源码：

        public static IEnumerable<TSource> Except<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second)
        {
            if (first == null) throw Error.ArgumentNull("first");
            if (second == null) throw Error.ArgumentNull("second");
            return ExceptIterator<TSource>(first, second, null);
        }
 
        public static IEnumerable<TSource> Except<TSource>(this IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer)
        {
            if (first == null) throw Error.ArgumentNull("first");
            if (second == null) throw Error.ArgumentNull("second");
            return ExceptIterator<TSource>(first, second, comparer);
        }
 
        static IEnumerable<TSource> ExceptIterator<TSource>(IEnumerable<TSource> first, IEnumerable<TSource> second, IEqualityComparer<TSource> comparer) {
            Set<TSource> set = new Set<TSource>(comparer);
            foreach (TSource element in second) set.Add(element);
            foreach (TSource element in first)
                if (set.Add(element)) yield return element;
        }