Stream.CopyToAsync Method (Stream, Int32, CancellationToken)

[ This article is for Windows Phone 8 developers. If you’re developing for Windows 10, see the latest documentation. ]

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)


Public Overridable Function CopyToAsync ( _
    destination As Stream, _
    bufferSize As Integer, _
    cancellationToken As CancellationToken _
) As Task
public virtual Task CopyToAsync(
    Stream destination,
    int bufferSize,
    CancellationToken cancellationToken


  • destination
    Type: System.IO..::.Stream
    The stream to which the contents of the current stream will be copied.
  • bufferSize
    Type: System..::.Int32
    The size, in bytes, of the buffer. This value must be greater than zero. The default size is 4096.

Return Value

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


Exception Condition

destination is nullNothingnullptra null reference (Nothing in Visual Basic).


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 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#.

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 overload.

Version Information

Windows Phone OS

Supported in: 8.1, 8.0

See Also


Stream Class

CopyToAsync Overload

System.IO Namespace