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

.NET Framework 4.5

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

Parameters

buffer
Type: System.Byte()

The buffer to write data from.

offset
Type: System.Int32

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

count
Type: System.Int32

The maximum number of bytes to write.

cancellationToken
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.
ExceptionCondition
ArgumentNullException

buffer is Nothing.

ArgumentOutOfRangeException

offset or count is negative.

ArgumentException

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

NotSupportedException

The stream does not support writing.

ObjectDisposedException

The stream has been disposed.

InvalidOperationException

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 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.

NoteNote

The HostProtectionAttribute attribute applied to this type or member has the following Resources property value: ExternalThreading. The HostProtectionAttribute does not affect desktop applications (which are typically started by double-clicking an icon, typing a command, or entering a URL in a browser). For more information, see the HostProtectionAttribute class or SQL Server Programming and Host Protection Attributes.

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

.NET Framework

Supported in: 4.5.1, 4.5

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

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