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

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);