MSDN Magazine - March 2008 - (Page 129)

Asynchronous Stream Processing STEPHEN TOUB Q I’ve read about some neat tricks you can play to ease the development of asynchronous implementations using C# iterators. (Jeffrey Richter was extolling the virtues of such techniques in a previous issue of MSDN® Magazine.) However, I was trying to figure out how to do similar things without such code transformations, and I was having a very hard time. For example, let’s say I want to copy from one stream to another, using only the asynchronous methods on Stream. I understand how to issue the asynchronous requests once (to do one read and one write), but how can I loop such requests continually in order to process the whole stream? Is that even possible? Yes, it’s possible. I’ll show how, using your example, with both C# and Visual Basic® (noting that Visual Basic doesn’t provide compiler support for iterators). To set the stage, Figure 1 shows a synchronous implementation of what we want to implement asynchronously, which could be called with code such as this: FileStream input = ; FileStream output = ; CopyStreamToStream(input, output, (src,dst,exc) => { src.Close(); dst.Close(); }); A With the synchronous implementation, CopyStreamToStream will block until the stream has been copied and the completed callback has been executed. You can easily implement this as an asynchronous invocation in a few ways. One would be to explicitly queue to the ThreadPool a work item that calls this method: ThreadPool.QueueUserWorkItem(delegate { CopyStreamToStream(input, output, (src,dst,exc) => { src.Close(); dst.Close(); }); }); But neither of those approaches is asynchronous in the way you’re looking for. Yes, they’re both asynchronous with respect to the thread starting their execution (meaning that the calling thread won’t be blocked waiting for their completion); however, in both cases the CopyStreamToStream method is just being run on a thread from the pool, and all of the reads and writes it’s issuing are synchronous with regard to the rest of the method call. You can have a If I were to launch Copytruly asynchronous StreamToStream asynchronously in this fashion 100 implementation that copies times, 100 threads (possibly all of the data from one reused) would be needed stream to another. to execute those copies. Not only is that significant overhead, but given the number of threads available in the thread pool and the rate at which the thread count is increased, a significant number of the copies probably wouldn’t begin execution until others had finished, increasing the overall time to completion. What you’re looking for is a true asynchronous implementation of CopyStreamToStream, where instead of issuing Read and Write calls on the Streams, you issue BeginRead and BeginWrite calls, with appropriately matched calls to EndRead and EndWrite. As you can see in the synchronous implementation, you need to issue a read request against the source Stream, take the retrieved data and write it to the destination Stream, and then rinse and reFigure 1 Synchronous CopyStreamToStream public static void CopyStreamToStream( Stream source, Stream destination, Action completed) { byte[] buffer = new byte[0x1000]; int read; try { while ((read = source.Read(buffer, 0, buffer.Length)) > 0) { destination.Write(buffer, 0, read); } if (completed != null) completed(source, destination, null); } catch (Exception exc) { if (completed != null) completed(source, destination, exc); } } Another would be to use asynchronous delegate invocation, which would have largely the same effect: delegate void CopyStreamToStreamHandler( Stream source, Stream destination, Action completed); CopyStreamToStreamHandler handler = CopyStreamToStream; handler.BeginInvoke(source, destination, (src,dst,exc) => { src.Close(); dst.Close(); }, iar => handler.EndInvoke(iar), null); march2008 129

Table of Contents Feed for the Digital Edition of MSDN Magazine - March 2008

MSDN Magazine - March 2008 Contents Toolbox CLR Inside Out Data Points Advanced Basics Office Space Introducing ASP.NET MVC Loosen Up CI Server Performance Office Development Test Run Security Briefs Extreme ASP.NET Foundations .NET Matters {End Bracket}

MSDN Magazine - March 2008

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