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FileStream.BeginRead Method

Begins an asynchronous read.

[Visual Basic]
Overrides Public Function BeginRead( _
   ByVal array() As Byte, _
   ByVal offset As Integer, _
   ByVal numBytes As Integer, _
   ByVal userCallback As AsyncCallback, _
   ByVal stateObject As Object _
) As IAsyncResult
[C#]
public override IAsyncResult BeginRead(
 byte[] array,
 int offset,
 int numBytes,
 AsyncCallback userCallback,
 object stateObject
);
[C++]
public: IAsyncResult* BeginRead(
 unsigned char array __gc[],
 int offset,
 int numBytes,
 AsyncCallback* userCallback,
 Object* stateObject
);
[JScript]
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.

Exceptions

Exception Type Condition
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.

Remarks

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(IntPtr, FileAccess, Boolean, Int32, Boolean) constructor.

Note   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 threadpool thread and become visible upon a call to EndRead.

Note   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. Writing Text to a File
Write to a text file. Writing Text to a File
Read from a text file. Reading Text from a File
Append text to a file. Opening and Appending 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. Reading and Writing to a Newly Created Data File
Write to a binary file. Reading and Writing to a Newly Created Data File
Create a directory. Directory.CreateDirectory

Directory.CreateDirectory

Example

[Visual Basic, C#, C++] This code example is part of a larger example provided for System.IO.FileStream.FileStream4.

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

    ' 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

[C#] 
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.
}

[C++] 
static void EndWriteCallback(IAsyncResult* asyncResult)
{
    State* tempState = 
        dynamic_cast<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(
        0, &FStream::EndReadCallback), tempState);

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

[JScript] No example is available for JScript. To view a Visual Basic, C#, or C++ example, click the Language Filter button Language Filter in the upper-left corner of the page.

Requirements

Platforms: Windows 98, Windows NT 4.0, Windows Millennium Edition, Windows 2000, Windows XP Home Edition, Windows XP Professional, Windows Server 2003 family, .NET Compact Framework, Common Language Infrastructure (CLI) Standard

See Also

FileStream Class | FileStream Members | System.IO Namespace | Working with I/O | Reading Text from a File | Writing Text to a File | Asynchronous File I/O

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