File.Copy在Parallel.ForEach [英] File.Copy in Parallel.ForEach

问题描述

您好我试图创建目录，并在Parallel.ForEach ...

Hi I try create dir and copy file (pdf) in Parallel.ForEach...

我创建简单的例子：

    private static void CreateFolderAndCopyFile(int index)
    {
        const string sourcePdfPath = "c:\\testdata\\test.pdf";
        const string rootPath = "c:\\testdata";

        string folderDirName = string.Format("Data{0}", string.Format("{0:00000000}", index));

        string folderDirPath = rootPath + @"\" + folderDirName;

        Directory.CreateDirectory(folderDirPath);

        string desPdfPath = folderDirPath + @"\" + "test.pdf";

        File.Copy(sourcePdfPath, desPdfPath, true);

    }

上面的方法创建新的文件夹和PDF文件复制到新的文件夹。
它创建这个目录树：

Method above create new folder and copy pdf file to new folder. It creates this dir tree:

TESTDATA
  -Data00000000
      -test.pdf
  -Data00000001
      -test.pdf
....
  -Data0000000N
      -test.pdf

我称之为Parallel.ForEach环法CreateFolderAndCopyFile。

I call method CreateFolderAndCopyFile in Parallel.ForEach loop.

    private static void Func<T>(IEnumerable<T> docs)
    {
        int index = 0;
        Parallel.ForEach(docs, doc =>
                                   {
                                       CreateFolderAndCopyFile(index);
                                       index++;
                                   });
    }

如果我尝试运行这段代码是错误结束：

If I try run this code it finish with error:

该进程无法访问该文件'C：\testdata\Data00001102\test.pdf'
，因为它正由另一个进程使用

The process cannot access the file 'c:\testdata\Data00001102\test.pdf' because it is being used by another process.

但首先它创造了1111年新的文件夹，并复制大约1111倍的test.pdf和我得到这个错误。

But first it created 1111 new folders and copy test.pdf about 1111 times and the I got this error.

什么原因此行为以及如何解决它。

What cause this behaviour and how to solve it?

编辑：

以上代码是玩具样品，比较遗憾的硬编码字符串
结论：并行方法SI慢

Code above was toy sample, sorry for hard coded strings Conclusion: Parallel method si slow.

明天我试着从一些方法如何在C＃写超级快速文件流的代码？。

特别：的 http://designingefficientsoftware.wordpress.com/2011/03/03/efficient-file-io-from-csharp/

推荐答案

您没有进行同步访问首页，这意味着你有就可以了比赛。这就是为什么你有错误。为了便于说明，可以避开种族和使用 Interlocked.Increment 。

You are not synchronizing access to index and that means you have a race on it. That's why you have the error. For illustrative purposes, you can avoid the race and keep this particular design by using Interlocked.Increment.

private static void Func<T>(IEnumerable<T> docs)
{
    int index = -1;
    Parallel.ForEach(
        docs, doc =>
        {
            int nextIndex = Interlocked.Increment(index);
            CreateFolderAndCopyFile(nextIndex);
        }
    );
}

然而，正如其他人建议，的 的ForEach ，提供了一个循环索引显然是一个更清洁的解决了这个特别的问题。

However, as others suggest, the alternative overload of ForEach that provides a loop index is clearly a cleaner solution to this particular problem.

但是，当你得到它的工作，你会发现，复制文件IO约束，而不是必然的处理器和我预测的并行代码会比串行慢代码。

But when you get it working you will find that copying files is IO bound rather than processor bound and I predict that the parallel code will be slower than the serial code.

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