FileOptions.Asynchronous导致FileStream.BeginRead阻塞

Question

为什么用FileOptions.Asynchronous创建FileStream会导致FileStream.BeginRead阻塞调用线程？

以下是代码片段：

private static Task<int> ReadFileAsync(string filePath)
  {
     var file = new FileStream(filePath, FileMode.Open, FileAccess.ReadWrite, FileShare.ReadWrite, 64 * 1024, FileOptions.Asynchronous);
     FileInfo fi = new FileInfo(filePath);
     byte[] buffer = new byte[fi.Length];
     Task<int> task = Task<int>.Factory.FromAsync(file.BeginRead, file.EndRead, buffer, 0, buffer.Length, null);         
     return task.ContinueWith(t =>
     {
        file.Close();
        Console.WriteLine("Done ReadFileAsync, read " + t.Result + " bytes.");
        return t.Result;
     });
  }

当使用JetBrains dotPeek研究微软FileStream代码时，他们的代码中似乎有一个bug：

      if (!this._isAsync)
    return base.BeginRead(array, offset, numBytes, userCallback, stateObject);
  else
    return (IAsyncResult) this.BeginReadAsync(array, offset, numBytes, userCallback, stateObject);

BeginRead方法实际上似乎通过调度一个任务来执行异步读取，但是BeginReadAsync方法实际上结束了同步读取。因此，他们的方法命名规则是倒退的，调用哪个方法的逻辑是错误的。如果this._isAsync ==为true，则应调用BeginRead。

因此，似乎要让FileStream.BeginRead立即返回(异步调度读取)，您实际上必须将cosntructor中的useAsync参数设置为false。

Answer 1

这里有一篇知识库文章，列出了所有可能导致您希望异步执行的代码实际上同步运行的方法。

Asynchronous Disk I/O Appears as Synchronous on Windows NT, Windows 2000, and Windows XP

清单上的内容是否适用于你的情况？

你有没有尝试过用.NET 4.5的ReadAsync method实现同样的行为？

我引用了MSDN的话：

在.NET Framework4及更早版本中，您必须使用BeginRead和EndRead等方法来实现异步I/O操作。这些方法在.NET Framework4.5中仍然可用来支持遗留代码；但是，ReadAsync、WriteAsync、CopyToAsync和FlushAsync等新的异步方法可以帮助您更轻松地实现异步I/O操作。

编辑我正在使用ICH10和Windows7在OCZ Vertex 2上重现256MB文件的问题。我必须生成该文件，重新启动PC以清除文件缓存，然后尝试读取相同的文件。

using System;
using System.Threading.Tasks;
using System.IO;
using System.Diagnostics;
using System.Threading;

namespace ConsoleApplication4
{
   class Program
   {
      static void Main(string[] args)
      {
         string fileName = @"C:\Temp\a1.txt";
         int arraySize = 512 * 1024 * 1024;
         var bytes = new byte[arraySize];
         new Random().NextBytes(bytes);

          // This prints false, as expected for async call
         var callback = new AsyncCallback(result =>
                             Console.WriteLine("Completed Synchronously: " + result.CompletedSynchronously));

         try
         {
            // Use this method to generate file...
            //WriteFileWithRandomBytes(fileName, arraySize, bytes, callback);

            Console.WriteLine("ReadFileAsync invoked at " + DateTimeOffset.Now);
            var task = ReadFileAsync(fileName);
            Console.WriteLine("ReadFileAsync completed at " + DateTimeOffset.Now);

            Task.WaitAll(task);
            Console.WriteLine("Wait on a read task completed at " + DateTimeOffset.Now);
         }
         finally
         {
            if (File.Exists(fileName))
               File.Delete(fileName);
         }
      }

      private static void WriteFileWithRandomBytes(string fileName, int arraySize, byte[] bytes, AsyncCallback callback)
      {
         using (var fileStream = new FileStream(fileName,
            FileMode.OpenOrCreate, FileAccess.Write, FileShare.None, 128 * 1024, FileOptions.Asynchronous))
         {
            Console.WriteLine("BeginWrite invoked at " + DateTimeOffset.Now);
            var asyncResult = fileStream.BeginWrite(bytes, 0, arraySize, callback, null);


            Console.WriteLine("BeginWrite completed at " + DateTimeOffset.Now);
            // completes in 6 seconds or so...  Expecting instantaneous return instead of blocking

            // I expect runtime to block here...
            Task.WaitAll(Task.Factory.FromAsync(asyncResult, fileStream.EndWrite));

            // or at least when flushing the stream on the following end-curly
         }
      }


      private static Task<int> ReadFileAsync(string filePath)
      {
         FileInfo fi = new FileInfo(filePath);
         byte[] buffer = new byte[fi.Length];

         var file = new FileStream(filePath, FileMode.Open, FileAccess.Read, FileShare.None, 64 * 1024, FileOptions.Asynchronous);
         Task<int> task = Task<int>.Factory.FromAsync(file.BeginRead, file.EndRead, buffer, 0, buffer.Length, null);
         return task.ContinueWith(t =>
         {
            file.Close();
            Console.WriteLine("Done ReadFileAsync, read " + t.Result + " bytes.");
            return t.Result;
         });
      }
   }
}

当所有其他方法都失败时，here's the reference to unmanaged API documentation。

Answer 2

您是否尝试传入有效的stateObject，而不是null？与http://msdn.microsoft.com/en-us/library/kztecsys(v=vs.100).aspx的代码进行比较