如何异步Files.ReadAllLines并等待结果？

我有以下代码，

    private void button1_Click(object sender, RoutedEventArgs e)
    {
        button1.IsEnabled = false;

        var s = File.ReadAllLines("Words.txt").ToList(); // my WPF app hangs here
        // do something with s

        button1.IsEnabled = true;
    }

Words.txt我将大量单词读入s变量，我正在尝试使用C＃5中的asyncandawait关键字，Async CTP Library因此WPF应用程序不会挂起。到目前为止，我有以下代码，

    private async void button1_Click(object sender, RoutedEventArgs e)
    {
        button1.IsEnabled = false;

        Task<string[]> ws = Task.Factory.FromAsync<string[]>(
            // What do i have here? there are so many overloads
            ); // is this the right way to do?

        var s = await File.ReadAllLines("Words.txt").ToList();  // what more do i do here apart from having the await keyword?
        // do something with s

        button1.IsEnabled = true;
    }

目标是以异步方式而不是同步方式读取文件，以避免冻结WPF应用程序。

任何帮助表示赞赏，谢谢！

更新：的异步版本File.ReadAll[Lines|Bytes|Text]，File.AppendAll[Lines|Text]并且File.WriteAll[Lines|Bytes|Text]现在已合并到.NET Core中，并随.NET Core 2.0一起提供。它们也包含在.NET Standard 2.1中。

对于异步包装Task.Run器来说Task.Factory.StartNew，使用本质上是包装器的代码是一种味道。

如果您不想通过使用阻塞函数来浪费CPU线程，则应等待真正的异步IO方法StreamReader.ReadToEndAsync，如下所示：

using (var reader = File.OpenText("Words.txt"))
{
    var fileText = await reader.ReadToEndAsync();
    // Do something with fileText...
}

这将得到整个文件，string而不是一个List<string>。如果您需要换行，则可以在以后轻松拆分字符串，如下所示：

using (var reader = File.OpenText("Words.txt"))
{
    var fileText = await reader.ReadToEndAsync();
    return fileText.Split(new[] { Environment.NewLine }, StringSplitOptions.None);
}

编辑：以下是一些方法来实现与相同的代码File.ReadAllLines，但以一种真正的异步方式。该代码基于File.ReadAllLines自身的实现：

using System.Collections.Generic;
using System.IO;
using System.Text;
using System.Threading.Tasks;

public static class FileEx
{
    /// <summary>
    /// This is the same default buffer size as
    /// <see cref="StreamReader"/> and <see cref="FileStream"/>.
    /// </summary>
    private const int DefaultBufferSize = 4096;

    /// <summary>
    /// Indicates that
    /// 1. The file is to be used for asynchronous reading.
    /// 2. The file is to be accessed sequentially from beginning to end.
    /// </summary>
    private const FileOptions DefaultOptions = FileOptions.Asynchronous | FileOptions.SequentialScan;

    public static Task<string[]> ReadAllLinesAsync(string path)
    {
        return ReadAllLinesAsync(path, Encoding.UTF8);
    }

    public static async Task<string[]> ReadAllLinesAsync(string path, Encoding encoding)
    {
        var lines = new List<string>();

        // Open the FileStream with the same FileMode, FileAccess
        // and FileShare as a call to File.OpenText would've done.
        using (var stream = new FileStream(path, FileMode.Open, FileAccess.Read, FileShare.Read, DefaultBufferSize, DefaultOptions))
        using (var reader = new StreamReader(stream, encoding))
        {
            string line;
            while ((line = await reader.ReadLineAsync()) != null)
            {
                lines.Add(line);
            }
        }

        return lines.ToArray();
    }
}

使用Stream.ReadAsync异步读取文件，

private async void Button_Click(object sender, RoutedEventArgs e)
{
    string filename = @"c:\Temp\userinputlog.txt";
    byte[] result;

    using (FileStream SourceStream = File.Open(filename, FileMode.Open))
    {
        result = new byte[SourceStream.Length];
        await SourceStream.ReadAsync(result, 0, (int)SourceStream.Length);
    }

    UserInput.Text = System.Text.Encoding.ASCII.GetString(result);
}

读取MSDN Stream.ReadAsync

如果要异步读取文件中的所有行，则可以使用来使用该async功能访问文件FileStream。

private static async Task<string[]> ReadAllLinesAsync(string filePath)
    {
        using (FileStream sourceStream = new FileStream(filePath,
            FileMode.Open, FileAccess.Read, FileShare.Read,
            bufferSize: 4096, useAsync: true))
        {
            StringBuilder sb = new StringBuilder();

            byte[] buffer = new byte[0x1000];
            int numRead;
            while ((numRead = await sourceStream.ReadAsync(buffer, 0, buffer.Length)) != 0)
            {
                string text = Encoding.Unicode.GetString(buffer, 0, numRead);
                sb.Append(text);
            }

            return sb.ToString().Split(new[] { Environment.NewLine },StringSplitOptions.None);
        }
    }

您可以async通过指定async事件处理函数来在事件处理函数中使用该方法。

这是您可以使用的方式，这不会让您的GUI线程冻结。

private async void button1_Click(object sender, RoutedEventArgs e)
{
    button1.IsEnabled = false;

    var s = await ReadAllLinesAsync("Words.txt").ToList();
    // do something with s

    button1.IsEnabled = true;
}

有关更多详细信息，请参阅MS Docs。

我还遇到了您的问题中所述的问题。在以前的答案中，我已经解决了这一问题：

string[] values;
StorageFolder folder = ApplicationData.Current.LocalFolder; // Put your location here.
IList<string> lines = await FileIO.ReadLinesAsync(await folder.GetFileAsync("Words.txt"););
lines.CopyTo(values, 0);

尝试这个：

private async void button1_Click(object sender, RoutedEventArgs e)
{
    button1.IsEnabled = false;
    try
    {
        var s = await Task.Run(() => File.ReadAllLines("Words.txt").ToList());
        // do something with s
    }
    finally
    {
        button1.IsEnabled = true;
    }
}

编辑：

您不需要最终尝试即可工作。实际上，这只是您需要更改的那一行。解释它是如何工作的：这产生了另一个线程（实际上是从线程池中获取一个）并获取该线程来读取文件。文件读取完成后，将（从GUI线程）调用button1_Click方法的其余部分，并返回结果。请注意，这可能不是最有效的解决方案，但这可能是对代码的最简单的更改，并且不会阻塞GUI。