Linq-SelectMany

        public static IEnumerable SelectMany(this IEnumerable source, Func> selector) {
            if (source == null) throw Error.ArgumentNull("source");
            if (selector == null) throw Error.ArgumentNull("selector");
            return SelectManyIterator(source, selector);
        }
 
        static IEnumerable SelectManyIterator(IEnumerable source, Func> selector) {
            foreach (TSource element in source) {
                foreach (TResult subElement in selector(element)) {
                    yield return subElement;
                }
            }
        }
 
        public static IEnumerable SelectMany(this IEnumerable source, Funcint, IEnumerable> selector) {
            if (source == null) throw Error.ArgumentNull("source");
            if (selector == null) throw Error.ArgumentNull("selector");
            return SelectManyIterator(source, selector);
        }
 
        static IEnumerable SelectManyIterator(IEnumerable source, Funcint, IEnumerable> 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，而且使用到了yield语法糖，建议使用selectMany而不是select。