Correct way to use GZipStream in dotNET C#

Question

I'm working with GZipStream at the moment using .net 3.5. I have two methods listed below. As input file I use text file which consists of chars 's'. Size of the file is 2MB. This code works fine if I use .net 4.5 but with .net 3.5 after compress and decompress I get file of size 435KB which of course isn't the same with source file. If I try to decompress file via WinRAR it is also looks good (the same with source file). If I try decompress file using GZipStream from .net4.5 (file compressed via GZipStream from .net 3.5) the result is bad.

UPD: In general I really need to read the file as several separate gzip chunks, in this case all the bytes of copressed files are read at one call of the Read() method so I still don't understand why decompressing doesn't works.

    public void CompressFile()
    {
        string fileIn = @"D:\sin2.txt";
        string fileOut = @"D:\sin2.txt.pgz";

        using (var fout = File.Create(fileOut))
        {
            using (var fin = File.OpenRead(fileIn))
            {
                using (var zip = new GZipStream(fout, CompressionMode.Compress))
                {
                    var buffer = new byte[1024 * 1024 * 10];
                    int n = fin.Read(buffer, 0, buffer.Length);
                    zip.Write(buffer, 0, n);
                }
            }
        }
    }
    public void DecompressFile()
    {
        string fileIn = @"D:\sin2.txt.pgz";
        string fileOut = @"D:\sin2.1.txt";

        using (var fsout = File.Create(fileOut))
        {
            using (var fsIn = File.OpenRead(fileIn))
            {
                var buffer = new byte[1024 * 1024 * 10];
                int n;
                while ((n = fsIn.Read(buffer, 0, buffer.Length)) > 0)
                {
                    using (var ms = new MemoryStream(buffer, 0, n))
                    {
                        using (var zip = new GZipStream(ms, CompressionMode.Decompress))
                        {
                            int nRead = zip.Read(buffer, 0, buffer.Length);
                            fsout.Write(buffer, 0, nRead);
                        }
                    }
                }
            }
        }
    }

Answer 1

You're trying to decompress each "chunk" as if it's a separate gzip file. Don't do that - just read from the GZipStream in a loop:

using (var fsout = File.Create(fileOut))
{
    using (var fsIn = File.OpenRead(fileIn))
    {
        using (var zip = new GZipStream(fsIn, CompressionMode.Decompress))
        {
            var buffer = new byte[1024 * 32];
            int bytesRead;

            while ((bytesRead = zip.Read(buffer, 0, buffer.Length)) > 0)
            {
                fsout.Write(buffer, 0, bytesRead);
            }
        }
    }
}

Note that your compression code should look similar, reading in a loop rather than assuming a single call to Read will read all the data.

(Personally I'd skip fsIn, and just use new GZipStream(File.OpenRead(fileIn)) but that's just a personal preference.)

Answer 2

First, as @Jon Skeet mentioned, you are not using Stream.Read method correctly. It doesn't matter if your buffer is big enough or not, the stream is allowed to return less bytes than requested, with zero indicating no more, so reading from stream should always be performed in a loop.

However the main problem in your decompress code is the way you share the buffer. Your read the input into a buffer, than wrap it in a MemoryStream (note that the constructor used does not make a copy of the passed array, but actually sets it as it's internal buffer), and then you try to read and write to that buffer at the same time. Taking into account that decompressing writes data "faster" than reading, it's surprising that your code works at all.

The correct implementation is quite simple

static void CompressFile()
{
    string fileIn = @"D:\sin2.txt";
    string fileOut = @"D:\sin2.txt.pgz";
    using (var input = File.OpenRead(fileIn))
    using (var output = new GZipStream(File.Create(fileOut), CompressionMode.Compress))
        Write(input, output);
}

static void DecompressFile()
{
    string fileIn = @"D:\sin2.txt.pgz";
    string fileOut = @"D:\sin2.1.txt";
    using (var input = new GZipStream(File.OpenRead(fileIn), CompressionMode.Decompress))
    using (var output = File.Create(fileOut))
        Write(input, output);
}

static void Write(Stream input, Stream output, int bufferSize = 10 * 1024 * 1024)
{
    var buffer = new byte[bufferSize];
    for (int readCount; (readCount = input.Read(buffer, 0, buffer.Length)) > 0;)
        output.Write(buffer, 0, readCount);
}