异步文件输入/输出

2025-06-17

使用异步操作可以执行资源密集型的I/O操作，而不会阻塞主线程。在 Windows 8.x 应用商店应用或桌面应用中，这种性能注意事项尤其重要，其中耗时的流作可能会阻止 UI 线程，并使你的应用看起来好像不起作用一样。

从 .NET Framework 4.5 开始，I/O 类型包括异步方法，以简化异步作。异步方法Async在其名称中包含，例如ReadAsync，、WriteAsync、CopyToAsync FlushAsync、ReadLineAsync和ReadToEndAsync。这些异步方法在流类（例如 Stream、FileStream 和 MemoryStream）以及用于从流读取或写入流的类（例如 TextReader 和 TextWriter）上实现。

在 .NET Framework 4 及更早版本中，必须使用诸如 BeginRead 和 EndRead 实现异步 I/O作等方法。这些方法在当前 .NET 版本中仍可用以支持旧代码;但是，异步方法可帮助你更轻松地实现异步 I/O作。

C# 和 Visual Basic 都有两个用于异步编程的关键字：

Async （Visual Basic）或 async （C#）修饰符，用于标记包含异步作的方法。
Await （Visual Basic）或 await （C#）运算符，该运算符应用于异步方法的结果。

若要实现异步 I/O作，请将这些关键字与异步方法结合使用，如以下示例所示。有关详细信息，请参阅使用 async 和 await 的异步编程（C#）或使用 Async 和 Await 进行异步编程（Visual Basic）。

以下示例演示如何使用两个 FileStream 对象将文件从一个目录异步复制到另一个目录。请注意， Click 控件的 Button 事件处理程序使用 async 修饰符进行标记，因为它调用异步方法。

using System;
using System.Threading.Tasks;
using System.Windows;
using System.IO;

namespace WpfApplication
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            string startDirectory = @"c:\Users\exampleuser\start";
            string endDirectory = @"c:\Users\exampleuser\end";

            foreach (string filename in Directory.EnumerateFiles(startDirectory))
            {
                using (FileStream sourceStream = File.Open(filename, FileMode.Open))
                {
                    using (FileStream destinationStream = File.Create(Path.Combine(endDirectory, Path.GetFileName(filename))))
                    {
                        await sourceStream.CopyToAsync(destinationStream);
                    }
                }
            }
        }
    }
}

Imports System.IO

Class MainWindow

    Private Async Sub Button_Click(sender As Object, e As RoutedEventArgs)
        Dim StartDirectory As String = "c:\Users\exampleuser\start"
        Dim EndDirectory As String = "c:\Users\exampleuser\end"

        For Each filename As String In Directory.EnumerateFiles(StartDirectory)
            Using SourceStream As FileStream = File.Open(filename, FileMode.Open)
                Using DestinationStream As FileStream = File.Create(EndDirectory + filename.Substring(filename.LastIndexOf("\"c)))
                    Await SourceStream.CopyToAsync(DestinationStream)
                End Using

            End Using
        Next
    End Sub

End Class

下一个示例类似于上一个示例，但使用 StreamReader 和 StreamWriter 对象以异步方式读取和写入文本文件的内容。

private async void Button_Click(object sender, RoutedEventArgs e)
{
    string UserDirectory = @"c:\Users\exampleuser\";

    using (StreamReader SourceReader = File.OpenText(UserDirectory + "BigFile.txt"))
    {
        using (StreamWriter DestinationWriter = File.CreateText(UserDirectory + "CopiedFile.txt"))
        {
            await CopyFilesAsync(SourceReader, DestinationWriter);
        }
    }
}

public async Task CopyFilesAsync(StreamReader Source, StreamWriter Destination)
{
    char[] buffer = new char[0x1000];
    int numRead;
    while ((numRead = await Source.ReadAsync(buffer, 0, buffer.Length)) != 0)
    {
        await Destination.WriteAsync(buffer, 0, numRead);
    }
}

Private Async Sub Button_Click(sender As Object, e As RoutedEventArgs)
    Dim UserDirectory As String = "c:\Users\exampleuser\"

