BeginRead Method

Stream.BeginRead Method (Byte[], Int32, Int32, AsyncCallback, Object)


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(
	byte[] buffer,
	int offset,
	int count,
	AsyncCallback callback,
	object state


Type: System.Byte[]

The buffer to read the data into.

Type: System.Int32

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

Type: System.Int32

The maximum number of bytes to read.

Type: System.AsyncCallback

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

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.

Exception Condition

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


One or more of the arguments is invalid.


Methods were called after the stream was closed.


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.

.NET Framework
Available since 1.1
Portable Class Library
Supported in: portable .NET platforms
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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