using System;

using System.Collections.Generic;

using System.Linq;

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"};

        // and some extra items: D and E

        var d = new Item {Name = "D"};

        var e = new Item {Name = "E"};

        var alternativeGroup1 = new ItemGroup {Id = 1};

        var alternativeGroup2 = new ItemGroup {Id = 2};

        var items = new List<Item>()

        {

            a,

            b,

            c,

            d,

            e

        };

        // any items that are alternatives to each other go in the same alternative group

        a.AlternativeGroupId = alternativeGroup1.Id;

        b.AlternativeGroupId = alternativeGroup1.Id;

        c.AlternativeGroupId = alternativeGroup1.Id;

        // let's add some more, but in a different group

        d.AlternativeGroupId = alternativeGroup2.Id;

        e.AlternativeGroupId = alternativeGroup2.Id;

        // let's print the alternatives

        foreach(var item in items)

        {

            var itemAlternatives = items.Where(x=> x != item && x.AlternativeGroupId == item.AlternativeGroupId);

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

            foreach(var alternative in itemAlternatives)

            {

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

            }

            Console.WriteLine();

            // Result:

            // Item A has the alternatives: B C 

            // Item B has the alternatives: A C 

            // Item C has the alternatives: A B 

            // Item D has the alternatives: E 

            // Item E has the alternatives: D 

            // adding a new alternative is as simple as assigning to it the ItemGroupId

        }

    }

}

​

public class Item 

{

    public string Name {get; set;}

    public long? AlternativeGroupId {get; set;}

}

​

public class ItemGroup

{

    public long Id {get; set;}

}