源码：

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

在看下

        public bool Add(TElement value) {
            return !Find(value, true);
        }
 
        // Check whether value is in set
        public bool Contains(TElement value) {
            return Find(value, false);
        }        bool Find(TElement value, bool add) {
            int hashCode = InternalGetHashCode(value);
            for (int i = buckets[hashCode % buckets.Length] - 1; i >= 0; i = slots[i].next) {
                if (slots[i].hashCode == hashCode && comparer.Equals(slots[i].value, value)) return true;
            }
            if (add) {
                int index;
                if (freeList >= 0) {
                    index = freeList;
                    freeList = slots[index].next;
                }
                else {
                    if (count == slots.Length) Resize();
                    index = count;
                    count++;
                }
                int bucket = hashCode % buckets.Length;
                slots[index].hashCode = hashCode;
                slots[index].value = value;
                slots[index].next = buckets[bucket] - 1;
                buckets[bucket] = index + 1;
            }
            return false;
        }