GZipStream.BeginRead Method (Byte(), Int32, Int32, AsyncCallback, Object)


Begins an asynchronous read operation. (Consider using the Stream.ReadAsync method instead; see the Remarks section.)

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

<HostProtectionAttribute(SecurityAction.LinkDemand, ExternalThreading := True)>
Public Overrides Function BeginRead (
	array As Byte(),
	offset As Integer,
	count As Integer,
	asyncCallback As AsyncCallback,
	asyncState As Object
) As IAsyncResult


Type: System.Byte()

The byte array to read the data into.

Type: System.Int32

The byte offset in array at which to begin reading data from the stream.

Type: System.Int32

The maximum number of bytes to read.

Type: System.AsyncCallback

An optional asynchronous callback, to be called when the read operation is complete.

Type: System.Object

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

Return Value

Type: System.IAsyncResult

An object that represents the asynchronous read operation, which could still be pending.

Exception Condition

The method tried to read asynchronously past the end of the stream, or a disk error occurred.


One or more of the arguments is invalid.


Methods were called after the stream was closed.


The current GZipStream implementation does not support the read operation.


A read operation cannot be performed because the stream is closed.

Starting with the .NET Framework 4.5, you can perform asynchronous read operations by using the Stream.ReadAsync method. The BeginRead method is still available in .NET Framework 4.5 to support legacy code; however, you can implement asynchronous I/O operations more easily by using the new async methods. For more information, see .

Pass the IAsyncResult return value to the EndRead method of the stream to determine how many bytes were read and to release operating system resources used for reading. You can do this either by using the same code that called BeginRead or in a callback passed to BeginRead.

The current position in the stream is updated when the asynchronous read or write is issued, not when the I/O operation completes.

Multiple simultaneous asynchronous requests render the request completion order uncertain.

Use the CanRead property to determine whether the current GZipStream object supports reading.

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 I/O request, occur on the thread pool thread and throw exceptions when calling EndRead.

The following code example shows how to use the GZipStream class to compress and decompress a file.

Imports System.IO
Imports System.IO.Compression
Module Module1

    Private directoryPath As String = "c:\temp"
    Public Sub Main()
        Dim directorySelected As New DirectoryInfo(directoryPath)

        For Each fileToDecompress As FileInfo In directorySelected.GetFiles("*.gz")
    End Sub

    Public Sub Compress(directorySelected As DirectoryInfo)
        For Each fileToCompress As FileInfo In directorySelected.GetFiles()
            Using originalFileStream As FileStream = fileToCompress.OpenRead()
                If (File.GetAttributes(fileToCompress.FullName) And FileAttributes.Hidden) <> FileAttributes.Hidden And fileToCompress.Extension <> ".gz" Then
                    Using compressedFileStream As FileStream = File.Create(fileToCompress.FullName & ".gz")
                        Using compressionStream As New GZipStream(compressedFileStream, CompressionMode.Compress)

                        End Using
                    End Using
                    Dim info As New FileInfo(directoryPath & "\" & fileToCompress.Name & ".gz")
                    Console.WriteLine("Compressed {0} from {1} to {2} bytes.", fileToCompress.Name,
                                      fileToCompress.Length.ToString(), info.Length.ToString())

                End If
            End Using
    End Sub

    Private Sub Decompress(ByVal fileToDecompress As FileInfo)
        Using originalFileStream As FileStream = fileToDecompress.OpenRead()
            Dim currentFileName As String = fileToDecompress.FullName
            Dim newFileName = currentFileName.Remove(currentFileName.Length - fileToDecompress.Extension.Length)

            Using decompressedFileStream As FileStream = File.Create(newFileName)
                Using decompressionStream As GZipStream = New GZipStream(originalFileStream, CompressionMode.Decompress)
                    Console.WriteLine("Decompressed: {0}", fileToDecompress.Name)
                End Using
            End Using
        End Using
    End Sub
End Module

.NET Framework
Available since 2.0
Portable Class Library
Supported in: portable .NET platforms
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