C#的异步文件操作

C#的异步文件操作2007-11-10 本站先来说说同步和异步操作之间的主要区别。在同步I/O操作中，方法将一直处于等待状态，直到I/O操作完成。而在异步I/O操作中，在开始了I/O操作后，程序的方法可以转移去执行其它的操作，这样大大提高了程序执行的效率。

由于是一个多任务的操作系统，在同一时刻系统可能会接受到多个I/O操作请求，要求对磁盘文件执行各种操作。如果采用同步方式，那么每时每刻最多只能有一个I/O操作在进行，而其它的任务都处于等待状态，系统的利用率将会大为降低。异步I/O操作则较好地解决了这种性能上的问题。

Stream类支持在同一个流中既可以进行同步读写，也可以进行异步读写。Stream类是一个抽象类，它为我们提供了BeginRead、BeginWrite、EndReader、EndWrite、Read、Write、Seek等成员方法，协同完成对流的读写操作。所有这些方法都是虚方法。因此，在我们自己设计Stream类的派生类时，我们在类用于读写的成员方法Read和Write中应该重载这些方法，并同时设计它们同步和异步的执行代码。BeginRead,EndRead,BeginWrite和EndWrite方法默认为我们提供的是异步读写操作方式，如果你的派生类的Read和Write方法执行同步操作时，那么程序提供的效率不会很好。只有当它们执行异步操作时，我们才能有效地提高程序的执行效率。

Stream类还提供了ReadByte和WriteByte方法，用于一次读写方式，这时我们就需要编写自己的方法来抛出一个异常。

下面的代码是在.NET联机帮助中提供的一个异步读写操作的例子，程序模拟了一个多处理器系统的工作。

程序清单17-8:

