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


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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 override IAsyncResult BeginWrite(
	byte[] array,
	int offset,
	int numBytes,
	AsyncCallback userCallback,
	object stateObject


Type: System.Byte[]

The buffer containing data to write to the current stream.

Type: System.Int32

The zero-based byte offset in array at which to begin copying bytes to the current stream.

Type: System.Int32

The maximum number of bytes to write.

Type: System.AsyncCallback

The method to be called when the asynchronous write operation is completed.

Type: System.Object

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

Return Value

Type: System.IAsyncResult

An object that references the asynchronous write.

Exception Condition

array length minus offset is less than numBytes.


array is null.


offset or numBytes is negative.


The stream does not support writing.


The stream is closed.


An I/O error occurred.

In the .NET Framework 4 and earlier versions, you have to use methods such as BeginWrite and EndWrite to implement asynchronous file 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 file operations more easily.

EndWrite must be called exactly once on every IAsyncResult from BeginWrite. EndWrite will block until the I/O operation has completed.

This method overrides BeginWrite.

FileStream provides two different modes of operation: synchronous I/O and asynchronous I/O. While either can be used, the underlying operating system resources might allow access in only one of these modes. By default, FileStream opens the operating system handle synchronously. In Windows, this slows down asynchronous methods. If asynchronous methods are used, use the FileStream(String, FileMode, FileAccess, FileShare, Int32, Boolean) constructor.

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 IO request, occur on the thread pool thread and become visible upon a call to EndWrite.

Multiple simultaneous asynchronous requests render the request completion order uncertain.

For a list of common file and directory operations, see Common I-O Tasks.

This code example is part of a larger example provided for the FileStream(String, FileMode, FileAccess, FileShare, Int32, Boolean) constructor.

static void Main()
    // Create a synchronization object that gets 
    // signaled when verification is complete.
    ManualResetEvent manualEvent = new ManualResetEvent(false);

    // Create random data to write to the file.
    byte[] writeArray = new byte[100000];
    new Random().NextBytes(writeArray);

    FileStream fStream = 
        new FileStream("Test#@@#.dat", FileMode.Create, 
        FileAccess.ReadWrite, FileShare.None, 4096, true);

    // Check that the FileStream was opened asynchronously.
    Console.WriteLine("fStream was {0}opened asynchronously.",
        fStream.IsAsync ? "" : "not ");

    // Asynchronously write to the file.
    IAsyncResult asyncResult = fStream.BeginWrite(
        writeArray, 0, writeArray.Length, 
        new AsyncCallback(EndWriteCallback), 
        new State(fStream, writeArray, manualEvent));

    // Concurrently do other work and then wait 
    // for the data to be written and verified.
    manualEvent.WaitOne(5000, false);

.NET Framework
Available since 1.1
Available since 2.0
Windows Phone Silverlight
Available since 7.0
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