Asynchronous File I/O
Asynchronous operations enable you to perform resource-intensive I/O operations without blocking the main thread. This performance consideration is particularly important in a Windows 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.
Starting with the .NET Framework 4.5, the I/O types include async methods to simplify asynchronous operations. An async method contains Async in its name, such as ReadAsync, WriteAsync, CopyToAsync, FlushAsync, ReadLineAsync, and ReadToEndAsync. 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 the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous I/O operations. 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.
Starting with Visual Studio 2012, Visual Studio provides two keywords for asynchronous programming:
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Async (Visual Basic) or async (C#) modifier, which is used to mark a method that contains an asynchronous operation.
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Await (Visual Basic) or await (C#) operator, which is applied to the result of an async method.
To implement asynchronous I/O operations, use these keywords in conjunction with the async methods, as shown in the following examples. For more information, see Asynchronous Programming with Async and Await (C# and Visual Basic).
The following example demonstrates how to use two FileStream objects to copy files asynchronously from one directory to another. Notice that the Click event handler for the Button control is marked with the async modifier because it calls an asynchronous method.
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
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 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
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 Store app, and read its contents by using an instance of the StreamReader class. It uses asynchronous methods to open the file as a stream and to read its contents.
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>
