yield bug.cs

yield bug.cs

using System;
using System.Collections.Generic;
using System.Linq;

public class Program
{
    public static void Main()
    {
        Console.WriteLine(MyLinqTest.LinqTest.Test());
    }
}

namespace MyLinqTest
{
    public class Slot
    {
        public readonly string Id;
        public Item ContainedItem;

        public Slot(string id)
        {
            Id = id;
        }

        public IEnumerable<Item> Items => ContainedItem == null ? new Item[0] : new[] {ContainedItem};
    }

    public class Item
    {
        public readonly Slot[] Slots;

        public Item(Slot[] slots)
        {
            Slots = slots;
        }
    }

    public static class LinqExtensions
    {
        public static IEnumerable<T> Flatten<T>(this IEnumerable<T> e, Func<T, IEnumerable<T>> f)
        {
            return e.Concat(e.SelectMany(c => f(c).Flatten(f)));
        }
    } 

    public static class LinqTest
    {
        static IEnumerable<Slot> GetSlots()
        {
            yield return new Slot("a");
            yield return new Slot("b");
            yield return new Slot("c");
            yield return new Slot("d");
            yield return new Slot("e");
            yield return new Slot("f");
            yield return new Slot("g") {
                ContainedItem = new Item(new[] {
                    new Slot("x"),
                    new Slot("y")
                })
            };
        }
        
        public static string Test()
        {
            var slots = GetSlots().Concat(new[] { new Slot("j") });
            
            var containers = slots
                .Flatten(container =>
                    container.Items.SelectMany(x => x.Slots)
                ).ToArray();

            return string.Join(", ", containers.Select(x => x == null ? "NULL" : x.Id).ToArray());
        }
    }
}