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


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


Type: System.Byte[]

The buffer to write data from.

Type: System.Int32

The byte offset in buffer from which to begin writing.

Type: System.Int32

The maximum number of bytes to write.

Type: System.AsyncCallback

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

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.

Exception Condition

Attempted an asynchronous write 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 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.

.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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