GZipStream Class

Updated: August 2009

Provides methods and properties used to compress and decompress streams.

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

'Declaration
Public Class GZipStream _
	Inherits Stream
'Usage
Dim instance As GZipStream

This class represents the gzip data format, which uses an industry standard algorithm for lossless file compression and decompression. The format includes a cyclic redundancy check value for detecting data corruption. The gzip data format uses the same algorithm as the DeflateStream class, but can be extended to use other compression formats. The format can be readily implemented in a manner not covered by patents. This class cannot be used to compress files larger than 4 GB.

Compressed GZipStream objects written to a file with an extension of .gz can be decompressed using many common compression tools; however, this class does not inherently provide functionality for adding files to or extracting files from .zip archives.

The compression functionality in DeflateStream and GZipStream is exposed as a stream. Data is read in on a byte-by-byte basis, so it is not possible to perform multiple passes to determine the best method for compressing entire files or large blocks of data. The DeflateStream and GZipStream classes are best used on uncompressed sources of data. If the source data is already compressed, using these classes may actually increase the size of the stream.

Notes to Inheritors:

When you inherit from GZipStream, you must override the following members: CanSeek, CanWrite, and CanRead.

The following example shows how to use the GZipStream class to compress and decompress a directory of files.

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

    Sub Main()
        ' Path to directory of files to compress. 
        Dim dirpath As String = "c:\users\public\reports" 

        Dim di As DirectoryInfo = New DirectoryInfo(dirpath)


        ' Compress the directory's files. 
        For Each fi As FileInfo In di.GetFiles()
            Compress(fi)
        Next 

        ' Decompress all *.gz files in the directory. 
        For Each fi As FileInfo In di.GetFiles("*.gz")
            Decompress(fi)
        Next 

    End Sub 

    ' Method to compress. 
    Private Sub Compress(ByVal fi As FileInfo)
        ' Get the stream of the source file. 
        Using inFile As FileStream = fi.OpenRead()
            ' Compressing: 
            ' Prevent compressing hidden and already compressed files. 

            If (File.GetAttributes(fi.FullName) And FileAttributes.Hidden) _
                <> FileAttributes.Hidden And fi.Extension <> ".gz" Then 
                ' Create the compressed file. 
                Using outFile As FileStream = File.Create(fi.FullName + ".gz")
                    Using Compress As GZipStream = New GZipStream(outFile, CompressionMode.Compress)
                        ' Copy the source file into the compression stream. 
                        Dim buffer As Byte() = New Byte(4096) {}
                        Dim numRead As Integer
                        numRead = inFile.Read(buffer, 0, buffer.Length)
                        Do While numRead <> 0
                            Compress.Write(buffer, 0, numRead)
                            numRead = inFile.Read(buffer, 0, buffer.Length)
                        Loop

                        Console.WriteLine("Compressed {0} from {1} to {2} bytes.", _
                                          fi.Name, fi.Length.ToString(), outFile.Length.ToString())

                    End Using 
                End Using 
            End If 
        End Using 
    End Sub 

    ' Method to decompress. 
    Private Sub Decompress(ByVal fi As FileInfo)
        ' Get the stream of the source file. 
        Using inFile As FileStream = fi.OpenRead()
            ' Get orignial file extension, for example "doc" from report.doc.gz. 
            Dim curFile As String = fi.FullName
            Dim origName = curFile.Remove(curFile.Length - fi.Extension.Length)

            ' Create the decompressed file. 
            Using outFile As FileStream = File.Create(origName)
                Using Decompress As GZipStream = New GZipStream(inFile, CompressionMode.Decompress)
                    ' Copy the compressed file into the decompression stream. 
                    Dim buffer As Byte() = New Byte(4096) {}
                    Dim numRead As Integer
                    numRead = Decompress.Read(buffer, 0, buffer.Length)
                    Do While numRead <> 0
                        outFile.Write(buffer, 0, numRead)
                        numRead = Decompress.Read(buffer, 0, buffer.Length)
                    Loop
                    Console.WriteLine("Decompressed: {0}", fi.Name)

                End Using 
            End Using 
        End Using 
    End Sub 
End Module

System.Object
  System.MarshalByRefObject
    System.IO.Stream
      System.IO.Compression.GZipStream

Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.

Windows 7, Windows Vista, Windows XP SP2, Windows XP Media Center Edition, Windows XP Professional x64 Edition, Windows XP Starter Edition, Windows Server 2008 R2, Windows Server 2008, Windows Server 2003, Windows Server 2000 SP4, Windows Millennium Edition, Windows 98, Windows CE, Windows Mobile for Smartphone, Windows Mobile for Pocket PC, Xbox 360, Zune

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

.NET Framework

Supported in: 3.5, 3.0, 2.0

.NET Compact Framework

Supported in: 3.5

XNA Framework

Supported in: 3.0

Date

History

Reason

August 2009

Improved code example.

Information enhancement.

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