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Stream.CopyToAsync Method (Stream, Int32, CancellationToken)

.NET Framework 4.6 and 4.5

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

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

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public virtual Task CopyToAsync(
	Stream destination,
	int bufferSize,
	CancellationToken cancellationToken


Type: System.IO.Stream

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

Type: System.Int32

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

Type: System.Threading.CancellationToken

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

Return Value

Type: System.Threading.Tasks.Task

A task that represents the asynchronous copy operation.

Exception Condition

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.

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.

Universal Windows Platform
Available since 4.5
.NET Framework
Available since 4.5
Portable Class Library
Supported in: portable .NET platforms
Windows Phone Silverlight
Available since 8.0
Windows Phone
Available since 8.1
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