FileStream.WriteAsync Method (Byte(), Int32, Int32, CancellationToken)

.NET Framework (current version)

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

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

<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)>
Public Overrides Function WriteAsync (
	buffer As Byte(),
	offset As Integer,
	count As Integer,
	cancellationToken As CancellationToken
) As Task


Type: System.Byte()

The buffer to write data from.

Type: System.Int32

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

Type: System.Int32

The maximum number of bytes to write.

Type: System.Threading.CancellationToken

The token to monitor for cancellation requests.

Return Value

Type: System.Threading.Tasks.Task

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 file 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 CanWrite property to determine whether the current instance supports reading.

If the operation is canceled before it completes, the returned task contains the Canceled value for the Status property. If the handle to the file is disposed, the returned task contains the ObjectDisposedException exception in the Exception property.

The following example shows how to write asynchronously to a file.

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

Universal Windows Platform
Available since 10
.NET Framework
Available since 4.5
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