Asynchronous events exception handling

[Sharon]

I have a remoting object, lets call it A, that fires an event to remotely
registered handlers, lets call it B.
The invocation is working OK and the remote handler B is handling the event
like it should.

But I need A to catch any exception thrown from the handler B, and also I
need that A will get the returned value returned by handler B.
For that I wrote:

private void ResultsHandler( IAsyncResult ar)
{
// Extract the delegate from the AsyncResult.
MyEventHandler handler =
(MyEventHandler )((System.Runti me.Remoting.Mes saging.AsyncRes ult)ar).AsyncDe legate;

// Obtain the result which I do not have because the delegate is void,
// but still need to that in order to catch exception caused in the.
handler.EndInvo ke(ar);
}

public void RemtinMethod(lo ng[] IDs)
{
// I'm using the AsyncCallback and state Object to be able to catch
exception caused in the delegate asynchronous invocation.
AsyncCallback cb = new AsyncCallback(R esultsHandler);
Object state = new Object();

if( RemoveDefectHan dler != null )
{
foreach( MyEventHandler handler in MyEventHandler. GetInvocationLi st() )
{
try
{
handler.BeginIn voke(IDs, cb, state);
}
catch( Exception )
{
throw;
}
}
}
}
But still I can not get the returned value, and I'm also not catching at A
any thrown exception from B.

What I'm I doing wrong?

Is there any difference if I using the VS2003 or the VS2005? If so, what is
it?
--------
Thanks
Sharon

[Sharon]

Hello again,
I have some more info that may help.

(1) In the code I have posted I don't need the returned value.
(to get the returned value I just need to add: int result =
handler.EndInvo ke(ar); at the ResultsHandler( ) )

(2) I can catch any exception thrown from B at the ResultsHandler( ), but the
ResultsHandler( ) is invoked from another thread and not the invoking thread
(A that invoke the handler.BeginIn voke(IDs, cb, state);),
But I need to catch the exception at the calling thread A.
How do I do that?
---------
Thanks
Sharon

[Luke Zhang [MSFT]]

Hello Sharon,

How about catching exception just in B (ResultsHandler ), and pass the
exception information back as result?

Luke

[Sharon]

Thanks Luke,

That would be great.
But how can I do that? they are in a different thread.
---------
Thanks
Sharon

[Luke Zhang [MSFT]]

As you have posted:

to get the returned value I just need to add: int result =
handler.EndInvo ke(ar); at the ResultsHandler( ) )

Luke

[Sharon]

That's good. But I need to catch also the exceptions and that also can be
done at the ResultsHandler( ). But the ResultsHandler( ) is invoked from
another thread then the thread the BegineInvoke() was called, and I need this
two values (returned value and caught exception) at the thread that invoked
the BegineInvoke() - calling thread A.

Is there a easy way to do that?
--
Thanks
Sharon

[Luke Zhang [MSFT]]

You may return a collection or array intead of a single object as returned
value. This is not perfect solution, but I don't there is a better one to
catch such an exception in another thread.

Luke

[Sharon]

But it's impossible to just return, the ResultsHandler( ) does not return any
value and even if does, where this returned value will go to. As the
ResultsHandler( ) is in different thread then the thread I need the returned
value and exception.

There must be a comfortable way to do that. What is it?

-------
Thanks
Sharon

[Joerg Jooss]

Hello Sharon,

But it's impossible to just return, the ResultsHandler( ) does not
return any value and even if does, where this returned value will go
to. As the ResultsHandler( ) is in different thread then the thread I
need the returned value and exception.

There must be a comfortable way to do that. What is it?

My preferred approach to communicate exceptions across threads is using events.
Assume you have two classes: class A uses an asynchronous service offered
by class B. B also has an "Error" event, for which A registers an event handler
before invoking any of B services. In all of B's relevant catch blocks, B
fires of its Error event to notify its clients (such as A) of an exception.

Cheers,
--
Joerg Jooss
ne********@joer gjooss.de

[Sharon]

Communicating among thread is fine, but I just thought that the asynchronous
delegates like I'm using by Remoting has this mechanism built in to catch
exception and return value in the calling thread and not in a callback at
another thread.

-------
Thanks
Sharon