AsyncDelegate CallBack Thread-Safety problem...


Hi!

I have an async delegate.
On the CallBack I need to set come properties on the form controls.

Doesn't callback already run o the main thread?
I'm having problems with that, raising an exception saying that the object is being changed from a different thread than the one it was created.

Thanks!
Alex
LVL 31
Alexandre SimõesManager / Technology SpecialistAsked:
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devsolnsConnect With a Mentor Commented:
My main question is "In which thread does an AsyncDelegate CallBack run? on the main thread or on the Delegate thread?"

Ok I think i see what your saying and yes the method callback would be running on the new thread that is processing the async method.  The callback is just a function pointer and whatever thread calls that (function pointer/delegate/callback) is the thread that will execute that function.


Run this to see,






using System;
using System.Collections;
using System.Collections.Generic;
using System.Threading;


    class Program
    {
        public delegate void FooDelegate();

        public void Foo()
        {
            Console.WriteLine("Thread id in Foo: {0}", AppDomain.GetCurrentThreadId());
        }

        public void FooCallBack(IAsyncResult ar)
        {
            Console.WriteLine("Thread id in FooCallBack: {0}", AppDomain.GetCurrentThreadId());
        }

        static void Main(string[] args)
        {
            try
            {
                Console.WriteLine("Thread id at main start: {0}", AppDomain.GetCurrentThreadId());
                Program p = new Program();
                FooDelegate fd = new FooDelegate(p.Foo);

                IAsyncResult ar = fd.BeginInvoke(new AsyncCallback(p.FooCallBack), null);

                fd.EndInvoke(ar);

            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
            }
            finally
            {
                Console.WriteLine("Done");
                Console.ReadKey();
            }
        }    
    }
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devsolnsCommented:
If you need to set things on the form then use Invoke() method of the form class.
0
 
devsolnsCommented:
Intead of typing a lot here is a good article explaining everything in detail.

http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnforms/html/winforms06112002.asp
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Alexandre SimõesManager / Technology SpecialistAuthor Commented:

Invoke makes the process Syncronous...
I need it to be Async.

I used the new BackgroundWorker Component and it worked without any problems.

My main question is "In which thread does an AsyncDelegate CallBack run? on the main thread or on the Delegate thread?"
Since... ever... I thought it wuns on the main thread...

Alex :p
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devsolnsCommented:
Right I was trying to point in in direction of the Invoke methods including 'BeginInvoke' which operates asynchrnously.
0
 
devsolnsCommented:
An asynchronous method runs on another thread different than the one currently executing, its just hidden from you.
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Alexandre SimõesManager / Technology SpecialistAuthor Commented:
I know that an Asyncronous Delegate runs on another thread... that's why it's called Async...
What was suposed to be is the CallBack running on the main thread already...

And as it seems... it does not...
That's what I'm wondering why!

Alex
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Alexandre SimõesManager / Technology SpecialistAuthor Commented:
Yep... I've done some testings too and conclude exactly the same thing...
I can tell you that I've been around with .net since its version 1.0 beta and always thought te callback would run on the main thread...

Thanks mate...
Alex
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