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

[Dennis Powell]

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

 

[Guest]

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