    Using SourceReader As StreamReader = File.OpenText(UserDirectory + "BigFile.txt")
        Using DestinationWriter As StreamWriter = File.CreateText(UserDirectory + "CopiedFile.txt")
            Await CopyFilesAsync(SourceReader, DestinationWriter)
        End Using
    End Using
End Sub

Public Async Function CopyFilesAsync(Source As StreamReader, Destination As StreamWriter) As Task
    Dim buffer(4095) As Char
    Dim numRead As Integer

    numRead = Await Source.ReadAsync(buffer, 0, buffer.Length)
    Do While numRead <> 0
        Await Destination.WriteAsync(buffer, 0, numRead)
        numRead = Await Source.ReadAsync(buffer, 0, buffer.Length)
    Loop

End Function

下一个示例展示了代码隐藏文件和 XAML 文件，这些文件用于在 Windows 8.x Store 应用中将文件以 Stream 的形式打开，并通过使用 StreamReader 类的实例来读取其内容。它使用异步方法以流的形式打开文件并读取其内容。

using System;
using System.IO;
using System.Text;
using Windows.Storage.Pickers;
using Windows.Storage;
using Windows.UI.Xaml;
using Windows.UI.Xaml.Controls;

namespace ExampleApplication
{
    public sealed partial class BlankPage : Page
    {
        public BlankPage()
        {
            this.InitializeComponent();
        }

        private async void Button_Click_1(object sender, RoutedEventArgs e)
        {
            StringBuilder contents = new StringBuilder();
            string nextLine;
            int lineCounter = 1;

            var openPicker = new FileOpenPicker();
            openPicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary;
            openPicker.FileTypeFilter.Add(".txt");
            StorageFile selectedFile = await openPicker.PickSingleFileAsync();

            using (StreamReader reader = new StreamReader(await selectedFile.OpenStreamForReadAsync()))
            {
                while ((nextLine = await reader.ReadLineAsync()) != null)
                {
                    contents.AppendFormat("{0}. ", lineCounter);
                    contents.Append(nextLine);
                    contents.AppendLine();
                    lineCounter++;
                    if (lineCounter > 3)
                    {
                        contents.AppendLine("Only first 3 lines shown.");
                        break;
                    }
                }
            }
            DisplayContentsBlock.Text = contents.ToString();
        }
    }
}

Imports System.Text
Imports System.IO
Imports Windows.Storage.Pickers
Imports Windows.Storage

NotInheritable Public Class BlankPage
    Inherits Page



    Private Async Sub Button_Click_1(sender As Object, e As RoutedEventArgs)
        Dim contents As StringBuilder = New StringBuilder()
        Dim nextLine As String
        Dim lineCounter As Integer = 1

        Dim openPicker = New FileOpenPicker()
        openPicker.SuggestedStartLocation = PickerLocationId.DocumentsLibrary

        openPicker.FileTypeFilter.Add(".txt")
        Dim selectedFile As StorageFile = Await openPicker.PickSingleFileAsync()

        Using reader As StreamReader = New StreamReader(Await selectedFile.OpenStreamForReadAsync())
            nextLine = Await reader.ReadLineAsync()
            While (nextLine <> Nothing)
                contents.AppendFormat("{0}. ", lineCounter)
                contents.Append(nextLine)
                contents.AppendLine()
                lineCounter = lineCounter + 1
                If (lineCounter > 3) Then
                    contents.AppendLine("Only first 3 lines shown.")
                    Exit While
                End If
                nextLine = Await reader.ReadLineAsync()
            End While
        End Using
        DisplayContentsBlock.Text = contents.ToString()
    End Sub
End Class

<Page
    x:Class="ExampleApplication.BlankPage"
    xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
    xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
    xmlns:local="using:ExampleApplication"
    xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
    xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
    mc:Ignorable="d">

    <StackPanel Background="{StaticResource ApplicationPageBackgroundBrush}" VerticalAlignment="Center" HorizontalAlignment="Center">
        <TextBlock Text="Display lines from a file."></TextBlock>
        <Button Content="Load File" Click="Button_Click_1"></Button>
        <TextBlock Name="DisplayContentsBlock"></TextBlock>
    </StackPanel>
</Page>

通过

异步文件输入/输出

另请参阅

其他资源