json serialization and immuatability

UnitTest1.cs

using System;
using System.Diagnostics;
using Newtonsoft.Json;
using Shouldly;
using Xunit;

namespace json
{
    public class UnitTest1
    {
        public class Person
        {
            public Person(string firstname, string lastname, DateTime dateOfBirth)
            {
                Firstname = firstname;
                Lastname = lastname;
                DateOfBirth = dateOfBirth;
            }

            public string Firstname { get; }
            public string Lastname { get; }
            public string Fullname => $"{Firstname} {Lastname}";
            public DateTime DateOfBirth { get; }
            public override string ToString()
            {
                return $"{Fullname} {DateOfBirth:dd-MMM-yyyy}";
            }
        }

        [Fact]
        public void Test1()
        {
            var p1 = new Person("James", "Bond", new DateTime(1950, 1, 1));
            var p1Json = JsonConvert.SerializeObject(p1);
            Debug.WriteLine(p1Json);
            p1Json.ShouldBe(@"{""Firstname"":""James"",""Lastname"":""Bond"",""Fullname"":""James Bond"",""DateOfBirth"":""1950-01-01T00:00:00""}");
            
            var p2Json = @"{""Lastname"":""Lieter"",""Firstname"":""Felix"",""DateOfBirth"":""1951-01-01T00:00:00""}";
            var p2 = JsonConvert.DeserializeObject<Person>(p2Json);
            Debug.WriteLine("p2=" + p2);
            p2.ToString().ShouldBe("Felix Lieter 01-Jan-1951");
        }
    }
}