what is an asynchronous event?

[ALI-R]

Hi All,

When we say events are processed asynchronously in for instance Sharepoint
,what dose it mean?

Thanks for your help.
Ali

[ALI-R]

Thanks Bruce,Like always your answers are informative.

Ali-R
"Bruce Wood" <br*******@cana da.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

I don't know about the particular routines you're calling, but usually
a synchronous invocation does not involve event handlers at all: you
call a method, it waits until some operation completes, then it returns
to you.

Of course, there may be an asynchronous invocation going on under the
covers. Some "synchronou s" method invocations are nothing more than
methods that send an event and don't return until the corresponding
reply event comes back. However, from the point of view of the caller,
a synchronous invocation doesn't need an event handler because you know
when the operation is done: when the method returns.

[Bruce Wood]

Thanks. :)

One more thing: I just remembered an example of a synchronous operation
that accepts a delegate, which is easily confused with an event handler
(since handling events is a common use for a delegate).

If you look at Image.GetThumbn ailImage(), you'll see that it accepts a
"callback delegate", even though it is a synchronous operation (that
is, it doesn't return to its caller until the thumbnail image is
complete).

Don't confuse this with an "event handler". GetThumbnailIma ge() calls
this delegate from time to time to see if the user wants to cancel the
operation. However, it's just a delegate. There are no events involved.
Same language construct, different purpose.

[ALI-R]

Bruse a quick question:

For innstance calling Update method from DataAdapter (According to your
explanation) must be classified as Synchronous invocation,righ t?
because the user is left waiting till he gets notified by the result of his
invoction.

Thanks
Ali-R
"Bruce Wood" <br*******@cana da.com> wrote in message
news:11******** *************@z 14g2000cwz.goog legroups.com...

Thanks. :)

One more thing: I just remembered an example of a synchronous operation
that accepts a delegate, which is easily confused with an event handler
(since handling events is a common use for a delegate).

If you look at Image.GetThumbn ailImage(), you'll see that it accepts a
"callback delegate", even though it is a synchronous operation (that
is, it doesn't return to its caller until the thumbnail image is
complete).

Don't confuse this with an "event handler". GetThumbnailIma ge() calls
this delegate from time to time to see if the user wants to cancel the
operation. However, it's just a delegate. There are no events involved.
Same language construct, different purpose.

[Bruce Wood]

Yes. I've used it myself, and it's synchronous: the caller is left
waiting until the database is updated and the number of affected rows
is returned.

I haven't tried this myself, but if you wanted an asynchronous Update
(update the database while "I" (the calling method) continue to do
something else, and let me know when you're done), it looks to me as
though you have to "roll your own" asynchrony by spinning off another
thread and asking for notification when that other thread finishes its
work (the Update). That way you could continue what you were doing
while the other thread blocked on the Update operation. When the
Updated completed, the other thread would then send you an event (via
Invoke) to tell you that it was done, and terminate.

Your thread (the main thread) could then continue working while the
Update was happening, and either pay little attention to the resulting
event coming back, or, if the user asked to do something that logically
required the update to be complete, block at that point, waiting for
the Update to finish before moving on. (Asynchrony doesn't mean that
you can't _ever_ wait for the operation to complete before moving on;
it just means that you have the choice not to wait if you don't want
to. In a synchronous world, you have no choice: you have to wait.)

You might want to read at least part of Jon Skeet's article on threads:
http://www.yoda.arachsys.com/csharp/threads/