private void read_xml_file(string filename)
        {
            XmlTextReader xml_file_reader = new XmlTextReader(filename);
            
            while (xml_file_reader.ReadToFollowing("album"))
            {
                XmlReader subtree_reader = xml_file_reader.ReadSubtree();
                Album new_album = new Album();
                while (subtree_reader.Read()) {
                    if (subtree_reader.Name == "title")
                    {
                        new_album.Title = subtree_reader.ReadString();
                    }
                    else if (subtree_reader.Name == "artist")
                    {
                        new_album.Artist = subtree_reader.ReadString();
                    }
                    else if (subtree_reader.Name == "track")
                    {
                        new_album.AddSong(subtree_reader.GetAttribute(0) + " - " + subtree_reader.ReadString());
                    }
                }
                _albums.Add(new_album);
                subtree_reader.Close();
            }
            xml_file_reader.Close();
        }