A set of tests proposal for Gilded Rose Kata.

GildedRoseTest.cs

using System;
using System.Collections.Generic;
using Gilded_rose;
using Xunit;

namespace KataGildedRose.Tests
{
    public class GildedRoseTest
    {
        string BACKSTAGE_NAME = "Backstage passes to a TAFKAL80ETC concert";
        string NORMAL_ITEM_NAME = "foo";
        string AGED_BRIE_NAME = "Aged Brie";
        string SULFURAS_NAME = "Sulfuras, Hand of Ragnaros";

        int QUALITY_CHANGE = 1;
        int NORMAL_ITEM_QUALITY = 5;
        int MINIMUM_QUALITY = 0;
        int MAXIMUM_QUALITY = 50;

        int SELL_IN_CHANGE = 1;
        int EXPIRED_SELL_IN = 0;
        int NORMAL_ITEM_SELL_IN = 15;
        int SELL_IN_IN_5_DAYS = 5;
        int SELL_IN_IN_10_DAYS = 10;

        [Fact]
        public void sell_in_decreases_every_day()
        {
            VerifySellIn(NORMAL_ITEM_SELL_IN - SELL_IN_CHANGE, NormalItem());
        }

        [Fact]
        public void quality_decreases_every_day()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY - QUALITY_CHANGE, NormalItem());
        }

        [Fact]
        public void quality_decreases_twice_as_fast_one_sell_in_has_passed()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY - 2 * QUALITY_CHANGE, ExpiredItem());
        }

        [Fact]
        public void quality_is_never_negative()
        {
            VerifyQuality(MINIMUM_QUALITY, ItemWithoutQuality());
        }

        [Fact]
        public void aged_brie_increases_quality()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY + QUALITY_CHANGE, AgedBrie());
        }

        [Fact]
        public void item_never_increase_quality_when_has_reached_the_maximum()
        {
            VerifyQuality(MAXIMUM_QUALITY, AgedBrieWithMaximumQuality());
        }

        [Fact]
        public void sulfuras_never_changes_the_quality()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY, Sulfuras());
        }

        [Fact]
        public void sulfuras_never_changes_the_sell_in()
        {
            VerifySellIn(NORMAL_ITEM_SELL_IN, Sulfuras());
        }

        [Fact]
        public void backstage_increase_quality()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY + QUALITY_CHANGE, Backstage());
        }

        [Fact]
        public void backstage_increase_quality_by_2_when_sell_in_is_10_or_less()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY + 2 * QUALITY_CHANGE, BackstageInIn10Days());
        }

        [Fact]
        public void backstage_increase_quality_by_3_when_sell_in_is_5_or_less()
        {
            VerifyQuality(NORMAL_ITEM_QUALITY + 3 * QUALITY_CHANGE, BackstageIn5Days());
        }

        [Fact]
        public void backstage_drops_to_0_quality_after_the_concert()
        {
            VerifyQuality(MINIMUM_QUALITY, BackstageAfterTheConcert());
        }

        [Fact]
        public void aged_brie_expired_gets_the_maximum_quality_when_has_the_quality_minus_1()
        {
            Item item = new Item {Name = AGED_BRIE_NAME, SellIn = EXPIRED_SELL_IN, Quality = MAXIMUM_QUALITY - 1};
            VerifyQuality(MAXIMUM_QUALITY, item);
        }

        private void VerifySellIn(int expectedSellIn, Item item)
        {
            GildedRose gildedRose = BuildGildedRose(item);
            gildedRose.UpdateQuality();
            AssertSellIn(expectedSellIn, gildedRose);
        }

        private void VerifyQuality(int expectedQuality, Item normalItem)
        {
            GildedRose gildedRose = BuildGildedRose(normalItem);
            gildedRose.UpdateQuality();
            AssertQuality(expectedQuality, gildedRose);
        }

        GildedRose BuildGildedRose(Item item)
        {
//        IList<Item> Items = new List<Item> {
            IList<Item> items = new List<Item> {item};
            GildedRose app = new GildedRose(items);
            return app;
        }

        private Item NormalItem()
        {
            return ItemWithNormalSellInAndQuality(NORMAL_ITEM_NAME);
        }

        private Item ExpiredItem()
        {
            return ItemWithNormalQuality(NORMAL_ITEM_NAME, EXPIRED_SELL_IN);
        }

        private Item ItemWithoutQuality()
        {
            return new Item {Name = NORMAL_ITEM_NAME, SellIn = NORMAL_ITEM_SELL_IN, Quality = MINIMUM_QUALITY};
        }

        private Item AgedBrie()
        {
            return ItemWithNormalSellInAndQuality(AGED_BRIE_NAME);
        }

        private Item AgedBrieWithMaximumQuality()
        {
            return new Item {Name = AGED_BRIE_NAME, SellIn = NORMAL_ITEM_SELL_IN, Quality = MAXIMUM_QUALITY};
        }

        private Item Sulfuras()
        {
            return ItemWithNormalSellInAndQuality(SULFURAS_NAME);
        }

        private Item Backstage()
        {
            return ItemWithNormalSellInAndQuality(BACKSTAGE_NAME);
        }

        private Item BackstageInIn10Days()
        {
            return ItemWithNormalQuality(BACKSTAGE_NAME, SELL_IN_IN_10_DAYS);
        }

        private Item BackstageIn5Days()
        {
            return ItemWithNormalQuality(BACKSTAGE_NAME, SELL_IN_IN_5_DAYS);
        }

        private Item BackstageAfterTheConcert()
        {
            return ItemWithNormalQuality(BACKSTAGE_NAME, EXPIRED_SELL_IN);
        }

        private Item ItemWithNormalSellInAndQuality(String name)
        {
            return ItemWithNormalQuality(name, NORMAL_ITEM_SELL_IN);
        }

        private Item ItemWithNormalQuality(String name, int sellIn)
        {
            return new Item {Name = name, SellIn = sellIn, Quality = NORMAL_ITEM_QUALITY};
        }

        private static void AssertSellIn(int expectedSellIn, GildedRose app)
        {
            Assert.Equal(expectedSellIn, app.Items[0].SellIn);
        }

        private static void AssertQuality(int expectedQuality, GildedRose gildedRose)
        {
            Assert.Equal(expectedQuality, gildedRose.Items[0].Quality);
        }
    }
}