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Stream.BeginRead Method

Begins an asynchronous read operation.

[Visual Basic]
Public Overridable Function BeginRead( _
   ByVal buffer() As Byte, _
   ByVal offset As Integer, _
   ByVal count As Integer, _
   ByVal callback As AsyncCallback, _
   ByVal state As Object _
) As IAsyncResult
public virtual IAsyncResult BeginRead(
 byte[] buffer,
 int offset,
 int count,
 AsyncCallback callback,
 object state
public: virtual IAsyncResult* BeginRead(
 unsigned char buffer __gc[],
 int offset,
 int count,
 AsyncCallback* callback,
 Object* state
public function BeginRead(
   buffer : Byte[],
 offset : int,
 count : int,
 callback : AsyncCallback,
 state : Object
) : IAsyncResult;


The buffer to read the data into.
The byte offset in buffer at which to begin writing data read from the stream.
The maximum number of bytes to read.
An optional asynchronous callback, to be called when the read is complete.
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 Type Condition
IOException Attempted an asynchronous read past the end of the stream, or a disk error occurs.
ArgumentException One or more or 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 Writing Text to a File. For an example of reading text from a file, see Reading Text from a File. For an example of reading from and writing to a binary file, see Reading and Writing 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. 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 threadpool thread and throw exceptions when calling EndRead.


Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework, Common Language Infrastructure (CLI) Standard

See Also

Stream Class | Stream Members | System.IO Namespace | Working with I/O | Reading Text from a File | Writing Text to a File | Asynchronous File I/O