Stream.CopyToAsync Method

Definition

Overloads

CopyToAsync(Stream)

Asynchronously reads the bytes from the current stream and writes them to another stream.

CopyToAsync(Stream, Int32)

Asynchronously reads the bytes from the current stream and writes them to another stream, using a specified buffer size.

CopyToAsync(Stream, Int32, CancellationToken)

Asynchronously reads the bytes from the current stream and writes them to another stream, using a specified buffer size and cancellation token.

CopyToAsync(Stream)

Asynchronously reads the bytes from the current stream and writes them to another stream.

[System.Runtime.InteropServices.ComVisible(false)]
public System.Threading.Tasks.Task CopyToAsync (System.IO.Stream destination);
Parameters
destination
Stream

The stream to which the contents of the current stream will be copied.

Returns

A task that represents the asynchronous copy operation.

Exceptions

destination is null.

Either the current stream or the destination stream is disposed.

The current stream does not support reading, or the destination stream does not support writing.

Examples

The following example demonstrates how to use two FileStream objects to asynchronously copy the files from one directory to another. The FileStream class derives from the Stream class. Notice that the Click event handler for the Button control is marked with the async modifier because it calls an asynchronous method

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

namespace WpfApplication
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
        }

        private async void Button_Click(object sender, RoutedEventArgs e)
        {
            string StartDirectory = @"c:\Users\exampleuser\start";
            string EndDirectory = @"c:\Users\exampleuser\end";

            foreach (string filename in Directory.EnumerateFiles(StartDirectory))
            {
                using (FileStream SourceStream = File.Open(filename, FileMode.Open))
                {
                    using (FileStream DestinationStream = File.Create(EndDirectory + filename.Substring(filename.LastIndexOf('\\'))))
                    {
                        await SourceStream.CopyToAsync(DestinationStream);
                    }
                }
            }
        }
    }
}
Imports System.IO

Class MainWindow

    Private Async Sub Button_Click(sender As Object, e As RoutedEventArgs)
        Dim StartDirectory As String = "c:\Users\exampleuser\start"
        Dim EndDirectory As String = "c:\Users\exampleuser\end"

        For Each filename As String In Directory.EnumerateFiles(StartDirectory)
            Using SourceStream As FileStream = File.Open(filename, FileMode.Open)
                Using DestinationStream As FileStream = File.Create(EndDirectory + filename.Substring(filename.LastIndexOf("\"c)))
                    Await SourceStream.CopyToAsync(DestinationStream)
                End Using

            End Using
        Next
    End Sub

End Class

Remarks

The CopyToAsync 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#.

Copying begins at the current position in the current stream.

CopyToAsync(Stream, Int32)

Asynchronously reads the bytes from the current stream and writes them to another stream, using a specified buffer size.

[System.Runtime.InteropServices.ComVisible(false)]
public System.Threading.Tasks.Task CopyToAsync (System.IO.Stream destination, int bufferSize);
Parameters
destination
Stream

The stream to which the contents of the current stream will be copied.

bufferSize
Int32

The size, in bytes, of the buffer. This value must be greater than zero. The default size is 81920.

Returns

A task that represents the asynchronous copy operation.

Exceptions

destination is null.

buffersize is negative or zero.

Either the current stream or the destination stream is disposed.

The current stream does not support reading, or the destination stream does not support writing.

Remarks

The CopyToAsync 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#.

Copying begins at the current position in the current stream.

For an example of copying between two streams, see the CopyToAsync(Stream) overload.

CopyToAsync(Stream, Int32, CancellationToken)

Asynchronously reads the bytes from the current stream and writes them to another stream, using a specified buffer size and cancellation token.

[System.Runtime.InteropServices.ComVisible(false)]
public virtual System.Threading.Tasks.Task CopyToAsync (System.IO.Stream destination, int bufferSize, System.Threading.CancellationToken cancellationToken);
Parameters
destination
Stream

The stream to which the contents of the current stream will be copied.

bufferSize
Int32

The size, in bytes, of the buffer. This value must be greater than zero. The default size is 81920.

cancellationToken
CancellationToken

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

Returns

A task that represents the asynchronous copy operation.

Exceptions

destination is null.

buffersize is negative or zero.

Either the current stream or the destination stream is disposed.

The current stream does not support reading, or the destination stream does not support writing.

Remarks

The CopyToAsync 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#.

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

Copying begins at the current position in the current stream.

For an example of copying between two streams, see the CopyToAsync(Stream) overload.