E/S de archivos asincrónicaAsynchronous File I/O

Las operaciones asincrónicas permiten realizar operaciones de E/S que hacen un uso intensivo de recursos sin bloquear el subproceso principal.Asynchronous operations enable you to perform resource-intensive I/O operations without blocking the main thread. Esta consideración de rendimiento es especialmente importante en una aplicación Tienda Windows 8.xWindows 8.x Store o una aplicación de escritorio en que una operación de streaming prolongada puede bloquear el subproceso de interfaz de usuario y hacer que parezca que una aplicación ha dejado de responder.This performance consideration is particularly important in a Tienda Windows 8.xWindows 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.

A partir de .NET Framework 4.5, los tipos de E/S incluyen métodos asincrónicos para simplificar las operaciones asincrónicas.Starting with the .NET Framework 4.5, the I/O types include async methods to simplify asynchronous operations. Un método asincrónico contiene Async en su nombre, como ReadAsync, WriteAsync, CopyToAsync, FlushAsync, ReadLineAsyncy ReadToEndAsync.An async method contains Async in its name, such as ReadAsync, WriteAsync, CopyToAsync, FlushAsync, ReadLineAsync, and ReadToEndAsync. Estos métodos asincrónicos se implementan en clases de secuencias, como Stream, FileStreamy MemoryStream, y en las clases de las que se usan para leer o escribir en secuencias, como TextReader y 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.

En .NET Framework 4 y versiones anteriores, es necesario usar métodos como BeginRead y EndRead para implementar operaciones de E/S asincrónicas.In the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous I/O operations. Estos métodos siguen estando disponibles en .NET Framework 4.5 para ser compatibles con el código heredado. Sin embargo, los métodos asincrónicos implementan las operaciones de E/S asincrónicas de una manera más sencilla.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# y Visual Basic tienen dos palabras clave para la programación asincrónica:C# and Visual Basic each have two keywords for asynchronous programming:

  • El modificadorAsync (Visual Basic) o async (C#), que se utiliza para marcar un método que contiene una operación asincrónica.Async (Visual Basic) or async (C#) modifier, which is used to mark a method that contains an asynchronous operation.

  • El operadorAwait (Visual Basic) o await (C#), que se aplica al resultado de un método asincrónico.Await (Visual Basic) or await (C#) operator, which is applied to the result of an async method.

Para implementar operaciones de E/S asincrónicas, use estas palabras clave junto con los métodos asincrónicos, como se muestra en los ejemplos siguientes.To implement asynchronous I/O operations, use these keywords in conjunction with the async methods, as shown in the following examples. Para obtener más información, vea Programación asincrónica con Async y Await (C#) y Programación asincrónica con Async y Await (Visual Basic).For more information, see Asynchronous programming with async and await (C#) or Asynchronous Programming with Async and Await (Visual Basic).

En el ejemplo siguiente se muestra cómo usar dos objetos FileStream para copiar archivos de forma asincrónica de un directorio en otro.The following example demonstrates how to use two FileStream objects to copy files asynchronously from one directory to another. Observe que el controlador de eventos Click para el control Button está marcado con el modificador async porque llama a un método asincrónico.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

El ejemplo siguiente es similar al anterior, pero utiliza objetos StreamReader y StreamWriter para leer y escribir el contenido de un archivo de texto de forma asincrónica.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(4096) 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

En el ejemplo siguiente se muestra el archivo de código subyacente y el archivo XAML que se usan para abrir un archivo como Stream en una aplicación de la Tienda Windows 8.xWindows 8.x Store y leer su contenido mediante una instancia de la clase StreamReader .The next example shows the code-behind file and the XAML file that are used to open a file as a Stream in a Tienda Windows 8.xWindows 8.x Store app, and read its contents by using an instance of the StreamReader class. Utiliza métodos asincrónicos para abrir el archivo como secuencia y leer su contenido.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>

Vea tambiénSee also