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StreamWriteAsync Method

Asynchronously writes a sequence of bytes to the current stream and advances the current position within this stream by the number of bytes written.

Namespace:   System.IO
Assembly:  mscorlib (in mscorlib.dll)

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public Task WriteAsync(
	byte[] buffer,
	int offset,
	int count
[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
Task^ WriteAsync(
	array<unsigned char>^ buffer,
	int offset,
	int count
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>]
member WriteAsync : 
        buffer:byte[] *
        offset:int *
        count:int -> Task
<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)>
Public Function WriteAsync (
	buffer As Byte(),
	offset As Integer,
	count As Integer
) As Task



The buffer to write data from.


The zero-based byte offset in buffer from which to begin copying bytes to the stream.


The maximum number of bytes to write.

Return Value


A task that represents the asynchronous write operation.

Exception Condition

buffer is null.


offset or count is negative.


The sum of offset and count is larger than the buffer length.


The stream does not support writing.


The stream has been disposed.


The stream is currently in use by a previous write operation.


The WriteAsync method enables you to perform resource-intensive I/O operations without blocking the main thread. 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. The async methods are used in conjunction with the async and await keywords in Visual Basic and C#.

Use the property to determine whether the current instance supports writing.


The following example shows how to write asynchronously to a file. The example uses the FileStream class, which derives from the Stream class.

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

namespace WpfApplication1
    public partial class MainWindow : Window
        public MainWindow()

        private async void Button_Click(object sender, RoutedEventArgs e)
            UnicodeEncoding uniencoding = new UnicodeEncoding();
            string filename = @"c:\Users\exampleuser\Documents\userinputlog.txt";

            byte[] result = uniencoding.GetBytes(UserInput.Text);

            using (FileStream SourceStream = File.Open(filename, FileMode.OpenOrCreate))
                SourceStream.Seek(0, SeekOrigin.End);
                await SourceStream.WriteAsync(result, 0, result.Length);
Imports System.IO
Imports System.Text

Class MainWindow
    Private Async Sub Button_Click(sender As Object, e As RoutedEventArgs)
        Dim uniencoding As UnicodeEncoding = New UnicodeEncoding()
        Dim filename As String = "c:\Users\exampleuser\Documents\userinputlog.txt"

        Dim result As Byte() = uniencoding.GetBytes(UserInput.Text)

        Using SourceStream As FileStream = File.Open(filename, FileMode.OpenOrCreate)
            SourceStream.Seek(0, SeekOrigin.End)
            Await SourceStream.WriteAsync(result, 0, result.Length)
        End Using
    End Sub
End Class
Version Information
Universal Windows Platform
Available since 8
.NET Framework
Available since 4.5
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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