FileStream.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 override IAsyncResult BeginWrite(
	byte[] array,
	int offset,
	int numBytes,
	AsyncCallback userCallback,
	Object stateObject
)

Parameters

array
Type: System.Byte[]

The buffer containing data to write to the current stream.

offset
Type: System.Int32

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

numBytes
Type: System.Int32

The maximum number of bytes to write.

userCallback
Type: System.AsyncCallback

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

stateObject
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.

ExceptionCondition
ArgumentException

array length minus offset is less than numBytes.

ArgumentNullException

array is null.

ArgumentOutOfRangeException

offset or numBytes is negative.

NotSupportedException

The stream does not support writing.

ObjectDisposedException

The stream is closed.

IOException

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.

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.

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

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0, 1.1, 1.0

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Supported in: Windows Phone 8.1

Supported in: Windows Phone Silverlight 8.1

Supported in: 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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