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GZipStream Class

Provides methods and properties used to compress and decompress streams.

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

Public Class GZipStream
	Inherits Stream
Dim instance As GZipStream

public class GZipStream extends Stream
public class GZipStream extends Stream
Not applicable.

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. The format for gzip is available from the RFC 1952, "GZIP ." 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. For an example of compressing and decompressing files in the gzip data format, see Compression Application Sample.

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

Imports System
Imports System.IO
Imports System.IO.Compression

Public Class GZipTest
    Shared msg As String
   Public Shared Function ReadAllBytesFromStream(stream As Stream, buffer() As Byte) As Integer
      ' Use this method is used to read all bytes from a stream.
      Dim offset As Integer = 0
      Dim totalCount As Integer = 0
      While True
         Dim bytesRead As Integer = stream.Read(buffer, offset, 100)
         If bytesRead = 0 Then
            Exit While
         End If
         offset += bytesRead
         totalCount += bytesRead
      End While
      Return totalCount
   End Function 'ReadAllBytesFromStream
   Public Shared Function CompareData(buf1() As Byte, len1 As Integer, buf2() As Byte, len2 As Integer) As Boolean
      ' Use this method to compare data from two different buffers.
        If len1 <> len2 Then
            msg = "Number of bytes in two buffer are different" & len1 & ":" & len2
            Return False
        End If
      Dim i As Integer
      For i = 0 To len1 - 1
         If buf1(i) <> buf2(i) Then
                msg = "byte " & i & " is different " & buf1(i) & "|" & buf2(i)
                Return False
         End If
        Next i
        msg = "All bytes compare."
        Return True
   End Function 'CompareData
   Public Shared Sub GZipCompressDecompress(filename As String)
        msg = "Test compression and decompression on file " & filename

        Dim infile As FileStream
         ' Open the file as a FileStream object.
         infile = New FileStream(filename, FileMode.Open, FileAccess.Read, FileShare.Read)
            Dim buffer(infile.Length - 1) As Byte
         ' Read the file to ensure it is readable.
            Dim count As Integer = infile.Read(buffer, 0, buffer.Length)
         If count <> buffer.Length Then
                msg = "Test Failed: Unable to read data from file"
         End If
         Dim ms As New MemoryStream()
         ' Use the newly created memory stream for the compressed data.
         Dim compressedzipStream As New GZipStream(ms, CompressionMode.Compress, True)
         compressedzipStream.Write(buffer, 0, buffer.Length)
         ' Close the stream.

            msg = "Original size: " & buffer.Length & ", Compressed size: " & ms.Length

         ' Reset the memory stream position to begin decompression.
         ms.Position = 0
         Dim zipStream As New GZipStream(ms, CompressionMode.Decompress)
         Dim decompressedBuffer(buffer.Length + 100) As Byte
         ' Use the ReadAllBytesFromStream to read the stream.
         Dim totalCount As Integer = GZipTest.ReadAllBytesFromStream(zipStream, decompressedBuffer)
            msg = "Decompressed " & totalCount & " bytes"

         If Not GZipTest.CompareData(buffer, buffer.Length, decompressedBuffer, totalCount) Then
                msg = "Error. The two buffers did not compare."

         End If
        Catch e As Exception
            msg = "Error: The file being read contains invalid data."
        End Try

   End Sub 'GZipCompressDecompress
    Public Shared Sub Main(ByVal args() As String)
        Dim usageText As String = "Usage: GZIPTEST <inputfilename>"
        'If no file name is specified, write usage text.
        If args.Length = 0 Then
            If File.Exists(args(0)) Then
            End If
        End If
    End Sub 'Main
End Class 'GZipTest 

import System.*;
import System.IO.*;
import System.IO.Compression.*;

public class GZipTest
    public static int ReadAllBytesFromStream(Stream stream, ubyte buffer[])
        // Use this method is used to read all bytes from a stream.
        int offSet = 0;
        int totalCount = 0;
        while (true) {
            int bytesRead = stream.Read(buffer, offSet, 100);
            if (bytesRead == 0) {
            offSet += bytesRead;
            totalCount += bytesRead;
        return totalCount;
    } //ReadAllBytesFromStream

