Asynchrone Datei-E/AAsynchronous File I/O

Mithilfe von asynchronen Vorgängen können Sie ressourcenintensive E/A-Vorgänge auszuführen, ohne den Hauptthread zu blockieren.Asynchronous operations enable you to perform resource-intensive I/O operations without blocking the main thread. Diese Überlegungen zur Leistung sind insbesondere in einer Windows 8.x Store-App oder Desktop-App wichtig, bei der ein zeitaufwendiger Streamingvorgang den UI-Thread blockieren kann und es dann den Anschein hat, dass Ihre App nicht funktioniert.This performance consideration is particularly important in a Windows 8.x Store app or desktop app where a time-consuming stream operation can block the UI thread and make your app appear as if it is not working.

Ab .NET Framework 4.5 enthalten die E/A-Typen asynchrone Methoden, um asynchrone Vorgänge zu vereinfachen.Starting with the .NET Framework 4.5, the I/O types include async methods to simplify asynchronous operations. Eine asynchrone Methode enthält Async in ihrem Namen, z. B. ReadAsync, WriteAsync, CopyToAsync, FlushAsync, ReadLineAsyncund ReadToEndAsync.An async method contains Async in its name, such as ReadAsync, WriteAsync, CopyToAsync, FlushAsync, ReadLineAsync, and ReadToEndAsync. Diese asynchronen Methoden werden für Streamklassen wie Stream, FileStreamund MemoryStreamsowie für Klassen implementiert, die für das Lesen und das Schreiben in Streams verwendet werden, z. B. TextReader und TextWriter.These async methods are implemented on stream classes, such as Stream, FileStream, and MemoryStream, and on classes that are used for reading from or writing to streams, such TextReader and TextWriter.

In .NET Framework 4 und vorherigen Versionen müssen Sie Methoden wie BeginRead und EndRead verwenden, um asynchrone E/A-Vorgänge zu implementieren.In the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous I/O operations. Diese Methoden sind in .NET Framework 4.5 weiterhin verfügbar, um Legacycode zu unterstützen; allerdings können Sie mithilfe der asynchronen Methoden leichter asynchrone E/A-Vorgänge implementieren.These methods are still available in the .NET Framework 4.5 to support legacy code; however, the async methods help you implement asynchronous I/O operations more easily.

C# und Visual Basic verfügen jeweils über zwei Schlüsselwörter für asynchrone Programmierung:C# and Visual Basic each have two keywords for asynchronous programming:

  • DerAsync -Modifizierer (Visual Basic) bzw. async -Modifizierer (C#), der verwendet wird, um eine Methode zu kennzeichnen, die einen asynchronen Vorgang enthält.Async (Visual Basic) or async (C#) modifier, which is used to mark a method that contains an asynchronous operation.

  • DerAwait -Operator (Visual Basic) bzw. await -Operator (C#), der auf das Ergebnis einer asynchronen Methode angewendet wird.Await (Visual Basic) or await (C#) operator, which is applied to the result of an async method.

Um asynchrone E/A-Vorgänge zu implementieren, verwenden Sie diese Schlüsselwörter in Verbindung mit den asynchronen Methoden, wie in den folgenden Beispielen gezeigt.To implement asynchronous I/O operations, use these keywords in conjunction with the async methods, as shown in the following examples. Weitere Informationen finden Sie unter Asynchrone Programmierung mit Async und Await (C#) oder Asynchrone Programmierung mit Async und Await (Visual Basic).For more information, see Asynchronous programming with async and await (C#) or Asynchronous Programming with Async and Await (Visual Basic).

Das folgende Beispiel zeigt, wie mithilfe von zwei FileStream -Objekten Dateien asynchron aus einem Verzeichnis in ein anderes kopiert werden.The following example demonstrates how to use two FileStream objects to copy files asynchronously from one directory to another. Beachten Sie, dass der Click -Ereignishandler für das Button -Steuerelement mit dem async -Modifizierer markiert wird, da er eine asynchrone Methode aufruft.Notice that the Click event handler for the Button control is marked with the async modifier because it calls an asynchronous method.

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(EndDirectory + filename.Substring(filename.LastIndexOf('\\'))))
                    {
                        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

Das folgende Beispiel ähnelt dem vorherigen, allerdings werden StreamReader - und StreamWriter -Objekte verwendet, um den Inhalt einer Textdatei asynchron zu lesen und zu schreiben.The next example is similar to the previous one but uses StreamReader and StreamWriter objects to read and write the contents of a text file asynchronously.

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

Im folgenden Beispiel werden die Code-Behind-Datei und die XAML-Datei gezeigt, mit denen eine Datei als ein Stream in einer App Windows 8.x Store geöffnet wird und der Inhalt mithilfe einer Instanz der StreamReader -Klasse gelesen wird.The next example shows the code-behind file and the XAML file that are used to open a file as a Stream in a Windows 8.x Store app, and read its contents by using an instance of the StreamReader class. Im Beispiel werden asynchrone Methoden verwendet, um die Datei als Stream zu öffnen und ihren Inhalt zu lesen.It uses asynchronous methods to open the file as a stream and to read its contents.

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>

Siehe auchSee also