異步檔案輸入/輸出

異步作可讓您執行耗用大量資源的 I/O 作業，而不會封鎖主要線程。在 Windows 8.x 市集應用程式或傳統型應用程式中，此效能考慮特別重要，其中耗時的串流作業可能會封鎖 UI 線程，並讓應用程式看起來好像無法運作。

從 .NET Framework 4.5 開始，I/O 類型包含異步方法，以簡化異步作。異步方法的名稱包含Async，例如 ReadAsync、WriteAsync、CopyToAsync、FlushAsync、ReadLineAsync 和 ReadToEndAsync。這些異步方法會在數據流類別上實作，例如Stream、、和 FileStream，以及用於讀取或寫入數據流的類別，例如 MemoryStream 和 TextReader TextWriter。

在 .NET Framework 4 和舊版中，您必須使用和等BeginRead EndRead方法來實作異步 I/O 作業。這些方法仍可在目前的 .NET 版本中使用，以支援舊版程序代碼;不過，異步方法可協助您更輕鬆地實作異步 I/O 作業。

C# 和 Visual Basic 各有兩個用於異步程式設計的關鍵詞：

Async （Visual Basic）或 async （C#）修飾詞，用來標記包含異步作的方法。
Await （Visual Basic）或 await （C#）運算符，其會套用至異步方法的結果。

若要實作異步 I/O 作業，請搭配異步方法使用這些關鍵詞，如下列範例所示。如需詳細資訊，請參閱使用 async 和 await 的異步編程（C#）或使用 Async 和 Await 的異步編程（Visual Basic）。

下列範例示範如何使用兩個 FileStream 物件，以異步方式將檔案從一個目錄複製到另一個目錄。請注意， Click 控件的 Button 事件處理程式會以 async 修飾詞標示，因為它會呼叫異步方法。

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

下一個範例類似於上一個範例，但會使用 StreamReader 和 StreamWriter 物件，以異步方式讀取和寫入文本文件的內容。

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

下一個範例顯示程式代碼後置檔案和 XAML 檔案，這些檔案用於在 Windows 8.x 市集應用程式中作為一個 Stream 開啟檔案，並透過使用 StreamReader 類別的實例來讀取其內容。它會使用異步方法來開啟檔案做為數據流，並讀取其內容。

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>

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Last updated on 2025-06-17

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異步檔案輸入/輸出

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