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Stream.CopyToAsync Method (Stream)

.NET Framework 4.5

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

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

public Task CopyToAsync(
	Stream destination


Type: System.IO.Stream

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

Return Value

Type: System.Threading.Tasks.Task
A task that represents the asynchronous copy operation.


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.

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 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.

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()

        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);

.NET Framework

Supported in: 4.6, 4.5

.NET for Windows Store apps

Supported in: Windows 8

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8

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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