FileStream.BeginRead Method

Begins an asynchronous read.

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

Public Overrides Function BeginRead ( _
	array As Byte(), _
	offset As Integer, _
	numBytes As Integer, _
	userCallback As AsyncCallback, _
	stateObject As Object _
) As IAsyncResult
Dim instance As FileStream
Dim array As Byte()
Dim offset As Integer
Dim numBytes As Integer
Dim userCallback As AsyncCallback
Dim stateObject As Object
Dim returnValue As IAsyncResult

returnValue = instance.BeginRead(array, offset, numBytes, userCallback, 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



The buffer to read data into.


The byte offset in array at which to begin reading.


The maximum number of bytes to read.


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


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


The array length minus offset is less than numBytes.


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


offset or numBytes is negative.


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.


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.


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



Rename or move a file.



Copy a file.



Get the size of a file.


Get the attributes of a file.


Set the attributes of a file.


Determine if a 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.



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

Private Shared Sub EndWriteCallback(asyncResult As IAsyncResult)
    Dim tempState As State = _
        DirectCast(asyncResult.AsyncState, State)
    Dim fStream As FileStream = tempState.FStream

    ' Asynchronously read back the written data.
    fStream.Position = 0
    asyncResult = fStream.BeginRead( _ 
        tempState.ReadArray, 0 , tempState.ReadArray.Length, _
        AddressOf EndReadCallback, tempState)

    ' Concurrently do other work, such as 
    ' logging the write operation.
End Sub

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

    // Asynchronously read back the written data.
    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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