How do I use GZipStream with System.IO.MemoryStream?

The way to compress and decompress to and from a MemoryStream is:

public static Stream Compress(
    Stream decompressed, 
    CompressionLevel compressionLevel = CompressionLevel.Fastest)
{
    var compressed = new MemoryStream();
    using (var zip = new GZipStream(compressed, compressionLevel, true))
    {
        decompressed.CopyTo(zip);
    }

    compressed.Seek(0, SeekOrigin.Begin);
    return compressed;
}

public static Stream Decompress(Stream compressed)
{
    var decompressed = new MemoryStream();
    using (var zip = new GZipStream(compressed, CompressionMode.Decompress, true))
    {
        zip.CopyTo(decompressed);
    }

    decompressed.Seek(0, SeekOrigin.Begin);
    return decompressed;
}

This leaves the compressed / decompressed stream open and as such usable after creating it.

What happens in your code is that you keep opening streams, but you never close them.

• In line 2, you create a GZipStream. This stream will not write anything to the underlying stream until it feels it’s the right time. You can tell it to by closing it.

• However, if you close it, it will close the underlying stream (outStream) too. Therefore you can’t use mStream.Position = 0 on it.

You should always use using to ensure that all your streams get closed. Here is a variation on your code that works.

var inputString = "“ ... ”";
byte[] compressed;
string output;

using (var outStream = new MemoryStream())
{
    using (var tinyStream = new GZipStream(outStream, CompressionMode.Compress))
    using (var mStream = new MemoryStream(Encoding.UTF8.GetBytes(inputString)))
        mStream.CopyTo(tinyStream);

    compressed = outStream.ToArray();
}

// “compressed” now contains the compressed string.
// Also, all the streams are closed and the above is a self-contained operation.

using (var inStream = new MemoryStream(compressed))
using (var bigStream = new GZipStream(inStream, CompressionMode.Decompress))
using (var bigStreamOut = new MemoryStream())
{
    bigStream.CopyTo(bigStreamOut);
    output = Encoding.UTF8.GetString(bigStreamOut.ToArray());
}

// “output” now contains the uncompressed string.
Console.WriteLine(output);

This is a known issue: http://blogs.msdn.com/b/bclteam/archive/2006/05/10/592551.aspx

I have changed your code a bit so this one works:

var mStream = new MemoryStream(new byte[100]);
var outStream = new System.IO.MemoryStream();

using (var tinyStream = new GZipStream(outStream, CompressionMode.Compress))
{
    mStream.CopyTo(tinyStream);           
}

byte[] bb = outStream.ToArray();

//Decompress                
var bigStream = new GZipStream(new MemoryStream(bb), CompressionMode.Decompress);
var bigStreamOut = new System.IO.MemoryStream();
bigStream.CopyTo(bigStreamOut);