Win8 Metro App GZipStream 用不明白了有高手否？ 压缩的数据无法解压

public static string CompressGzip(string st)
{
List<byte> li = new List<byte>();

using (MemoryStream msf = new MemoryStream(Encoding.UTF8.GetBytes(st)))
{
using (MemoryStream mst = new MemoryStream())
{
using (GZipStream zip = new GZipStream(mst, CompressionMode.Compress))
{
msf.CopyTo(zip);
//return string.Join(",", mst.ToArray());
li.AddRange(mst.ToArray());
}
}
}

using (MemoryStream msf = new MemoryStream(li.ToArray()))
{
using (MemoryStream mst = new MemoryStream())
{
using (GZipStream zip = new GZipStream(msf, CompressionMode.Decompress))
{
zip.CopyTo(mst);
//byte[] byt = mst.ToArray();
//return Encoding.UTF8.GetString(byt, 0, byt.Length);
}
}
}
return "aa";
}