Stream.BeginRead Method (System.IO)

Switch View :
Classic
Lightweight
ScriptFree
.NET Framework Class Library
Stream.BeginRead Method

Begins an asynchronous read operation.

Namespace:  System.IO
Assembly:  mscorlib (in mscorlib.dll)
Syntax
Visual Basic
<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)> _
Public Overridable Function BeginRead ( _
	buffer As Byte(), _
	offset As Integer, _
	count As Integer, _
	callback As AsyncCallback, _
	state As Object _
) As IAsyncResult
C#
[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public virtual IAsyncResult BeginRead(
	byte[] buffer,
	int offset,
	int count,
	AsyncCallback callback,
	Object state
)
Visual C++
[HostProtectionAttribute(SecurityAction::LinkDemand, ExternalThreading = true)]
public:
virtual IAsyncResult^ BeginRead(
	array<unsigned char>^ buffer, 
	int offset, 
	int count, 
	AsyncCallback^ callback, 
	Object^ state
)
F#
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>]
abstract BeginRead : 
        buffer:byte[] * 
        offset:int * 
        count:int * 
        callback:AsyncCallback * 
        state:Object -> IAsyncResult 
[<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)>]
override BeginRead : 
        buffer:byte[] * 
        offset:int * 
        count:int * 
        callback:AsyncCallback * 
        state:Object -> IAsyncResult

Parameters

buffer
Type: 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.
Exceptions
Exception Condition
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.

Remarks

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.

Note Note

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.
Version Information

.NET Framework

Supported in: 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
Platforms

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows XP SP2 x64 Edition, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

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

Reference

Stream Class
System.IO Namespace

Other Resources

File and Stream I/O
How to: Read Text from a File
How to: Write Text to a File
Asynchronous File I/O
Community Content

kwaclaw
Current position not always updated in BeginRead

The documentation above states that "The current position in the stream is updated when the asynchronous read or write is issued, not when the I/O operation completes.". This is not true for the default implementation on Stream, it works however for FileStream.

If you look at it with Reflector you will notice that BeginRead/EndRead is using the synchronous Read implementation scheduled through delegate.BeginInvoke. This means that Read will execute on a threadpool thread some time later, and it is during the execution of Read that the stream position is updated. In this case, the updated stream position can only read reliably when the completion callback  is executed (delegate passed to BeginRead). This works because only one read is allowed to be in progress at a time. When EndRead is called, the next Read is released for scheduling.

This behaviour can be an issue if you want to store the read position with the read data. For the default async implementation you have to do it in the call-back (before calling EndRead), for the FileStream implementation you have to do it just before BeginRead (using a lock around the code). I have not found a way to detect which behaviour is present, so as to code accordingly.