Re: GZip Compression :(
- From: "Steve Gerrard" <mynamehere@xxxxxxxxxxx>
- Date: Fri, 28 Mar 2008 19:15:52 -0700
Carlo Razzeto wrote:
Hello there,
I'm having an odd issue with GZIP compression (having followed
example code found on MSDN). Basically, after running through the
compression routine I end up with a byte array several times larger
than the source text file, full of zero data. Below is the code used
to do the compression, it's a part of a web service to retreive a
file, there's a compress option prior to base64 encoding the data. In
the following code all undeclared variables you see are properties,
compress repersents a compress attribute specified in the xml
request, FileName is a relitive path to the file on the server inside
the webroot.
Response.ContentType = "text/xml"
If Not File.Exists(Server.MapPath(FileName)) Then
Throw New GetBinaryFileException(FileName,
GetBinaryFileException.GetBinaryFileError.FileNotFound)
End If
Dim FileData() As Byte = Nothing
Dim FStream As New FileStream(Server.MapPath(FileName),
FileMode.Open, FileAccess.Read, FileShare.ReadWrite)
If Compress Then
Dim TempData(FStream.Length - 1) As Byte
FStream.Read(TempData, 0, FStream.Length)
Dim MStream As New MemoryStream
Dim Compressor As New GZipStream(MStream,
CompressionMode.Compress, True)
Compressor.Write(TempData, 0, TempData.Length)
ReDim FileData(MStream.Length - 1)
Dim BytesRead As Integer = MStream.Read(FileData, 0,
MStream.Length)
MStream.Close()
MStream.Dispose()
Compressor.Close()
Compressor.Dispose()
Else
ReDim FileData(FStream.Length - 1)
FStream.Read(FileData, 0, FStream.Length)
End If
FStream.Close()
FStream.Dispose()
Dim Base64 As String = Convert.ToBase64String(FileData)
Dim FileDataNode As XmlNode =
XmlExchangeLib.GetOrSetXmlNode("FileData", Root)
XmlExchangeLib.AddAttributeWithValue(FileDataNode,
"Compressed", Compress.ToString().ToLower())
FileDataNode.InnerText = Base64
XmlResponse.Save(Response.OutputStream)
It took a little while, but I was able to lookup my own use of a GZip stream.
I believe your problem stems from the state of MStream after you have filled it
with the compressed data. Unless you do MStream.Position = 0, it will not be at
the start of the stream, so your MStream.Read(FileData, ... will not put any
data into FileData, and FileData will therefore stay all 0's.
You could just do MStream.Position = 0 before reading it, and that would
probably fix it, but you could also just do FileData = MStream.GetBuffer, which
seems like a more straightforward way to get the bytes.
In my testing, I found that if the result was less than 4K, the zip was no good.
I had to test for the size, and redo it with MStream.Capacity = 4096, to get a
minimum size that would produce a useable zip version.
.
- References:
- GZip Compression :(
- From: Carlo Razzeto
- GZip Compression :(
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