it is thread unsafe.
I've always used PostMessage for safe interthread communication, now it
appears it is not that safe.

Sergei

[sergei]

Willy Denoyette [MVP] wrote:

"Sergei" <se****@nospam.summertime.mtu-net.ruwrote in message
news:%2****************@TK2MSFTNGP02.phx.gbl...
| Can anyone explain why PostMessage to a Windows Form
| is considered unsafe and raises InvalidOperationException?
| I can get rid of that setting CheckForIllegalCrossThreadCalls
| to false and everything works just fine. Still I feel somewhat
| uncomfortable.
|
| Sergei

IMO this is not caused by the PostMessage API call, but what I know is that
you are somehow "touching" the UI from a thread other than the UI thread.

Yes, I call it from another than UI thread.

This is something you should not do, when you need to access UI elements
from another thread, you have to marshal the call's to the control's owning
thread using one of the Control.Invoke or BeginInvoke methods.

I do not access any elements, I just post WM_USER message to a windows
handle.
And I still can't understand what might be unsafe in doing that.

Sergei

[Ben Newsam]

On 4 Sep 2006 05:36:46 -0700, se****@summertime.mtu-net.ru wrote:

>Stoitcho Goutsev (100) wrote:

>Sergei,

What is this method? AFAIK Widnows Forms doesn't provide such a method. Are
you talking about the API method that can be used through PInvoke? If you
are then this is the answer of your question why it is unsafe.

Brief cosulting with the MSDN tells me that there is PostMessage for
WindowsCE. Is it the one that you are asking for?

No, it's API and I call it from another the UI thread.
Still I don't understand why it is thread unsafe.
I've always used PostMessage for safe interthread communication, now it
appears it is not that safe.

I think it is not really so much "unsafe" as "unmanaged", which is not
the same at all. That being said, if you post a message to another
application, you have no idea what is going to happen, so from that
point of view it is definitely "unsafe".

--
Posted via a free Usenet account from http://www.teranews.com

[Willy Denoyette [MVP]]

<se****@summertime.mtu-net.ruwrote in message
news:11**********************@m79g2000cwm.googlegr oups.com...
|
| Willy Denoyette [MVP] wrote:
| "Sergei" <se****@nospam.summertime.mtu-net.ruwrote in message
| news:%2****************@TK2MSFTNGP02.phx.gbl...
| | Can anyone explain why PostMessage to a Windows Form
| | is considered unsafe and raises InvalidOperationException?
| | I can get rid of that setting CheckForIllegalCrossThreadCalls
| | to false and everything works just fine. Still I feel somewhat
| | uncomfortable.
| |
| | Sergei
| >
| IMO this is not caused by the PostMessage API call, but what I know is
that
| you are somehow "touching" the UI from a thread other than the UI
thread.
|
| Yes, I call it from another than UI thread.
|
| This is something you should not do, when you need to access UI elements
| from another thread, you have to marshal the call's to the control's
owning
| thread using one of the Control.Invoke or BeginInvoke methods.
|
| I do not access any elements, I just post WM_USER message to a windows
| handle.
| And I still can't understand what might be unsafe in doing that.

This exception is not thrown by the PostMessage call (it simply can't), IMO
somehow, you are getting the handle to be used in your PostMessage call on
the same (non UI) thread and this is when you get the exception, this handle
was created on the UI thread and the frameword guards against cross-thread
accesses to HWND's.
Say you have a Control instance MyControl on the UI, and you have written:

PostMessage (MyControl.Handle,...);

this will throw an exception when executed on another thread, but it's not
PostMessage who throws, it's the property getter MyControl.Handle. who
throws CheckForIllegalCrossThreadCalls.
Now, in this case it's not an error to use the handle, but it's better to
correct the code such that you are calling the API on the UI thread. Another
option is to cache the handle on the UI thread and read the IntPtr (wrapping
the handle) from other threads.

Willy.

[Stoitcho Goutsev \(100\)]

Oh, you are say that you get some threading error. You can call PostMessage
from any thread. It is not thread dependent. I believe your problem is
somewhere else.

Can you post some compilable sample code? I don't think the problem is
thread related. However calling unmanaged code (API) is unsafe operation and
can be executed only when the application is running with full trust.
--
HTH
Stoitcho Goutsev (100)

<se****@summertime.mtu-net.ruwrote in message
news:11**********************@74g2000cwt.googlegro ups.com...

