AsyncCallback method accessing WebBrowser control causes exception

[Artie]

Hi,

We have an app which uses an AsyncCallback method to handle the return
from a COM call.

In this callback, if we try to do anything with a WebBrowser control,
we get the following exception:

"InvalidCastExc eption was unhandled
Specified cast is not valid"

If we remove all code which tries to access the WebBrowser, the
exception doesn't occur.

Has anyone got any idea why this would happen? We can access any
other controls in our app and no exception is thrown.

Any help much appreciated.

Artie

[Marc Gravell]

Re: AsyncCallback method accessing WebBrowser control causesexception

First-off; have you used Invoke/BeginInvoke to get back to the UI
thread before touching the controls?

Otherwise, I'd guess that you simply have a bug - i.e. you are
genuinely doing an invalid cast, perhaps picking up the wrong element
from a DHTML query?

Marc

[Marc Gravell]

Re: AsyncCallback method accessing WebBrowser control causesexception

Ah - confusion due to the very confusing Delegate.[Begin]Invoke vs
Control.[Begin]Invoke; I meant the latter, which pushes work back to
the UI thread. Your EndInvoke usage sounds like Delegate.EndInv oke...

What I meant was something like:

void MyCallback(IAsy ncResult result)
{
// (note: this bit runs on a worker thread)
// TODO: usual stuff to get the answer from result

// now jump to UI thread before touching controls
this.Invoke((Me thodInvoker)del egate
{
// (note: this bit runs on the UI thread)
// TODO: things that impact your WebBrowser, e.g.
webBrowser1.Lef t += 5;
});
}

[Marc Gravell]

Re: AsyncCallback method accessing WebBrowser control causes exception

I don't pretend that it is an answer to "why", but perhaps one good thing
about this approach is that it avoids accidental and overly inefficient
code - i.e.

x.Foo = 123;
// ... lots more, perhaps involving a loop
x.Bar = "abc";

If each of those lines (executed on a worker thread) silently did an
Invoke() behind your back, you could get some *seriously* poor performance.
However, wrap that up a bit (for not much extra work) and it should perform
OK:

x.Invoke((Metho dInvoker) {
x.Foo = 123;
// ... lots more, perhaps involving a loop
x.Bar = "abc";
});

I suspect that this is more by accident than by intent, however...

Marc