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Stream.BeginRead Method

Begins an asynchronous read operation.

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

public virtual IAsyncResult BeginRead (
	byte[] buffer,
	int offset,
	int count,
	AsyncCallback callback,
	Object state
)
public IAsyncResult BeginRead (
	byte[] buffer, 
	int offset, 
	int count, 
	AsyncCallback callback, 
	Object state
)
public function BeginRead (
	buffer : byte[], 
	offset : int, 
	count : int, 
	callback : AsyncCallback, 
	state : Object
) : IAsyncResult

Parameters

buffer

The buffer to read the data into.

offset

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

count

The maximum number of bytes to read.

callback

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

state

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

Return Value

An IAsyncResult that represents the asynchronous read, which could still be pending.
Exception typeCondition

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.

For an example of creating a file and writing text to a file, see How to: Write Text to a File. For an example of reading text from a file, see How to: Read Text from a File. For an example of reading from and writing to a binary file, see How to: Read and Write to a Newly Created Data File. For an example of asynchronous file I/O, see Asynchronous File I/O.

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.

Windows 98, Windows 2000 SP4, Windows CE, Windows Millennium Edition, Windows Mobile for Pocket PC, Windows Mobile for Smartphone, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see System Requirements.

.NET Framework

Supported in: 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0
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