using System;

using System.Linq;

using System.Collections.Generic;

using Newtonsoft.Json;

public class Program

{

    public static void Main()

    {

        var input = new Foo{

            Name = "Monica",

            Pets = new List<Pet>{

                new Pet{},

                new Pet{},

                new Pet{},

            }       

        } ;

        var jsonInput = JsonConvert.SerializeObject(input);

        Console.WriteLine(jsonInput);

        var result = JsonConvert.DeserializeObject<HFoo_IEqualityComparer>(jsonInput);

        result.Dump();

        var result2= JsonConvert.DeserializeObject<HFoo_IEquatable>(jsonInput);

        result2.Dump();

    }

​

    public class HFoo_IEqualityComparer{    

        public string Name {get;set;}

        public HashSet<Pet> Pets {get;set;} = new HashSet<Pet>(new CustomComparer());

    }

    public class HFoo_IEquatable{   

        public string Name {get;set;}

        public HashSet<Pet> Pets {get;set;}

    }

    public class Foo{

        public string Name {get;set;}

        public List<Pet> Pets {get;set;}

    }

​

    public class Pet: IEquatable<Pet>{

        public string Name{get;set;}

        public int Id{get;set;}

        public DateTime BirthDate{get;set;}

        public bool Equals(Pet obj1 ){

            Console.WriteLine("Called");

            return true;

        }

​

    }

    public class CustomComparer : IEqualityComparer<Pet>{

        public int GetHashCode(Pet obj) {

            return 1;

        }       

        public bool Equals(Pet obj1, Pet obj2) {        

            return true;

        }

    }   

​

}