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FileStream::ReadAsync Method (array<Byte>, Int32, Int32, CancellationToken)

.NET Framework 4.5

Asynchronously reads a sequence of bytes from the current stream, advances the position within the stream by the number of bytes read, and monitors cancellation requests.

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

[ComVisibleAttribute(false)]
[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
public:
virtual Task<int>^ ReadAsync(
	array<unsigned char>^ buffer, 
	int offset, 
	int count, 
	CancellationToken cancellationToken
) override

Parameters

buffer
Type: array<System::Byte>

The buffer to write the data into.

offset
Type: System::Int32

The byte offset in buffer at which to begin writing data from the stream.

count
Type: System::Int32

The maximum number of bytes to read.

cancellationToken
Type: System.Threading::CancellationToken

The token to monitor for cancellation requests.

Return Value

Type: System.Threading.Tasks::Task<Int32>
A task that represents the asynchronous read operation. The value of the TResult parameter contains the total number of bytes read into the buffer. The result value can be less than the number of bytes requested if the number of bytes currently available is less than the requested number, or it can be 0 (zero) if the end of the stream has been reached.

ExceptionCondition
ArgumentNullException

buffer is nullptr.

ArgumentOutOfRangeException

offset or count is negative.

ArgumentException

The sum of offset and count is larger than the buffer length.

NotSupportedException

The stream does not support reading.

ObjectDisposedException

The stream has been disposed.

InvalidOperationException

The stream is currently in use by a previous read operation.

The ReadAsync method enables you to perform resource-intensive file 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#.

Use the CanRead property to determine whether the current instance supports reading.

If the operation is canceled before it completes, the returned task contains the Canceled value for the Status property. If the handle to the file is disposed, the returned task contains the ObjectDisposedException exception in the Exception property.

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 shows how to read from a file asynchronously.

No code example is currently available or this language may not be supported.

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5

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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