GZipStream And DeflateStream will not decompress a

Question

I was in need of a way to compress images in .net so i looked into using the .net GZipStream class (or DeflateStream). However i found that decompression was not always successful, sometimes the images would decompress fine and other times i would get a GDI+ error that something is corrupted.

After investigating the issue i found that the decompression was not giving back all the bytes it compressed. So if i compressed 2257974 bytes i would sometimes get back only 2257870 bytes (real numbers).

The most funny thing is that sometimes it would work. So i created this little test method that compresses only 10 bytes and now i don't get back anything at all.

I tried it with both compression classes GZipStream and DeflateStream and i double checked my code for possible errors. I even tried positioning the stream to 0 and flushing all the streams but with no luck.

Here is my code:

    public static void TestCompression()
    {
        byte[] test = new byte[] { 0, 1, 2, 3, 4, 5, 6, 7, 8, 9 };

        byte[] result = Decompress(Compress(test));

        // This will fail, result.Length is 0
        Debug.Assert(result.Length == test.Length);
    }

    public static byte[] Compress(byte[] data)
    {
        var compressedStream = new MemoryStream();
        var zipStream = new GZipStream(compressedStream, CompressionMode.Compress);
        zipStream.Write(data, 0, data.Length);
        return compressedStream.ToArray();
    }

    public static byte[] Decompress(byte[] data)
    {
        var compressedStream = new MemoryStream(data);
        var zipStream = new GZipStream(compressedStream, CompressionMode.Decompress);
        var resultStream = new MemoryStream();

        var buffer = new byte[4096];
        int read;

        while ((read = zipStream.Read(buffer, 0, buffer.Length)) > 0) {
            resultStream.Write(buffer, 0, read);
        }

        return resultStream.ToArray();
    }

Answer 1

You need to Close() the ZipStream after adding all the data you want to compress; it retains a buffer of unwritten bytes internally (even if you Flush()) that needs to be written.

More generally, Stream is IDisposable, so you should also be using each... (yes, I know that MemoryStream isn't going to lose any data, but if you don't get into this habit, it will bite you with other Streams).

public static byte[] Compress(byte[] data)
{
    using (var compressedStream = new MemoryStream())
    using (var zipStream = new GZipStream(compressedStream, CompressionMode.Compress))
    {
        zipStream.Write(data, 0, data.Length);
        zipStream.Close();
        return compressedStream.ToArray();
    }
}

public static byte[] Decompress(byte[] data)
{
    using(var compressedStream = new MemoryStream(data))
    using(var zipStream = new GZipStream(compressedStream, CompressionMode.Decompress))
    using (var resultStream = new MemoryStream())
    { ... }
}

[edit : updated re comment] Re not using things like MemoryStream - this is always a fun one, with lots of votes on either side of the fence: but ultimatey...

(rhetorical - we all know the answer...) How is MemoryStream implemented? is it a byte[] (owned by .NET)? is it a memory-mapped file (owned by the OS)?

The reason you aren't using it is because you are letting knowledge of internal implementation details change how you code against a public API - i.e. you just broke the laws of encapsulation. The public API says: I am IDisposable; you "own" me; therefore, it is your job to Dispose() me when you are through.

Answer 2

Also - keep in mind the DeflateStream in System.IO.Compression does not implement the most efficient deflate algorithm. If you like, there is an alternative to the BCL GZipStream and DeflateStream; it is implemented in a fully-managed library based on zlib code, that performs better than the built-in {Deflate,GZip}Stream in this respect. [ But you still need to Close() the stream to get the full bytestream. ]

These stream classes are shipped in the DotNetZlib assembly, available in the DotNetZip distribution at http://DotNetZip.codeplex.com/.