Re: Async delegates, illegal cross thread UI control access

Thanks everyone for their reply, and doubt. I suppose I should have
mentioned I have a habit of running without debugging (Start->Start Without
Debugging). It seems that makes all the difference.

I created this little app:

          delegate string GetStringDelegate();

        private void button1_Click(object sender, EventArgs e)
        {
            Trace.WriteLine("button1_Click " +
                Thread.CurrentThread.ManagedThreadId.ToString());
            GetStringDelegate del = new GetStringDelegate(GetString);
            del.BeginInvoke(OnGetStringComplete, del);
        }

        private string GetString()
        {
            Trace.WriteLine("GetString " +
                Thread.CurrentThread.ManagedThreadId.ToString());
            return "Hello world";
        }

        private void OnGetStringComplete(IAsyncResult ar)
        {
            Trace.WriteLine("OnGetStringComplete " +
                Thread.CurrentThread.ManagedThreadId.ToString());
            GetStringDelegate del = ar.AsyncState as GetStringDelegate;
            button1.Text = del.EndInvoke(ar);
        }

        private void Form1_Load(object sender, EventArgs e)
        {
            Trace.WriteLine("Main thread ID: " +
                Thread.CurrentThread.ManagedThreadId.ToString());
        }

Ran it without debugging, and didn't get the illegal cross thread call.
I ran a second time with debugging and got the the InvalidOperationException
At the EndInvoke, which is what I was looking for.

Any reason why the exception is getting swallowed when run with debugging
would be nice to know.

But thanks,

Ron Young

-----Original Message-----
From: Discussion forum for developers using Windows Forms to build apps and
controls [mailto:[email protected]] On Behalf Of Fabian
Schmied
Sent: Friday, March 30, 2007 2:13 AM
To: [email protected]
Subject: Re: [DOTNET-WINFORMS] 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.

When you use "pictureBox.Refresh()" instead of "pictureBox.Image =
image", does this cause the error to appear?

If yes, there are two possibilities:
1 - It could be a small bug or oversight (i.e. they just forgot to add
that check), or
2 - It could be actually legal to set pictureBox.Image from different
threads, at least in the current implementation.

A quick glance via Reflector shows that the PictureBox actually does a
lot of locking, as itself seems to use a background thread for
animation. However, it also shows that not all of the code is
protected against multi-threading; therefore, I'd vote for version 1.
And even if 2 were correct, it's not documented and thus could not be
relied upon.

Fabian