The following example will return the byte[] data of a zipped file containing the files provided to it, without needing access to the file system.

public static byte[] ZipFiles(Dictionary<string, byte[]> files)
{
    using (MemoryStream ms = new MemoryStream())
    {
        using (ZipArchive archive = new ZipArchive(ms, ZipArchiveMode.Update))
        {
            foreach (var file in files)
            {
                ZipArchiveEntry orderEntry = archive.CreateEntry(file.Key); //create a file with this name
                using (BinaryWriter writer = new BinaryWriter(orderEntry.Open()))
                {
                    writer.Write(file.Value); //write the binary data
                }
            }
        }
        //ZipArchive must be disposed before the MemoryStream has data
        return ms.ToArray();
    }
}