using System;using System.IO;using System.Threading;using BenchUtil;public class BulkImageProcAsync{public const String ImageBaseName="tmpImage-";public const int numImages=200;public const int numPixels=512*512;//ProcessImage has a simple O（N）loop,and we can vary the number//of times we repeat that loop to make the app more CPU-bound or more IO-bound.public static int processImageRepeats=20; //Threads must decrement NumImagesToFinish,and protect //their access to it via a mutex. public static int NumImagesToFinish=numImages; public static Object NumImagesMutex=new Object[0]; //WaitObject is signalled when all image processing is done. public static Object WaitObject=new Object[0]; internal static PerfTimer Pf=new PerfTimer（"Asynchronous Bulk Image Processor"）;public class ImageStateObject{ public byte[] pixels; public int imageNum;}public static void MakeImageFiles（）{ int sides=（int）Math.Sqrt（numPixels）; Console.Write（"Making"+numImages+""+sides+"x"+sides+"images... "）; byte[] pixels=new byte[numPixels]; for（int i=0;i＜numPixels;i++）pixels[i]=（byte）i; for（int i=0;i＜numImages;i++）{FileStream fs=new FileStream（ImageBaseName+i+".tmp",FileMode.Create,FileAccess.Write,FileShare.None,8192,false）;fs.Write（pixes,0,pixels.Length）; FlushFileBuffers（fs.GetHandle（））;fs.Close（）; }Console.WriteLine（"Done."）;}public static void ReadInImageCallback（IAsyncResult asyncResult）{ImageStateObject state=（ImageStateObject）asyncResult.AsyncState;Console.WriteLine（"Image"+state.imageNum+"was read."+（asyncResult.CompletedSynchronously？"synchronously":"asyncchronously"））;Stream stream=（Stream）asyncResult.AsyncObject;int bytesRead=stream.EndRead（asyncResult）;if（bytesRead！=numPixels）throw new Exception（"In ReadInImageCallback,got wrong number of bytesfrom the image！ got:"+bytesRead）;ProcessImages（state.pixels,state.imageNum）; stream.Close（）;//Now write out the image.//using async IO here probably swamps the threadpool,since//there are blocked threadpool threads on soon-to-be-//spawned//threadpool threadsFileStream fs=new FileStream（ImageBaseName+state.imageNum+".done",FileMode.Create,FileAccess.Write,FileShare.None,4096,false）;fs.Write（state.pixels,0,numPixels）;//IAsyncResult writeResult=fs.BeginWrtie（state.pixels,//0,numPixels,null,null）;//fs.EndWrite（writeResult）;fs.Close（）;//Release memory as soon as possible,especially global state.state.pixels=null;//Record that an image is done now.lock（NumImageMutex）{ NumImagesToFinish--;if（NumImagesToFinish==0）{ Monitor.Enter（WaitObject）; Monitor.Pulse（WaitObject）; Monitor.Exit（WaitObject）; }}}public static void ProcessImage（byte[] pixels,int imageNum）{ //Console.WriteLine（"ProcessImage"+imageNum）; //Do some CPU-intensive operation on the image. for（int i=0;i＜processImageReports;i++）for（int j=0;j＜numPixels;j++） pixels[j]+=1; //Console.WriteLine（"ProcessImage"+imageNum+"done."）; }public static void ProcessImagesInBulk（）{Console.WriteLine（"Processing images..."）;//int timer=Pf.StartTimer（"ProcessImages"）;int timer=Pf.StartTimer（"Total Time"）;NumImagesToFinish=numImages;AsyncCallback readImagesCallback=new AsyncCallback（ReadInImageCallback）;for（int i=0;i＜numImages;i++）{ImageStateObject state=new ImageStateObject（）;state.pixels=new byte[numPixels];state.imageNum=i;//Because very large items are read only once,the buffer//on the file stream can be very small to save memory.FileStream fs=new FileStream（ImageBaseName+i+".tmp",FileMode.Open,FileAccess.Read,FileShare.Read,1,true）;fs.BeginRead（state.pixels,0,numPixels,readImageCallback,state）;} //Ensure all image processing is done.//If not,block until all are finished.bool mustBlock=false; lock（NumImagesMutex） if（NumImagesToFinish>0）mustBlock=true; } if（mustBlock）{ Console.WriteLine（"All worker threads are queued...Blocking until theycomplete. numLeft:"+NumImagesToFinish）; Monitor.Enter（WaitObject）;Monitor.Wait（WaitObject）; Monitor.Exit（WaitObject）; }Pf.StopTimer（timer）;Pf.OutputStoppedTime（）;}public static void Cleanup（）{for（int i=0;i＜numImages;i++）{ File.Delete（ImageBaseName+i+".tmp"）; File.Delete（ImageBaseName+i+".done"）; }}public static void TryToClearDiskCache（）{ //Try to force all pending writes to disk,AND to clear the //disk cache of any data.byte[] bytes=new byte[100*（1<<20）]; for（int i=0;i＜bytes.Length;i++）bytes[i]=0; bytes=null;GC.Collect（）;Thread.Sleep（2000）; }public static void Main（String[] args）{ Console.WriteLine（"Bulk image processing sample application,using asycn IO"）; Console.WriteLine（"Simulates applying a simple transformation to "+numImages+""imgaes""）; Console.WriteLine（"ie,Async FileStream ＆ Threadpool benchmark）"）; Console.Writeline（"Warning - this test requires"+（numPixels*numImages*2）+"bytes of tmp space"）; if（args.length==1）{processImageRepeats=Int32.Parse（args[0]）;Console.WriteLine（"ProcessImage inner loop - "+processImageRepeats）; } MakeImageFiles（）; TryToClearDiskCache（）; ProcessImageInBulk（）; Cleanup（）;}[dllimport（"KERNEL32",SetlastError=true）]static extern void FlushFileBuffers（int handle）;}这里是采用同步方法实现同样功能的程序。程序清单17-9:using System;using System.IO;using System.Threading;using BenchUtil;public class BulkImageProcSync{ public const String ImageBaseName="tmpImage-";public const int numImages=200;public const int numPixels=512*512; //ProcessImage has a simple O（N） loop,and we can vary the number //of times we repeat that loop to make the app more CPU-bound or more IO-bound.public static int processImageRepeats=20;internal static PerfTimer Pf=new PerfTimer（"Synchronous Bulk Image Processor"）;public static void MakeImageFiles（）{int sides=（int）Math.Sqrt（numPixels）;Console.Write（"Making"+numImages+""+sides+"x"+sides+"images..."）;byte[] pixels=new byte[numPixels];for（int i=0;i＜numPixels;i++）pixels[i]=（byte）i;for（int i=0;i＜numImages;i++）{ FileStream fs=new FileStream（ImageBaseName+i+".tmp",FileMode.Create,FileAccess.Write,FileShare.None,8192,false）; fs.Write（pixels,0,pixels.Length）; FlushFileBuffers（fs.GetHandle（））; fs.Close（）;}Console.WriteLine（"Done."）;}public static void ProcessImage（byte[] pixels,int imageNum）{//Console.WriteLine（"ProcessImage"+imageNum）;//Do some CPU-intensive operation on the imagefor（int i=0;i＜processImageReports;i++） for（int j=0;j＜numPixels;j++）pixels[j]+=1;//Console.WriteLine（"processImage"+imageNum+"//done."）; }public static void ProcessImagesInBulk（）{Console.WriteLine（"Processing images..."）;int timer=Pf.StartTimer（"Total Time"）;byte[] pixels=new byte[numPixels];for（int i=0;i＜numImages;i++）{ FileStream input=new FileStream（ImageBaseName+i+".tmp",FileMode.Open,FileAccess.Read,FileShare.Read,4196,false）; input.Read（pixels,0,numPixels）; input:Close（）;ProcessImage（pixels,i）;FileStream output=new FileStream（ImageBaseName+i+".done",FileMode.Create,FileAccess.Write,FileShare.None,4196,false）; output.Write（pixels,0,numPixels）; output.Close（）; } Pf.StopTimer（timer）; Pf.OutputStoppedTime（）; } public static void Cleanup（） {for（int i=0;i＜numImages;i++）{File.Delete（ImageBaseName+i+".tmp"）;File.Delete（ImageBaseName+i+".done"）; } } public static void TryToClearDiskCache（）{ bute[] bytes=new byte[100*（1<<20）]; for（int i=0;i＜bytes.Length;i++） bytes[i]=0; bytes=null; GC.Collect（）; Thread.Sleep（2000）;}public static void Main（String[] args）{Console.WriteLine（"Bulk image processing sample application,using synchronous IO"）;Console.WriteLine（"Simulates applying a simple transformation to "+numImages+""images""）;Console.WriteLine（"ie,Sync FileStream benchmark）"）;Console.WriteLine（"Warning - this test requires"+（numPixels*numImages*2）+"bytes of tmp space"）;if（args.Length==1）{ processImageRepeats=Int32.Parse（args[0]）; Console.WriteLine（"ProcessImage inner loop -"+processImageRepeats）;}MakeImageFiles（）;TryToClearDiskCache（）; ProcessImagesInBulk（）; Cleanup（）;}[dllimport"KERNEL32",SetlastError=true）]static extern void FlushFileBuffers（int handlw）;}