Stream.CopyToAsync Method (Stream)

.NET Framework 4.6 and 4.5

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

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

[ComVisibleAttribute(false)]
[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public Task CopyToAsync(
	Stream destination
)

Parameters

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.

ExceptionCondition
ArgumentNullException

destination is null.

ObjectDisposedException

Either the current stream or the destination stream is disposed.

NotSupportedException

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.

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 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);
                    }
                }
            }
        }
    }
}

.NET Framework

Supported in: 4.6, 4.5

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: Windows Phone Silverlight 8
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