GZipStream machine dependence

Question

I'm running into some strange machine/OS dependent GZipStream behavior in .NET 4.0. This is the relevant code:

public static string Compress(string input) {
    using(var ms = new MemoryStream(Encoding.UTF8.GetBytes(input)))
    using(var os = new MemoryStream()) {
        using(var gz = new GZipStream(os,CompressionMode.Compress,true)) {
            ms.CopyTo(gz);
        }
        return string.Join("",os.ToArray().Select(b=>b.ToString("X2")));
    }
}

Running Compress("freek") gives me

1F8B08000000000004004B2B4A4DCD06001E33909D05000000

on Windows 7 and

1F8B0800000000000400ECBD07601C499625262F6DCA7B7F4AF54AD7E074A10880601324D8904010ECC188CDE692EC1D69472329AB2A81CA6556655D661640CCED9DBCF7DE7BEFBDF7DE7BEFBDF7BA3B9D4E27F7DFFF3F5C6664016CF6CE4ADAC99E2180AAC81F3F7E7C1F3F22CEEB3C7FFBFF040000FFFF1E33909D05000000

on Windows Server 2008R2. Both are 64bit. I would expect the results to be the same.

Both machines give the correct result when I decompress either result. I already found out that on W7 ms.Length == 25 while on W2K8R2 ms.Length==128, but no clue why.

What's going on?

Answer 1

It was announced that .NET 4.5 Beta includes zip compression improvements to reduce the size:

Starting with the .NET Framework 4.5 RC, the DeflateStream class uses the zlib library for compression. As a result, it provides a better compression algorithm and, in most cases, a smaller compressed file than it provides in earlier versions of the .NET Framework.

Do you perhaps have .Net 4.5+ installed on the Win7 machine?