Re: Async delegates, illegal cross thread UI control access

Yeah, to be sure I set CheckForIllegal.... to true in Form_Load. I remember
when I starting VS2005 and did some async calls which ultimately updated the
UI, that I got that warning and thought that was pretty conveniant, and it
taught me the correct way to do it. But now, I can't get that warning back.

I'm almost sure that if I attempted the same functionality using a Thread
versus async delegates, that I'd raise the error.

Ron

----- Original Message -----
From: "Peter Ritchie" <[email protected]>
To: <[email protected]>
Sent: Thursday, March 29, 2007 2:02 PM
Subject: Re: [DOTNET-WINFORMS] Async delegates, illegal cross thread UI
control access


> That sounds odd.  IIRC, There's really only three instances where you
> wouldn't get the cross-thread exception: the control hasn't fully been
> created yet, CheckForIllegalCrossThreadCalls is set to false, or the
> framework thinks your calling a thread-safe call.
>
> On Thu, 29 Mar 2007 13:49:51 -0500, Ron <[email protected]> wrote:
>
>>Hi all,
>>
>>I've encountered a situation where I thought VS2005 would warn me about a
>>illegal cross thread UI call. I actually wanted the error to happen
> because
>>I'm doing a demo for my peers on the Asynchronous Programming Model.
>>
>>WinForms app, long running web service call of 3-5 seconds. Don't want to
>>synchronously invoke the service call, and not using the asynchronous
>>methods in the proxy yet.
>>
>>The app is demonstrating the Asynchronous Programming Model ala MSDN docs
> to
>>my peers.
>>
>>Step 1. Create a delegate to match the service call.
>>
>>delegate Image GetMapImageDelegate();
>>
>>Step 2. Create a delegate instance, invoke it
>>
>>void btnGetMap_Click()
>>{
>> GetMapImageDelegate del = new GetMapImageDelegate(GetMapImage);
>> del.BeginInvoke(OnGetMapImageComplete, del);
>>}
>>
>>Image GetMapImage()
>>{
>> // long running service call here
>> // return image;
>>}
>>
>>Step 3. In the callback, end the async invocation, getting the results
>>
>>void OnGetMapImageComplete(IAsyncResult ar)
>>{
>> GetMapImageDelegate del = ar.AsycnState as GetMapImageDelegate;
>> Image image = del.EndInvoke(ar);
>>
>> // i expect this to cause the cross-thread error
>> pictureBox.Image = image;
>>}
>>
>>My understanding is that the thread in OnGetMapImageComplete is a thread
>>pool thread, and therefore accessing the pictureBox control should be
>>illegal. I traced the Thread.CurrentThread.ManagedThreadID in each method,
>>and showed that there are two different threads: btnGetMap_Click had ID
> 10,
>>all others were 11.
>>
>>I was all prepared to create yet another delegate to handle displaying the
>>image with the check for InvokeRequired pattern, but if the error (cross
>>thread access to UI control) doesn't exist, then...well it isnt' necessary
>>to do that.
>>
>>Any explanation as to why this is not occuring is appreciated.
>