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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)

[ComVisibleAttribute(false)]
[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public override Task WriteAsync(
	byte[] buffer,
	int offset,
	int count,
	CancellationToken cancellationToken
)

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.

Exception Condition
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.

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.

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);
            }
        }
    }
}

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