using System;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        // we have 3 items: A, B and C

        var a = new Item {Name = "A"};

        var b = new Item {Name = "B"};

        var c = new Item {Name = "C"};

        var items = new List<Item>()

        {

            a,

            b,

            c,

        };

        // add B as an alternative to A

        a.Alternatives.Add(b);

        // that means A is an alternative to B

        b.Alternatives.Add(a);

        // add C as an alternative to B

        b.Alternatives.Add(c);

        // that means B is an alternative to C

        c.Alternatives.Add(b);

        // and now the complication: it means that C is an alternative to A too, because A is alternative to B

        a.Alternatives.Add(c);

        // and it means A is an alternative to C too

        c.Alternatives.Add(a);

​

        // let's print the alternatives

        foreach(var item in items)

        {

            Console.Write($"Item {item.Name} has the alternatives: ");

            foreach(var alternative in item.Alternatives)

            {

                Console.Write($"{alternative.Name} ");

            }

            Console.WriteLine();

        }

        // then trying to add numerous alternatives... would make all thing way to complicated

    }

}

​

public class Item 

{

  public string Name {get; set;}

  public List<Item> Alternatives {get; set;} = new();

}