TDD Gilded Rose Kata test solution in C# without add changes to the Item

GildedRoseTest.cs

using NUnit.Framework;

namespace Gilded_rose.Test;

public class GildRoseTest
{
    private const int MinQuality = 0;
    private const int MaxQuality = 50;
    
    [Test]
    public void regular_items_quality_decreases_every_day()
    {
        Item item = RegularItem(1, 3);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(0));
        Assert.That(item.Quality, Is.EqualTo(2));
    }

    [Test]
    public void expired_regular_items_quality_decreases_twice_as_fast_once_sellIn_has_passed()
    {
        Item item = RegularItem(0, 3);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(1));
    }

    [Test]
    public void regular_items_quality_never_decreases_below_the_minimum_quality()
    {
        Item item = RegularItem(5, 0);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(4));
        Assert.That(item.Quality, Is.EqualTo(MinQuality));
    }

    [Test]
    public void expired_regular_items_quality_never_decreases_below_the_minimum_quality()
    {
        Item item = RegularItem(0, 1);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(MinQuality));
    }

    [Test]
    public void aged_brie_increases_quality_by_1()
    {
        Item item = AgedBrie(5, 8);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(4));
        Assert.That(item.Quality, Is.EqualTo(9));
    }

    [Test]
    public void aged_brie_quality_never_increases_over_the_maximun_quality()
    {
        Item item = AgedBrie(5, MaxQuality);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(4));
        Assert.That(item.Quality, Is.EqualTo(MaxQuality));
    }

    [Test]
    public void expired_aged_brie_quality_never_increases_over_the_maximun_quality()
    {
        Item item = AgedBrie(0, MaxQuality - 1);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(MaxQuality));
    }

    [Test]
    public void expired_aged_brie_increases_quality_by_2()
    {
        Item item = AgedBrie(0, 8);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(10));
    }

    [Test]
    public void sulfuras_never_changes()
    {
        Item item = Sulfuras(1, 80);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(1));
        Assert.That(item.Quality, Is.EqualTo(80));
    }

    [Test]
    public void expired_sulfuras_never_changes()
    {
        Item item = Sulfuras(-1, 80);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(80));
    }

    [Test]
    public void backstage_increase_quality_by_1_when_sellIn_is_greater_than_10()
    {
        Item item = BackstagePasses(11, 6);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(10));
        Assert.That(item.Quality, Is.EqualTo(7));
    }

    [Test]
    public void backstage_increase_quality_by_2_when_sellIn_is_between_10_and_6()
    {
        Item item1 = BackstagePasses(10, 6);
        Item item2 = BackstagePasses(6, 20);
        GildedRose shop = new GildedRose(new[] { item1, item2 });

        shop.UpdateQuality();

        Assert.That(item1.SellIn, Is.EqualTo(9));
        Assert.That(item1.Quality, Is.EqualTo(8));
        Assert.That(item2.SellIn, Is.EqualTo(5));
        Assert.That(item2.Quality, Is.EqualTo(22));
    }

    [Test]
    public void backstage_increase_quality_by_3_when_sellIn_is_between_5_and_1()
    {
        Item item1 = BackstagePasses(5, 6);
        Item item2 = BackstagePasses(1, 20);
        GildedRose shop = new GildedRose(new[] { item1, item2 });

        shop.UpdateQuality();

        Assert.That(item1.SellIn, Is.EqualTo(4));
        Assert.That(item1.Quality, Is.EqualTo(9));
        Assert.That(item2.SellIn, Is.EqualTo(0));
        Assert.That(item2.Quality, Is.EqualTo(23));
    }

    [Test]
    public void backstage_quality_drops_to_0_after_the_concert()
    {
        Item item = BackstagePasses(0, 6);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(-1));
        Assert.That(item.Quality, Is.EqualTo(MinQuality));
    }

    [Test]
    public void backstage_quality_never_increases_over_the_maximun_quality_when_increasing_by_1()
    {
        Item item = BackstagePasses(11, MaxQuality);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(10));
        Assert.That(item.Quality, Is.EqualTo(MaxQuality));
    }

    [Test]
    public void backstage_quality_never_increases_over_the_maximun_quality_when_increasing_by_2()
    {
        Item item = BackstagePasses(8, MaxQuality - 1);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(7));
        Assert.That(item.Quality, Is.EqualTo(MaxQuality));
    }

    [Test]
    public void backstage_quality_never_increases_over_the_maximun_quality_when_increasing_by_3()
    {
        Item item = BackstagePasses(4, MaxQuality - 2);
        GildedRose shop = new GildedRose(new[] { item });

        shop.UpdateQuality();

        Assert.That(item.SellIn, Is.EqualTo(3));
        Assert.That(item.Quality, Is.EqualTo(MaxQuality));
    }
    
    [Test]
    public void update_several_items()
    {
        Item item1 = RegularItem(1, 3);
        Item item2 = AgedBrie(2, 3);
        GildedRose shop = new GildedRose(new[] { item1, item2 });

        shop.UpdateQuality();

        Assert.That(item1.SellIn, Is.EqualTo(0));
        Assert.That(item1.Quality, Is.EqualTo(2));
        Assert.That(item2.SellIn, Is.EqualTo(1));
        Assert.That(item2.Quality, Is.EqualTo(4));
    }

    private Item RegularItem(int sellIn, int quality)
    {
        return new Item("foo", sellIn, quality);
    }

    private Item AgedBrie(int sellIn, int quality)
    {
        return new Item("Aged Brie", sellIn, quality);
    }

    private Item Sulfuras(int sellIn, int quality)
    {
        return new Item("Sulfuras, Hand of Ragnaros", sellIn, quality);
    }

    private Item BackstagePasses(int sellIn, int quality)
    {
        return new Item("Backstage passes to a TAFKAL80ETC concert", sellIn, quality);
    }
}