TakeTopN

    public static class LinqExtensions
    {
        public static IEnumerable<T> TakeTopN<T, TKey>(this IEnumerable<T> list, int n, Func<T, TKey> keySelector, bool ascending = true) where TKey : IComparable<TKey>
        {
            IComparer<TKey> comparer = Comparer<TKey>.Default;
            if (ascending == false) comparer = new ReverseComparer<TKey>(comparer);

            List<T> values = new List<T>(n + 1);
            List<TKey> keys = new List<TKey>(n + 1);

            TKey max = keySelector(list.First());

            foreach (var item in list)
            {
                var key = keySelector(item);

                if (keys.Count < n)
                {
                    int index = FindIndex<TKey>(keys, key, comparer);
                    keys.Insert(index, key);
                    values.Insert(index, item);
                    max = keySelector(values[values.Count - 1]);
                    continue;
                }

                if (comparer.Compare(key,max) < 0)
                {
                    int index = FindIndex<TKey>(keys, key, comparer);
                    keys.Insert(index, key);
                    values.Insert(index, item);
                    if (keys.Count > n)
                    {
                        keys.RemoveAt(n);
                        values.RemoveAt(n);
                    }
                    max = keySelector(values[values.Count - 1]);
                }
            }

            return values;
        }

        //Needed for stable sort...
        private static int FindIndex<TKey>(List<TKey> keys, TKey key, IComparer<TKey> comparer) where TKey : IComparable<TKey>
        {
            int index = keys.BinarySearch(key, comparer);
            if (index < 0) index = ~index;
            while (index < keys.Count && comparer.Compare(keys[index],key) == 0) index++;
            return index;
        }

        class ReverseComparer<T> : IComparer<T>
        {
            IComparer<T> _Comparer;

            public ReverseComparer(IComparer<T> comparer)
            {
                _Comparer = comparer;
            }

            public int Compare(T x, T y)
            {
                return _Comparer.Compare(y, x);
            }
        }
    }