Simple BDD {Based on a 2009 blog post from Garry Shutler]

BehaviourTest.cs

namespace TestsXunit.Framework;

public abstract class BehaviourTest
{
    protected void Setup()
    {
        Given();
        When();
    }

    protected abstract void Given();
    protected abstract void When();
}

RecentlyUsedList.cs

namespace BDDTestConsole;

public class RecentlyUsedList<T>
{
    private readonly List<T> _data;
    private readonly int _capacity;

    public RecentlyUsedList(int capacity)
    {
        _data = new List<T>();
        _capacity = capacity;
    }

    public void Add(T item)
    {
        _data.Remove(item);
        if (_data.Count == _capacity)
        {
            _data.RemoveAt(0); 
        }
        _data.Add(item);
    }

    public IReadOnlyList<T> Items => _data.AsReadOnly();
}

WhenAddingAnItemToANewList.cs

using BDDTestConsole;
using TestsXunit.Framework;

namespace TestsXunit.RecentlyUsedList;

public class WhenAddingAnItemToANewList : BehaviourTest
{
    private const string AddedItem = "Item1";
    private RecentlyUsedList<string> _sut;

    public WhenAddingAnItemToANewList()
    {
        Setup();
    }
    
    [Fact]
    public void ShouldContainOneItem()
    {
        Assert.Equal(1, _sut.Items.Count);
    }
    
    [Fact]
    public void ShouldContainExpectedItem()
    {
        var expected = new List<string> { AddedItem };
        Assert.Equal(expected, _sut.Items);
    }

    protected override void Given()
    {
        _sut = new RecentlyUsedList<string>(3);
    }

    protected override void When()
    {
        _sut.Add(AddedItem);
    }
}

WhenCreatingANewList.cs

using TestsXunit.Framework;

namespace TestsXunit.RecentlyUsedList;

public class WhenCreatingANewList : BehaviourTest
{
    private RecentlyUsedList<string> _sut;
    
    public WhenCreatingANewList()
    {
        Setup();
    }
    
    [Fact]
    public void ShouldContainNoItems()
    {
        Assert.Equal(0, _sut.Items.Count);
    }

    protected override void Given()
    {
        _sut = new RecentlyUsedList<string>(3);
    }

    protected override void When()
    {
        // Nothing to do
    }
}