FileStream.WriteAsync Method

Definition

Namespace:

System.IO

Assembly:

System.Runtime.dll

Overloads

WriteAsync(Byte[], Int32, Int32, CancellationToken)	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.
WriteAsync(ReadOnlyMemory<Byte>, CancellationToken)	Asynchronously writes a sequence of bytes from a memory region to the current file stream, advances the current position within this file stream by the number of bytes written, and monitors cancellation requests.

WriteAsync(Byte[], Int32, Int32, CancellationToken)

Source:

FileStream.cs

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.

public:
 override System::Threading::Tasks::Task ^ WriteAsync(cli::array <System::Byte> ^ buffer, int offset, int count, System::Threading::CancellationToken cancellationToken);

public override System.Threading.Tasks.Task WriteAsync (byte[] buffer, int offset, int count, System.Threading.CancellationToken cancellationToken);

override this.WriteAsync : byte[] * int * int * System.Threading.CancellationToken -> System.Threading.Tasks.Task

Public Overrides Function WriteAsync (buffer As Byte(), offset As Integer, count As Integer, cancellationToken As CancellationToken) As Task

Parameters

buffer

Byte[]

The buffer to write data from.

offset

Int32

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

count

Int32

The maximum number of bytes to write.

cancellationToken

CancellationToken

The token to monitor for cancellation requests.

Returns

Task

A task that represents the asynchronous write operation.

Exceptions

ArgumentNullException

buffer is null.

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.

OperationCanceledException

The cancellation token was canceled. This exception is stored into the returned task.

Examples

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

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()
        {
            InitializeComponent();
        }

        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

Remarks

The WriteAsync method enables you to perform resource-intensive file operations without blocking the main thread. This performance consideration is particularly important in apps where a time-consuming stream operation can block the UI thread and make your app appear as if it isn't working.

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

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.

This method stores in the task it returns all non-usage exceptions that the method's synchronous counterpart can throw. If an exception is stored into the returned task, that exception will be thrown when the task is awaited. Usage exceptions, such as ArgumentException, are still thrown synchronously. For the stored exceptions, see the exceptions thrown by Write(Byte[], Int32, Int32).

See also

• Cancellation

WriteAsync(ReadOnlyMemory<Byte>, CancellationToken)

Source:

FileStream.cs

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

public override System.Threading.Tasks.ValueTask WriteAsync (ReadOnlyMemory<byte> buffer, System.Threading.CancellationToken cancellationToken = default);

override this.WriteAsync : ReadOnlyMemory<byte> * System.Threading.CancellationToken -> System.Threading.Tasks.ValueTask

Public Overrides Function WriteAsync (buffer As ReadOnlyMemory(Of Byte), Optional cancellationToken As CancellationToken = Nothing) As ValueTask

Parameters

buffer

ReadOnlyMemory<Byte>

The region of memory to write data from.

cancellationToken

CancellationToken

The token to monitor for cancellation requests. The default value is None.

Returns

ValueTask

A task that represents the asynchronous write operation.

Exceptions

OperationCanceledException

The cancellation token was canceled. This exception is stored into the returned task.

Remarks

The WriteAsync method lets you perform resource-intensive I/O operations without blocking the main thread. This performance consideration is particularly important in apps where a time-consuming stream operation can block the UI thread and make your app appear as if it isn't working.

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

If the operation is canceled before it completes, the returned task contains the TaskStatus.Canceled value for the Task.Status property.