using System;

using System.Collections;

using System.Collections.Generic;

public class Program

{

    public static void Main()

    {

        var item = new Item { Id = 1 };

        IList list = new List<Item> { item };

        IList array = new[] { item };

        var newItem = new Item { Id = 1 };

        var lIndex = list.IndexOf(newItem);

        var aIndex = array.IndexOf(newItem);

        Console.WriteLine(lIndex);

        Console.WriteLine(aIndex);

    }

    public class Item : IEquatable<Item>

    {

        public int Id { get; set; }

        public bool Equals(Item other) => other != null && other.Id == Id;

    }

}