Equality and Symmetry

program.cs

using System;

namespace symmetry
{
    public class Person : IEquatable<Person>
    {
        public string FirstName {get;set;}
        public string LastName {get;set;}

        public bool Equals(Person other)
        {
            // Guard against 'other' being a type derived from Person:
            if (other.GetType() != typeof(Person))
                return false;
            return this.FirstName == other?.FirstName && this.LastName == other?.LastName;
        }
    }

    public class Student : Person, IEquatable<Student>
    {
        public int Level {get;set;}

        public bool Equals(Student other)
        {
            // Guard against 'other' being a type derived from Student
            if (other.GetType() != typeof(Student))
                return false;
            return base.Equals(other) && this.Level == other?.Level;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            Person p = new Person
            {
                FirstName = "Bill",
                LastName = "Wagner"
            };
            Student s = new Student
            {
                FirstName = "Bill",
                LastName = "Wagner",
                Level = 17
            };


            Console.WriteLine(p.Equals(s));
            Console.WriteLine(s.Equals(p));
        }
    }
}