public PlaylistsApiController(IAPIUserService _userService, UserManager <User> _userManager,
                               IPlaylistService _playlistService, IGeneratePlaylistService _generateService,
                               IPixaBayImageService _imageService)
 {
     this.playlistService = _playlistService;
     this.generateService = _generateService;
     this.imageService    = _imageService;
     this.userService     = _userService;
     this.userManager     = _userManager;
 }
        private readonly UserManager <User> userManager; //?? which class

        public UsersApiController(IAPIUserService _userService, UserManager <User> _userManager)
        {
            this.userService = _userService;
            this.userManager = _userManager;
        }