Does the .net framework provides async methods for

问题:

Does the .net framework has an async built-in library/assembly which allows to work with the file system (e.g. File.ReadAllBytes, File.WriteAllBytes)?

Or do I have to write my own library using the async Methods of StreamReader and StreamWriter?

Something like this would be pretty nice:

var bytes = await File.ReadAllBytes("my-file.whatever");

回答1:

Does the .net framework has an async built-in library/assembly which allows to work with the file system

Yes. There are async methods for working with the file system but not for the helper methods on the static File type. They are on FileStream.

So, there's no File.ReadAllBytesAsync but there's FileStream.ReadAsync, etc. For example:

byte[] result;
using (FileStream stream = File.Open(@"C:\file.txt", FileMode.Open))
{
    result = new byte[stream.Length];
    await stream.ReadAsync(result, 0, (int)stream.Length);
}

回答2:

It already does it. See for example Using Async for File Access MSDN article.

private async Task WriteTextAsync(string filePath, string text)
{
    byte[] encodedText = Encoding.Unicode.GetBytes(text);

    using (FileStream sourceStream = new FileStream(filePath,
        FileMode.Append, FileAccess.Write, FileShare.None,
        bufferSize: 4096, useAsync: true))
    {
        await sourceStream.WriteAsync(encodedText, 0, encodedText.Length);
    };
}

回答3:

Does the .net framework has an async built-in library/assembly which allows to work with the file system (e.g. File.ReadAllBytes, File.WriteAllBytes)?

Unfortunately, the desktop APIs are a bit spotty when it comes to asynchronous file operations. As you noted, a number of the nice convenience methods do not have asynchronous equivalents. Also missing are asynchronous opening of files (which is especially useful when opening files over a network share).

I'm hoping these APIs will be added as the world moves to .NET Core.

Or do I have to write my own library using the async Methods of StreamReader and StreamWriter?

That's the best approach right now.

Note that when using ReadAsync/WriteAsync and friends, you must explicitly open the file for asynchronous access. The only way to do this (currently) is to use a FileStream constructor overload that takes a bool isAsync parameter (passing true) or a FileOptions parameter (passing FileOptions.Asynchronous). So you can't use the convenience open methods like File.Open.

回答4:

In .NET core (since version 2.0) there are now all the async flavors of corresponding ReadAll/WriteAll/AppendAll methods like:

File.(Read|Write|Append)All(Text|Lines|Bytes)Async

https://docs.microsoft.com/en-us/dotnet/api/system.io.file.readallbytesasync?view=netcore-2.1

Unfortunately, they are still missing from .NET standard 2.0.

回答5:

If you have to write a file in a background task you can write it sincronously in an async method.

static void Main(string[] args)
{
    Console.WriteLine("1");
    // thread 1, sync
    byte[] dataToWrite = System.Text.ASCIIEncoding.ASCII.GetBytes("my custom data");
    Task.Run(() =>
    {
        Console.WriteLine("2");
        // aspetto un secondo
        System.Threading.Thread.Sleep(1000);
        // thread 2, async code
        System.IO.File.WriteAllBytes(@"c:\temp\prova.txt", dataToWrite);
        Console.WriteLine("2, end");
    });

    Console.WriteLine("3, exit");

    Console.ReadLine();
    // thread 1
}

the execution output will be: 1,3,2,2 end