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Stream.BeginWrite Method

Begins an asynchronous write operation. (Consider using WriteAsync instead; see the Remarks section.)

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

[HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading = true)]
public virtual IAsyncResult BeginWrite(
	byte[] buffer,
	int offset,
	int count,
	AsyncCallback callback,
	Object state
)

Parameters

buffer
Type: System.Byte[]

The buffer to write data from.

offset
Type: System.Int32

The byte offset in buffer from which to begin writing.

count
Type: System.Int32

The maximum number of bytes to write.

callback
Type: System.AsyncCallback

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

state
Type: System.Object

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

Return Value

Type: System.IAsyncResult
An IAsyncResult that represents the asynchronous write, which could still be pending.

ExceptionCondition
IOException

Attempted an asynchronous write 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 write operation.

In the .NET Framework 4 and earlier versions, you have to use methods such as BeginWrite and EndWrite 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 BeginWrite on a stream calls the Write method synchronously, which means that Write 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 BeginWrite will not block on those streams. You can override BeginWrite (by using async delegates, for example) to provide asynchronous behavior.

Pass the IAsyncResult returned by the current method to EndWrite to ensure that the write completes and frees resources appropriately. EndWrite must be called once for every call to BeginWrite. You can do this either by using the same code that called BeginWrite or in a callback passed to BeginWrite. If an error occurs during an asynchronous write, an exception will not be thrown until EndWrite is called with the IAsyncResult returned by this method.

If a stream is writable, writing at the end of the stream expands the stream.

The current position in the stream is updated when you issue the asynchronous read or write, not when the I/O operation completes. Multiple simultaneous asynchronous requests render the request completion order uncertain.

Use the CanWrite property to determine whether the current instance supports writing.

If a stream is closed or you pass an invalid argument, exceptions are thrown immediately from BeginWrite. Errors that occur during an asynchronous write request, such as a disk failure during the I/O request, occur on the thread pool thread and throw exceptions when calling EndWrite.

NoteNote

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.

.NET Framework

Supported in: 4.5, 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

.NET for Windows Phone apps

Supported in: Windows Phone 8.1, Windows Phone Silverlight 8.1, Windows Phone Silverlight 8

Windows Phone 8.1, Windows Phone 8, Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

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

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