In asynchronous model, IAsyncResult.AsyncWaitHandle signaled first, or AsyncCallback invoked first?

[Morgan Cheng]

In asynchronous model, BeginXXX method returns a IAsnycResult-derived
object. IAsyncResult.AsyncWaitHandle will be signaled when the
asynchronous job is complete; and BeginXXX method has a argument as
AsyncCallback to be invoked when the job is complete.

I am wondering. When the asynchronous is complete, is
IAsyncResult.AsyncWaitHandle be signaled first, or AsyncCallback be
invoked first?

[Peter Duniho]

Morgan Cheng wrote:

[...]
I am wondering. When the asynchronous is complete, is
IAsyncResult.AsyncWaitHandle be signaled first, or AsyncCallback be
invoked first?

Surely they are effectively simultaneous. That is, obviously one
happens before the other. But they likely occur in practically
immediate sequence, and because of the way thread scheduling works on
Windows, it's entirely possible that if you have both a callback and a
thread waiting on the WaitHandle, that either the code in the callback
or the code waiting on the WaitHandle could get scheduled next.

In other words, even if one or the other is technically always done
first, the observed result could in fact be the opposite. I would
suggest that this is one of those things that you should not assume is
well-defined and should not rely on a specific behavior in code,
especially if you are asking as a general question related to anything
that uses IAsyncResult (even if the behavior is well-defined for a
particular class, I'm not sure you could depend on it being well-defined
for _all_ classes that implement the asynchronous paradigm).

I'm curious if there's a specific scenario in which this question came
up? It seems like an interesting enough academic question, but since
you would normally use _either_ the WaitHandle _or_ a callback, not both
together, it's not clear why this would come up in actual code.

Pete

[Morgan Cheng]

On Aug 2, 2:34 pm, Peter Duniho <[e-mail]:

Morgan Cheng wrote:

[...]
I am wondering. When the asynchronous is complete, is
IAsyncResult.AsyncWaitHandle be signaled first, or AsyncCallback be
invoked first?

Surely they are effectively simultaneous. That is, obviously one
happens before the other. But they likely occur in practically
immediate sequence, and because of the way thread scheduling works on
Windows, it's entirely possible that if you have both a callback and a
thread waiting on the WaitHandle, that either the code in the callback
or the code waiting on the WaitHandle could get scheduled next.

In other words, even if one or the other is technically always done
first, the observed result could in fact be the opposite. I would
suggest that this is one of those things that you should not assume is
well-defined and should not rely on a specific behavior in code,
especially if you are asking as a general question related to anything
that uses IAsyncResult (even if the behavior is well-defined for a
particular class, I'm not sure you could depend on it being well-defined
for _all_ classes that implement the asynchronous paradigm).

I'm curious if there's a specific scenario in which this question came
up? It seems like an interesting enough academic question, but since
you would normally use _either_ the WaitHandle _or_ a callback, not both
together, it's not clear why this would come up in actual code.

Pete

I did some googling and find this post
http://www.bluebytesoftware.com/blog...tAddendum.aspx.
The correct sequence should be
1) set IsCompleted to true,
2) signal the handle,
3) invoke the callback.

This is reasonable, because the callback function might wait on the
AsyncWaitHandle. If the sequence of 2) & 3) is inverted, it would be a
deadlock.
Even the callback doesn't depends on the AsyncWaitHandle, it might be
a time-costing operation, which can delay signaling of
AsyncWaitHandle. And, if other operations depends on AsyncWaitHandle,
they are delayed. That's not good.

With the help of Lutz Roeder's .Net Reflector, I read the reflected
code of System.Web.Services.Protocols.WebClientAsyncResult , which is a
subclass of IAsyncResult. Its Complete function does in the sequence
as mentioned above.

Of course the implementation can be like this
1) set IsCompleted to true;
2) invoke the callback with BeginInvoke;
3) signal the handle
But this involves a unnecessary threading switch.