using System;

using System.Collections.Generic;

using System.Linq;

public class Item

{

    public int Id {get; set;}

    public int Test {get; set;}

}

​

public class FactorComparer : IEqualityComparer<Item>

{

    public bool Equals(Item a, Item b)

    {

        return a is not null && b is not null && a.Id == b.Id;

    }

    public int GetHashCode(Item f)

    {

        return f.Id.GetHashCode();

    }

}

​

public class Program

{

    public static void Main()

    {

        var a = new List<Item> 

        {

            new Item() { Id = 1, Test = 1 },

            new Item() { Id = 2, Test = 1 },

            new Item() { Id = 3, Test = 1 },

        };

        var b = new List<Item> 

        {

            new Item() { Id = 3, Test = 2 },

            new Item() { Id = 4, Test = 2 },

            new Item() { Id = 5, Test = 2 },

        };

​

        var union = a.Union(b, new FactorComparer());

        var distinct = a.Union(b).Distinct(new FactorComparer());

​

        foreach (Item f in union)

        {

            Console.Write("({0} {1})", f.Id, f.Test);

        }

        Console.WriteLine();

​

        foreach (Item f in distinct)

        {

            Console.Write("({0} {1})", f.Id, f.Test);

        }

    }

}