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DeflateStream Class

Updated: August 2009

Provides methods and properties for compressing and decompressing streams using the Deflate algorithm.

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

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

This class represents the Deflate algorithm, an industry standard algorithm for lossless file compression and decompression. It uses a combination of the LZ77 algorithm and Huffman coding. Data can be produced or consumed, even for an arbitrarily long, sequentially presented input data stream, using only previously bound amount of intermediate storage. The format can be implemented readily in a manner not covered by patents. For more information, see RFC 1951. "DEFLATE Compressed Data Format Specification version 1.3." This class cannot be used to compress files larger than 4 GB.

This class does not inherently provide functionality for adding files to or extracting files from .zip archives.

The GZipStream class uses the gzip data format, which includes a cyclic redundancy check value for detecting data corruption. The gzip data format uses the same compression algorithm as the DeflateStream class.

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.

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 *.cmp files in the directory. 
        For Each fi As FileInfo In di.GetFiles("*.cmp")
            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 <> ".cmp" Then 
                ' Create the compressed file. 
                Using outFile As FileStream = File.Create(fi.FullName + ".cmp")
                    Using Compress As DeflateStream = New DeflateStream(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.cmp. 
            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 DeflateStream = New DeflateStream(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.DeflateStream

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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