Hi!

I'm designing a system for communicating with an USB device. Now I'm a
bit stuck with Microsoft's asynchronous operation concept in .NET. You
know, the stuff with a normal method (eg. Receive()) and two
asynchronous methods (eg. BeginReceive() and EndReceive()).

Now I'm wondering about the exact semantics of this concept. Maybe
someone with more experience can help me answer these questions:

- What happens if an error occurs during the asynchronous transmission?
Is it reported by EndReceive()?
- Do IAsyncResult.AsyncWaitHandle and IsCompleted both need to be set
regardless of the outcome of the operation?
- What about multiple simultaneous calls to BeginReceive()? Is this
allowed? Or can I decided whether to support it on a case-per-case basis?

Thanks,
-Markus-

Hello, Markus!

ME> I'm designing a system for communicating with an USB device. Now I'm a
ME> bit stuck with Microsoft's asynchronous operation concept in .NET. You
ME> know, the stuff with a normal method (eg. Receive()) and two
ME> asynchronous methods (eg. BeginReceive() and EndReceive()).

ME> Now I'm wondering about the exact semantics of this concept. Maybe
ME> someone with more experience can help me answer these questions:

ME> - What happens if an error occurs during the asynchronous transmission?
ME> Is it reported by EndReceive()?

Yes, it can be reported via throwing an exception on EndReceive(...), or with an out parameter in EndReceive(...)

ME> - Do IAsyncResult.AsyncWaitHandle and IsCompleted both need to be set
ME> regardless of the outcome of the operation?

From the docs:
"
Notes to Implementers The object that implements IAsyncResult does not need to create the WaitHandle until the AsyncWaitHandle property is read. It is the choice of the IAsyncResult implementer. However, if the implementer creates AsyncWaitHandle, it is the responsibility of the implementer to signal the WaitHandle that will terminate the wait at the appropriate time. For example, System.Runtime.Remoting.Messaging.AsyncResult terminates the wait on behalf of the caller when an asynchronously invoked method returns. Once created, AsyncWaitHandle should be kept alive until the user calls the method that concludes the asynchronous operation. At that time the object behind AsyncWaitHandle can be discarded.
"

So, AsyncWaitHandle and IsCompleted need to be set as callers will excepect appropriate (documented) behavior .

ME> - What about multiple simultaneous calls to BeginReceive()? Is this
ME> allowed?

Yes, every call to BeginReceive will spawn ( get thread from threadpool ) new thread.
However, callers typically will call BeginReceive/EndReceive sequentially

ME> Or can I decided whether to support it on a case-per-case basis?
You mean to block BeginReceive until previous operation is completed? If so, then BeginReceive will be the same as Receive...

Vadym Stetsyak wrote:
Hm, it makes sense, but I've never seen that described anywhere. Do you
indidentally still happen to know where you found that?

Alright, gues I overlooked that paragraph, sorry ;)

I had planned on adding a timeout argument to the Receive() method, but
I guess that probably wasn't such a good idea after all. As I understand
it now, Receive() is supposed to return immediately. If less than the
requested number of bytes is available, only that much data is returned,
just like the Stream.Read() methods.

Thanks,
-Markus-

Hello, Markus!

ME> Hm, it makes sense, but I've never seen that described anywhere. Do you
ME> indidentally still happen to know where you found that?

As an  example of such behavior consider async socket operations. Socket.BeginReceive/EndReceive

ME> I had planned on adding a timeout argument to the Receive() method, but
ME> I guess that probably wasn't such a good idea after all. As I
ME> understand it now, Receive() is supposed to return immediately. If less
ME> than the requested number of bytes is available, only that much data is
ME> returned, just like the Stream.Read() methods.

Yes, Receive returns immediately if data is available, it returns number of received data, if there is no data - it blocks...

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