Functional c# Gilded Rose

GuildedRose.cs

using System;
using System.Collections.Generic;

namespace GildedRose
{
    public class GildedRose
    {
        private static readonly Dictionary<string, Action<Item>> Products = new Dictionary<string, Action<Item>>
            {
                {"Aged Brie", Product.Adjust(Adjusters.QualityAdjustedBy(1))},
                {"Backstage passes to a TAFKAL80ETC concert", Product.Adjust(Adjusters.Backstage)},
                {"Sulfuras, Hand of Ragnaros", Product.NoAdjustment},
                {"Conjured Mana Cake", Product.Adjust(Adjusters.QualityAdjustedBy(-2))},
                {"NORMAL ITEM", Product.Adjust(Adjusters.QualityAdjustedBy(-1))}
            };

        public static void UpdateQuality(List<Item> items)
        {
            items.ForEach(q => Products[q.Name](q));
        }
    }

    public class Product
    {
        public static Action<Item> Adjust(Func<Item, int> adjustment)
        {
            return item => {
                               item.SellIn = item.SellIn - 1;
                               item.Quality = item.Quality + adjustment(item);
                               item.Quality = Math.Max(item.Quality, 0);
                               item.Quality = Math.Min(item.Quality, 50);
            };
        }

        public static Action<Item> NoAdjustment = item => {};
    }

    public class Adjusters
    {
        public static int Backstage(Item item)
        {
            if (item.SellIn < 0)
                return -item.Quality;
            if (item.SellIn < 5)
                return +3;
            if (item.SellIn < 10)
                return +2;
            return +1;
        }

        public static Func<Item, int> QualityAdjustedBy(int adjustment)
        {
            return item => item.SellIn < 0 ? adjustment * 2 : adjustment;
        }
    }
}