Problems decompressing a gzip archive in .NET

Question

I grabbed the following code somewhere off the Internet, and I am using it to decompress gzip files, such as http://wwwmaster.postgresql.org/download/mirrors-ftp/pgadmin3/release/v1.8.4/src/pgadmin3-1.8.4.tar.gz, but when I run It, I get an exception, stating that the magic number doesn't match.

public byte[] Download(string pUrl) {
    WebClient wc = new WebClient();
    byte[] bytes = wc.DownloadData(pUrl);
    return UnGzip(bytes, 0);
}

private static byte[] UnGzip(byte[] data, int start) {
    int size = BitConverter.ToInt32(data, data.Length - 4);
    byte[] uncompressedData = new byte[size];
    MemoryStream memStream = new MemoryStream(data, start, (data.Length - start));
    memStream.Position = 0;
    GZipStream gzStream = new GZipStream(memStream, CompressionMode.Decompress);

    try {
        gzStream.Read(uncompressedData, 0, size);
    } catch (Exception gzError) {
        throw;
    }

    gzStream.Close();
    return uncompressedData;
}

What's wrong with the code that would cause this problem?

Answer 1

The problem is that the URL you specified in your question doesn't actually give a gzip file. It takes the browser to a page where you select a mirror.

If you temporarily change your Download method to use:

string text = wc.DownloadString(pUrl);
Console.WriteLine(text);

you'll see all the HTML for mirror selection.

If you use a URL which is the actual gz file, e.g. http://wwwmaster.postgresql.org/redir/170/h/pgadmin3/release/v1.8.4/src/pgadmin3-1.8.4.tar.gz then it works fine.

Answer 2

I couldn't get the GZipStream to read the file you linked, but it seems to decompress other GZip files just fine. For example:

ftp://gnu.mirror.iweb.com/gnu/bash/bash-1.14.0-1.14.1.diff.gz

ftp://gnu.mirror.iweb.com/gnu/emacs/elisp-manual-21-2.8.tar.gz

Perhaps the file you linked is corrupt? Or maybe it uses a non-standard or new GZip format.

Answer 3

I've used DotNetZip with some success with .zip files. According to the docs it also works with GZip. You might give this guy's library a try.

Answer 4

.NET does not support the tar format. gzip is simply a byte compressor. Tar is the container format.

Formats like ZIP and RAR, combines both these steps for you, but they are not good for streaming compression.

If I remember correctly, SharpZipLib has support for tar.