public void Add(Person friend)
 {
      _friends.Add(friend);
 }
 public void Add(Song song,Person friend)
 {
     _friends.FirstOrDefault(thisFriend => thisFriend.Id ==friend.Id).Songs.Add(song);
 }
 public InMamoryUserDataRepository()
 {
     SelectedSong = new Song();
     SelectedPerson = new Person();
 }