System.IO.Stream.BeginRead Method

Begins an asynchronous read operation. (Consider using Stream.ReadAsync(Byte[], int, int) instead; see the Remarks section.)

Syntax

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

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.

Returns

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

Exceptions

TypeReason
System.IO.IOExceptionAn I/O error occurred.
NotSupportedExceptionThe current System.IO.Stream does not support reading.
ObjectDisposedExceptionThe stream is closed.

Remarks

In the .NET Framework 4 and earlier versions, you have to use methods such as Stream.BeginRead(Byte[], int, int, AsyncCallback, object) and Stream.EndRead(IAsyncResult) to implement asynchronous I/O operations. These methods are still available in the net_v45 to support legacy code; however, the new async methods, such as Stream.ReadAsync(Byte[], int, int), Stream.WriteAsync(Byte[], int, int), Stream.CopyToAsync(Stream), and Stream.FlushAsync, help you implement asynchronous I/O operations more easily.

The default implementation of BeginRead on a stream calls the Stream.Read(Byte[], int, int) 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 Stream.EndRead(IAsyncResult) method of the stream to determine how many bytes were read and to release operating system resources used for reading. Stream.EndRead(IAsyncResult) must be called once for every call to Stream.BeginRead(Byte[], int, int, AsyncCallback, object). 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 Stream.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.

Requirements

Namespace: System.IO
Assembly: mscorlib (in mscorlib.dll)
Assembly Versions: 1.0.5000.0, 2.0.0.0, 4.0.0.0