Correct way to use GZipStream in dotNET C#

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);
                        }
                    }
                }
            }
        }
    }
Jon Skeet
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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.)

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