MSDN Magazine - July 2008 - (Page 131)

STEPHEN TOUB .NET MaTTErs Asynchronous I/O With WebClient Q A I really like the static helper methods on the System.IO.File class for reading and writing data: ReadAllText, ReadAllBytes, WriteAllText, WriteAllBytes, and so on. However, these methods are synchronous, and I’d like to be able to use them asynchronously such that under the covers they’re also using asynchronous I/O. I know the System.IO.FileStream class supports asynchronous I/O; does File? Figure 1 Asynchronous Stream Copying public static void CopyStreamToStream( Stream source, Stream destination, Action completed) { byte[] buffer = new byte[0x1000]; if (completed == null) completed = delegate {}; AsyncCallback rc = null; rc = readResult => { try { int read = source.EndRead(readResult); if (read > 0) { destination.BeginWrite(buffer, 0, read, writeResult => { try { destination.EndWrite(writeResult); source.BeginRead( buffer, 0, buffer.Length, rc, null); } catch (Exception exc) { completed(exc); } }, null); } else completed(null); } catch (Exception exc) { completed(exc); } }; } source.BeginRead(buffer, 0, buffer.Length, rc, null); File’s methods only support synchronous operation. But the functionality to implement asynchronous methods like the ones you describe certainly exists. In this column, I’ll walk through two ways to do it, starting with the more complicated, but more efficient, of the two. First, I’ll begin by defining the APIs I actually want to imple ment. The original signatures you cite look like this: public static byte [] ReadAllBytes(string path); public static string ReadAllText(string path); public static void WriteAllBytes(string path, byte [] bytes); public static void WriteAllText(string path, string contents); I want asynchronous versions of these, and I’ll define them as: public static void ReadAllBytesAsync( string path, Action success, Action failure); public static void ReadAllTextAsync( string path, Action success, Action failure); public static void WriteAllBytesAsync( string path, byte[] bytes, Action success, Action failure); public static void WriteAllTextAsync( string path, string contents, Action success, Action failure); These signatures are very similar to the originals. Instead of returning data synchronously, the Read* methods accept two delegates, one for successful execution and one for exceptional execution, where the delegate for the former is passed the read data and where the delegate for the latter is passed any resulting exception. The Write* methods also accept two delegates, but the success delegate is parameterless, since there’s no expected output (the original Write* methods return void). The more complicated approach involves implementing these directly with the Asynchronous Programming Model (APM) pat tern exposed by the System.IO.Stream class, from which FileStream Send your questions and comments for Stephen to netqa@microsoft.com. Code download available at msdn.microsoft.com/magazine/cc135911. derives. In my column in the March 2008 issue of MSDN® Magazine (msdn.microsoft.com/magazine/cc337900), I demonstrated implement ing a method CopyStreamToStream, which asynchronously cop ies from one Stream to another Stream using each Stream’s APM implementation; for reference, that implementation is shown in Figure 1, and I’m going to reuse it here. (The implementation is slightly simplified and changed from the last time you saw it. This included removing the usage of System.ComponentModel.Async OperationManager and AsyncOperation, preferring to move their usage into the higher level APIs being implemented in this column. I’ll discuss why in a moment.) That implementation represents the most difficult aspect of implementing these asynchronous methods. On top of Copy StreamToStream, I’ll implement two helper methods, one for read ing bytes from a file asynchronously and one for writing bytes to a file asynchronously. They do the bulk of the work (see Figure 2). ReadAllBytesAsyncInternal creates an input FileStream to work with the underlying stream asynchronously and a MemoryStream to July 2008 131 http://msdn.microsoft.com/magazine/cc337900 http://msdn.microsoft.com/magazine/cc135911

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

MSDN Magazine - July 2008 Contents Toolbox CLR Inside Out Flex Your Data Data Points Advanced Basics Office Space Cutting Edge Data Services ADO.NET Data and WPF Transactions WCF P2P Test Run Security Briefs Foundations .NET Matters {End Bracket}

MSDN Magazine - July 2008

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