Stoitcho Goutsev (100) wrote:

>Sergei,

What is this method? AFAIK Widnows Forms doesn't provide such a method.
Are
you talking about the API method that can be used through PInvoke? If you
are then this is the answer of your question why it is unsafe.

Brief cosulting with the MSDN tells me that there is PostMessage for
WindowsCE. Is it the one that you are asking for?

No, it's API and I call it from another the UI thread.
Still I don't understand why it is thread unsafe.
I've always used PostMessage for safe interthread communication, now it
appears it is not that safe.

Sergei

[Sergei]

Stoitcho Goutsev (100) wrote:

Oh, you are say that you get some threading error. You can call PostMessage
from any thread. It is not thread dependent. I believe your problem is
somewhere else.

Can you post some compilable sample code? I don't think the problem is
thread related. However calling unmanaged code (API) is unsafe operation and
can be executed only when the application is running with full trust.

Alas. I left that job on 31.08 and it's the second day in another
company. Moreover here I work with VS.2003 while there it
was 2005. And the error is VS version specific. On 2005 certain
operations on WinForms raise CheckForIllegalCrossThreadCalls
exceptions if performed on another than the form's thread.
I couldn't understand why a PostThread API call would raise the
exception.

Anyway thanks for responding
Sergei

[Stoitcho Goutsev \(100\)]

Sergei,

It seems you found a solution to your problem :-P
Good luck with your new job.

--
Stoitcho Goutsev (100)
"Sergei" <se****@summertime.mtu-net.ruwrote in message
news:11**********************@d34g2000cwd.googlegr oups.com...

Stoitcho Goutsev (100) wrote:

>Oh, you are say that you get some threading error. You can call
PostMessage
from any thread. It is not thread dependent. I believe your problem is
somewhere else.

Can you post some compilable sample code? I don't think the problem is
thread related. However calling unmanaged code (API) is unsafe operation
and
can be executed only when the application is running with full trust.

Alas. I left that job on 31.08 and it's the second day in another
company. Moreover here I work with VS.2003 while there it
was 2005. And the error is VS version specific. On 2005 certain
operations on WinForms raise CheckForIllegalCrossThreadCalls
exceptions if performed on another than the form's thread.
I couldn't understand why a PostThread API call would raise the
exception.

Anyway thanks for responding
Sergei

[Sergei]

Willy Denoyette [MVP] wrote:

<se****@summertime.mtu-net.ruwrote in message
news:11**********************@m79g2000cwm.googlegr oups.com...
|
| Willy Denoyette [MVP] wrote:
| "Sergei" <se****@nospam.summertime.mtu-net.ruwrote in message
| news:%2****************@TK2MSFTNGP02.phx.gbl...
| | Can anyone explain why PostMessage to a Windows Form
| | is considered unsafe and raises InvalidOperationException?
| | I can get rid of that setting CheckForIllegalCrossThreadCalls
| | to false and everything works just fine. Still I feel somewhat
| | uncomfortable.
| |
| | Sergei
| >
| IMO this is not caused by the PostMessage API call, but what I know is
that
| you are somehow "touching" the UI from a thread other than the UI
thread.
|
| Yes, I call it from another than UI thread.
|
| This is something you should not do, when you need to access UI elements
| from another thread, you have to marshal the call's to the control's
owning
| thread using one of the Control.Invoke or BeginInvoke methods.
|
| I do not access any elements, I just post WM_USER message to a windows
| handle.
| And I still can't understand what might be unsafe in doing that.

This exception is not thrown by the PostMessage call (it simply can't), IMO
somehow, you are getting the handle to be used in your PostMessage call on
the same (non UI) thread and this is when you get the exception, this handle
was created on the UI thread and the frameword guards against cross-thread
accesses to HWND's.
Say you have a Control instance MyControl on the UI, and you have written:

PostMessage (MyControl.Handle,...);

this will throw an exception when executed on another thread, but it's not
PostMessage who throws, it's the property getter MyControl.Handle. who
throws CheckForIllegalCrossThreadCalls.
Now, in this case it's not an error to use the handle, but it's better to
correct the code such that you are calling the API on the UI thread. Another
option is to cache the handle on the UI thread and read the IntPtr (wrapping
the handle) from other threads.

I can't reproduce this today because I changed the job and in the
new place only have VS.2003 but I remember I tried this converting
the Forms handle into int, and than in the worker thread converting
it back into IntPtr. It didn't work.

Anyway thanks for responding
Sergei