public static RecommenderResult RecommendUser(
            this IMatrixFactorizationRecommender recommender,
            IEnumerable <string> items)
        {
            if (recommender == null)
            {
                throw new ArgumentNullException(nameof(recommender));
            }

            if (items == null)
            {
                throw new ArgumentNullException(nameof(items));
            }

            return(recommender.RecommendUser(recommender.ComputeUserFactors(items)));
        }
        public static RecommenderResult RankUsers(
            this IMatrixFactorizationRecommender recommender,
            string userId,
            IEnumerable <KeyValuePair <string, IEnumerable <string> > > userItems)
        {
            if (recommender == null)
            {
                throw new ArgumentNullException(nameof(recommender));
            }

            if (userId == null)
            {
                throw new ArgumentNullException(nameof(userId));
            }

            if (userItems == null)
            {
                throw new ArgumentNullException(nameof(userItems));
            }

            return(recommender.RankUsers(userId, userItems.Select(o => new KeyValuePair <string, UserFeatures>(o.Key, recommender.ComputeUserFactors(o.Value))).ToList()));
        }