Linq-SelectMany

        public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) {
            if (source == null) throw Error.ArgumentNull("source");
            if (selector == null) throw Error.ArgumentNull("selector");
            return SelectManyIterator<TSource, TResult>(source, selector);
        }
 
        static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, IEnumerable<TResult>> selector) {
            foreach (TSource element in source) {
                foreach (TResult subElement in selector(element)) {
                    yield return subElement;
                }
            }
        }
 
        public static IEnumerable<TResult> SelectMany<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector) {
            if (source == null) throw Error.ArgumentNull("source");
            if (selector == null) throw Error.ArgumentNull("selector");
            return SelectManyIterator<TSource, TResult>(source, selector);
        }
 
        static IEnumerable<TResult> SelectManyIterator<TSource, TResult>(IEnumerable<TSource> source, Func<TSource, int, IEnumerable<TResult>> selector) {
            int index = -1;
            foreach (TSource element in source) {
                checked { index++; }
                foreach (TResult subElement in selector(element, index)) {
                    yield return subElement;
                }
            }
        }

返回的是，IEnumerable<TResult>，而且使用到了yield语法糖，建议使用selectMany而不是select。