    public static boolean CompareData(ubyte buf1[], int len1, 
        ubyte buf2[], int len2)
        // Use this method to compare data from two different buffers.
        if (len1 != len2) {
            Console.WriteLine("Number of bytes in two buffer are "
                + "different {0}:{1}", System.Convert.ToString(len1), 
            return false;
        for (int i = 0; i < len1; i++) {
            if (!(buf1.get_Item(i).Equals(buf2.get_Item(i)))) {
                Console.WriteLine("byte {0} is different {1}|{2}", 
                return false;
        Console.WriteLine("All bytes compare.");
        return true;
    } //CompareData

    public static void GZipCompressDecompress(String fileName)
        Console.WriteLine("Test compression and decompression on file {0}",
        FileStream inFile;
        try {
            // Open the file as a FileStream object.
            inFile = new FileStream(fileName, FileMode.Open, 
                FileAccess.Read, FileShare.Read);
            ubyte buffer[] = new ubyte[(ubyte)inFile.get_Length()];

            // Read the file to ensure it is readable.
            int count = inFile.Read(buffer, 0, buffer.length);
            if (count != buffer.length) {
                Console.WriteLine("Test Failed: Unable to read data from file");
            MemoryStream ms = new MemoryStream();

            // Use the newly created memory stream for the compressed data.
            GZipStream compressedZipStream = 
                new GZipStream(ms, CompressionMode.Compress, true);
            compressedZipStream.Write(buffer, 0, buffer.length);

            // Close the stream.
            Console.WriteLine("Original size: {0}, Compressed size: {1}", 

            // Reset the memory stream position to begin decompression.
            GZipStream zipStream = 
                new GZipStream(ms, CompressionMode.Decompress);
            ubyte decompressedBuffer[] = new ubyte[buffer.length + 100];

            // Use the ReadAllBytesFromStream to read the stream.
            int totalCount = GZipTest.ReadAllBytesFromStream(zipStream, 
            Console.WriteLine("Decompressed {0} bytes", 
            if (!(GZipTest.CompareData(buffer, buffer.length, 
                decompressedBuffer, totalCount))) {
                Console.WriteLine("Error. The two buffers did not compare.");
        catch (InvalidDataException exp) {
            Console.WriteLine("Error: The file being read contains"
                + " invalid data.");
        catch (FileNotFoundException exp) {
            Console.WriteLine("Error:The file specified was not found.");
        catch (ArgumentException exp) {
            Console.WriteLine("Error: path is a zero-length string, contains "
                + "only white space, or contains one or more invalid characters");
        catch (PathTooLongException exp) {
            Console.WriteLine("Error: The specified path, file name, or both "
                + "exceed the system-defined maximum length. For example, on"
                + " Windows-based platforms, paths must be less than 248 "
                + "characters, and file names must be less than 260 characters.");
        catch (DirectoryNotFoundException exp) {
            Console.WriteLine("Error: The specified path is invalid, such"
                + " as being on an unmapped drive.");
        catch (IOException exp) {
            Console.WriteLine("Error: An I/O error occurred while "
                + "opening the file.");
        catch (UnauthorizedAccessException exp) {
            Console.WriteLine("Error: path specified a file that is read-only,"
                + " the path is a directory, or caller does not have"
                + " the required permissions.");
        catch (IndexOutOfRangeException exp) {
            Console.WriteLine("Error: You must provide parameters for MyGZIP.");
    } //GZipCompressDecompress

    public static void main(String[] args)
        String usageText = "Usage: MYGZIP <inputfilename>";

        //If no file name is specified, write usage text.
        if (args.length == 0) {
        else {
            if (File.Exists(args[0])) {
    } //main
} //GZipTest


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 98, Windows Server 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 Microsoft .NET Framework 3.0 is supported on Windows Vista, Microsoft Windows XP SP2, and Windows Server 2003 SP1.

.NET Framework

Supported in: 3.0, 2.0