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Stream::BeginRead Method

Begins an asynchronous read operation. (Consider using ReadAsync instead; see the Remarks section.)

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

[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
public:
virtual IAsyncResult^ BeginRead(
	array<unsigned char>^ buffer, 
	int offset, 
	int count, 
	AsyncCallback^ callback, 
	Object^ state
)

Parameters

buffer
Type: array<System::Byte>

The buffer to read the data into.

offset
Type: System::Int32

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

count
Type: System::Int32

The maximum number of bytes to read.

callback
Type: System::AsyncCallback

An optional asynchronous callback, to be called when the read is complete.

state
Type: System::Object

A user-provided object that distinguishes this particular asynchronous read request from other requests.

Return Value

Type: System::IAsyncResult
An IAsyncResult that represents the asynchronous read, which could still be pending.

ExceptionCondition
IOException

Attempted an asynchronous read past the end of the stream, or a disk error occurs.

ArgumentException

One or more of the arguments is invalid.

ObjectDisposedException

Methods were called after the stream was closed.

NotSupportedException

The current Stream implementation does not support the read operation.

In the .NET Framework 4 and earlier versions, you have to use methods such as BeginRead and EndRead to implement asynchronous I/O operations. These methods are still available in the .NET Framework 4.5 to support legacy code; however, the new async methods, such as ReadAsync, WriteAsync, CopyToAsync, and FlushAsync, help you implement asynchronous I/O operations more easily.

The default implementation of BeginRead on a stream calls the Read method synchronously, which means that Read might block on some streams. However, instances of classes such as FileStream and NetworkStream fully support asynchronous operations if the instances have been opened asynchronously. Therefore, calls to BeginRead will not block on those streams. You can override BeginRead (by using async delegates, for example) to provide asynchronous behavior.

Pass the IAsyncResult return value to the EndRead method of the stream to determine how many bytes were read and to release operating system resources used for reading. EndRead must be called once for every call to BeginRead. You can do this either by using the same code that called BeginRead or in a callback passed to BeginRead.

The current position in the stream is updated when the asynchronous read or write is issued, not when the I/O operation completes.

Multiple simultaneous asynchronous requests render the request completion order uncertain.

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

If a stream is closed or you pass an invalid argument, exceptions are thrown immediately from BeginRead. Errors that occur during an asynchronous read request, such as a disk failure during the I/O request, occur on the thread pool thread and throw exceptions when calling EndRead.

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.

.NET Framework

Supported in: 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, 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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