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FileStream.BeginRead Method

Begins an asynchronous read.

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

public override IAsyncResult BeginRead (
	byte[] array,
	int offset,
	int numBytes,
	AsyncCallback userCallback,
	Object stateObject
)
public IAsyncResult BeginRead (
	byte[] array, 
	int offset, 
	int numBytes, 
	AsyncCallback userCallback, 
	Object stateObject
)
public override function BeginRead (
	array : byte[], 
	offset : int, 
	numBytes : int, 
	userCallback : AsyncCallback, 
	stateObject : Object
) : IAsyncResult

Parameters

array

The buffer to read data into.

offset

The byte offset in array at which to begin reading.

numBytes

The maximum number of bytes to read.

userCallback

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

stateObject

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

Return Value

An IAsyncResult that references the asynchronous read.
Exception typeCondition

ArgumentException

The array length minus offset is less than numBytes.

ArgumentNullException

array is a null reference (Nothing in Visual Basic).

ArgumentOutOfRangeException

offset or numBytes is negative.

IOException

An asynchronous read was attempted past the end of the file.

EndRead must be called exactly for every call to BeginRead. Failing to end a read process before beginning another read can cause undesirable behavior such as deadlock.

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.

NoteNote

Use the CanRead property to determine whether the current instance supports reading. For additional information, see CanRead.

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

NoteNote

On Windows, all I/O operations smaller than 64 KB will complete synchronously for better performance. Asynchronous I/O might hinder performance for buffer sizes smaller than 64 KB.

EndRead must be called with this IAsyncResult to find out how many bytes were read.

Multiple simultaneous asynchronous requests render the request completion order uncertain.

The following table lists examples of other typical or related I/O tasks.

To do this...

See the example in this topic...

Create a text file.

How to: Write Text to a File

Write to a text file.

How to: Write Text to a File

Read from a text file.

How to: Read Text from a File

Append text to a file.

How to: Open and Append to a Log File

File.AppendText

FileInfo.AppendText

Rename or move a file.

File.Move

FileInfo.MoveTo

Copy a file.

File.Copy

FileInfo.CopyTo

Get the size of a file.

FileInfo.Length

Get the attributes of a file.

File.GetAttributes

Set the attributes of a file.

File.SetAttributes

Determine if a file exists.

File.Exists

Read from a binary file.

How to: Read and Write to a Newly Created Data File

Write to a binary file.

How to: Read and Write to a Newly Created Data File

Create a directory.

Directory.CreateDirectory

Directory.CreateDirectory

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

static void EndWriteCallback(IAsyncResult asyncResult)
{
    State tempState = (State)asyncResult.AsyncState;
    FileStream fStream = tempState.FStream;
    fStream.EndWrite(asyncResult);

    // Asynchronously read back the written data.
    fStream.Position = 0;
    asyncResult = fStream.BeginRead(
        tempState.ReadArray, 0 , tempState.ReadArray.Length, 
        new AsyncCallback(EndReadCallback), tempState);

    // Concurrently do other work, such as 
    // logging the write operation.
}

static void EndWriteCallback(IAsyncResult asyncResult)
{
    State tempState = ((State)(asyncResult.get_AsyncState()));
    FileStream fStream = tempState.get_FStream();
    fStream.EndWrite(asyncResult);

    // Asynchronously read back the written data.
    fStream.set_Position(0);
    asyncResult = fStream.BeginRead(tempState.get_ReadArray(), 0,
        tempState.get_ReadArray().length , 
        new AsyncCallback(EndReadCallback), tempState);

    // Concurrently do other work, such as 
    // logging the write operation.
} //EndWriteCallback

Windows 98, Windows 2000 SP4, Windows Millennium Edition, Windows Server 2003, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP SP2, Windows XP Starter Edition

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

.NET Framework

Supported in: 2.0, 1.1, 1.0

.NET Compact Framework

Supported in: 2.0, 1.0
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