using System;

using System.Collections.Generic;

​

public class Program

{

    public static void Main()

    {

        var hashset = new HashSet<Rate>();

        var eeRate = new Rate(){Country="EE", Price=3};

        hashset.Add(eeRate);

        Console.WriteLine(hashset.Count + " " + hashset.Contains(eeRate));

        eeRate.Price = 2; // it changes GetHashCode, so the object gets lost in hashset. 

        Console.WriteLine(hashset.Count + " " + hashset.Contains(eeRate));

    }

    //Mutable object

    class Rate{

        public string Country{get;set;}

        public int Price{get;set;}

        public override int GetHashCode()

        {

            return Country.GetHashCode() *2 + Price.GetHashCode() * 561;

        }

    }

}