using System;

using System.Text;

using System.IO;

using System.IO.Compression;

public class Program

{

    public static void Main()

    {

        string input = "läölå";

        Console.WriteLine(Decompress(Compress(input)));

    }

    public static byte[] Compress(string s)

        {

            var bytes = Encoding.Unicode.GetBytes(s);

            using (var msi = new MemoryStream(bytes))

            using (var mso = new MemoryStream())

            {

                using (var gs = new GZipStream(mso, CompressionMode.Compress))

                {

                    msi.CopyTo(gs);

                }

                return mso.ToArray();

            }

        }

​

        public static string Decompress(byte[] s)

        {

            using (var msi = new MemoryStream(s))

            using (var mso = new MemoryStream())

            {

                using (var gs = new GZipStream(msi, CompressionMode.Decompress))

                {

                    gs.CopyTo(mso);

                }

                return Encoding.Unicode.GetString(mso.ToArray());

            }

        }

}