RE: Has anyone tried using zlib.dll in VB .Net?

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From: Dennis (Dennis_at_discussions.microsoft.com)
Date: 07/23/04 

Date: Thu, 22 Jul 2004 18:43:02 -0700

This works!

'DATACOMPRESS CLASS - VB.Net
'**********************************************************************************************************
' METHODS:
' CompressBytes(Source ByteArray, Optional Temporay Byte Array
' Compresses ByteArray into Temporary Byte Array or into ByteArray if Temporary Byte Array not input
'
' DeCompressBytes (Source ByteArray, Original Array Size before compression as integer, Optional Temporary Byte Array)
' Decompresses ByteArray into Temporary Byte Array or into ByteArray if Temporary Byte Array not input
'
'***********************************************************************************************************
Imports System.Runtime.InteropServices

Public Class DataCompress

    'Declare zlib functions "Compress" and "Uncompress" for compressing Byte Arrays
    <DllImport("zlib.DLL", EntryPoint:="compress")> _
Private Shared Function CompressByteArray(ByVal dest As Byte(), ByRef destLen As Integer, ByVal src As Byte(), ByVal srcLen As Integer) As Integer
        ' Leave function empty - DLLImport attribute forwards calls to CompressByteArray to compress in zlib.dLL
    End Function
    <DllImport("zlib.DLL", EntryPoint:="uncompress")> _
Private Shared Function UncompressByteArray(ByVal dest As Byte(), ByRef destLen As Integer, ByVal src As Byte(), ByVal srcLen As Integer) As Integer
        ' Leave function empty - DLLImport attribute forwards calls to UnCompressByteArray to Uncompress in zlib.dLL
    End Function

    Public Sub New()
        MyBase.New()
    End Sub

    Public Function CompressBytes(ByRef Data() As Byte, Optional ByRef TempBuffer() As Byte = Nothing) As Integer
        'Compresses Data into a temp buffer
        'Returns compressed Data in Data if TempBuff not specified
        'Returns Result = Size of compressed data if ok, -1 if not
        Dim OriginalSize As Long = UBound(Data) + 1
        'Allocate temporary Byte Array for storage
        Dim result As Integer
        Dim usenewstorage As Boolean
        If TempBuffer Is Nothing Then usenewstorage = False Else usenewstorage = True
        Dim BufferSize As Integer = UBound(Data) + 1
        BufferSize = CInt(BufferSize + (BufferSize * 0.01) + 12)
        ReDim TempBuffer(BufferSize)
        'Compress data byte array
        result = CompressByteArray(TempBuffer, BufferSize, Data, UBound(Data) + 1)
        'Store results
        If result = 0 Then
            If usenewstorage Then
                'Return results in TempBuffer
                ReDim Preserve TempBuffer(BufferSize - 1)
            Else
                'Return compressed Data in original Data Array
                ' Resize original data array to compressed size
                ReDim Data(BufferSize - 1)
                ' Copy Array to original data array
                Array.Copy(TempBuffer, Data, BufferSize)
                'Release TempBuffer STorage
                TempBuffer = Nothing
            End If
            Return BufferSize
        Else
            Return -1
        End If
    End Function
    Public Function DeCompressBytes(ByRef Data() As Byte, ByVal Origsize As Integer, Optional ByRef TempBuffer() As Byte = Nothing) As Integer
        'DeCompresses Data into a temp buffer..note that Origsize must be the size of the original data before compression
        'Returns compressed Data in Data if TempBuff not specified
        'Returns Result = Size of decompressed data if ok, -1 if not
        'Allocate memory for buffers
        Dim result As Integer
        Dim usenewstorage As Boolean
        Dim Buffersize As Integer = CInt(Origsize + (Origsize * 0.01) + 12)
        If TempBuffer Is Nothing Then usenewstorage = False Else usenewstorage = True
        ReDim TempBuffer(Buffersize)

        'Decompress data
        result = UncompressByteArray(TempBuffer, Origsize, Data, UBound(Data) + 1)

        'Truncate buffer to compressed size
        If result = 0 Then
            If usenewstorage Then
                'Return decoompressed data in TempBuffer
                ReDim Preserve TempBuffer(Origsize - 1)
            Else
                'Return decompressed data in original source data file
                ' Truncate to compressed size
                ReDim Data(Origsize - 1)
                ' Copy Array to original data array
                Array.Copy(TempBuffer, Data, Origsize)
                'Release TempBuffer STorage
                TempBuffer = Nothing
            End If
            Return Origsize
        Else
            Return -1
        End If
    End Function

    Protected Overrides Sub Finalize()
        MyBase.Finalize()
    End Sub
    'Test routines for this class:
    'Testing - Remove
    'Dim sdata As Byte() = New Byte() {}
    'Dim fs As FileStream = New FileStream("C:\Programming\VS-Help Docs\Misc NewsGroup Articles.doc", FileMode.Open, FileAccess.Read)
    'Dim rnew As BinaryReader = New BinaryReader(fs)
    ' sdata = rnew.ReadBytes(CInt(fs.Length))
    ' r.Close()
    ' fs.Close()
    'Dim ucomp As Integer = UBound(sdata, 1) + 1
    'Dim compsize As Integer = cmp.CompressBytes(sdata)
    'Dim uncompsize As Integer = cmp.DeCompressBytes(sdata, ucomp)
    'End TEsting

End Class 

-- 
Dennis in Houston
"Dennis Powell" wrote:
> Does anyone have a successful implementaion of the zlib.dll in VB. Net they
> can show me.  I'm writting a class encaplsulating zlib functionality and I
> keep getting a System.NullReferenceException (Object reference not set to an
> instance of an object) when I try to call the dll's compress function.
> 
> The  function in the dll is declared as such:
> int compress (Bytef *dest, uLongf *destLen, const Bytef *source, uLong
> sourceLen)
> 
> I've declared it as such in my class:
> Private Declare Function Shrink Lib "zlib.dll" Alias "compress" _
>                 (ByRef strCompressed As String, ByRef lngCompressedLen As
> Long, _
>                 ByVal strToCompress As String, ByVal strToCompressLen As
> Long) As Long
> 
> Implemented in the class:
> Public Sub Compress()
> Dim rVal as Long     'return value from the function
> 
> ' all values passed to zlib.dll are class private member variables
> 
> ' m_strCompressed - a buffer(string) at least 1% + 12 characters larger than
> the string to compress (required by the compress function) that the
> compressed string  ' is returned in.  Initialized in the StrToCompress
> Property Set this way: m_strCompressed = New String(" ",
> (Len(m_strToCompress) * 1.01) + 13)
> ' m_lngCompressedLen - the length of the return buffer when called and holds
> the actual length when it returns.
> ' m_strToCompress - the string that will be compressed
> ' m_lngStringLen - the length of the string that will be compressed
> 
> rVal = Shrink(m_strCompressed, m_lngCompressedLen, m_strToCompress,
> m_lngStringLen)
> I get the exception on the above line:
> 
> I used the zlib.dll in the VB6 version of my program without any problems.
> The only difference is the lngCompressedLen was declared as 'As Any' which
> is not possible in .Net.  I'm re-writing the program in VB .Net and need to
> maintain compatibility with the VB6 Version as far as compressing and
> decompressing files is concerned.
> 
> Any help would be greatly appreciated.
> 
> Thanks,
> Dennis
> 
> 
>
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