using System;

​

public class Program

{

    class MyClass : IEquatable<MyClass> {

      public int X { get; }

      public int Y { get; }

      public MyClass(int x = 0, int y = 0) { X = x; Y = y; }

      public override bool Equals(object obj) {

        var other = obj as MyClass;

        return other is null ? false : X == other.X && Y == other.Y;

      }

      public bool Equals(MyClass o) => Equals((object)o);

      public static bool operator ==(MyClass o1, MyClass o2) => object.Equals(o1, o2);

      public static bool operator !=(MyClass o1, MyClass o2) => !(o1 == o2);

      public override int GetHashCode() => X ^ Y;

    }

​

    class MyDerived : MyClass, IEquatable<MyDerived> {

      public int Z { get; }

      public int K { get; }

      public MyDerived(int x = 0, int y = 0, int z=0, int k=0) : base(x, y) { Z = z; K = k; }

      public override bool Equals(object obj) {

        var other = obj as MyDerived;

        return other is null ? false : base.Equals((object)other) && Z == other.Z && K == other.K;

      }

      public bool Equals(MyDerived other) => Equals((object)other);

      public static bool operator ==(MyDerived o1, MyDerived o2) => object.Equals(o1, o2);

      public static bool operator !=(MyDerived o1, MyDerived o2) => !(o1 == o2);

      public override int GetHashCode() => base.GetHashCode() ^ Z ^ K;

    }

​

    public static void Main()

    {

        MyDerived d1 = new MyDerived(0, 1, 2, 3), d2 = new MyDerived(0, 1, 4, 5), d3 = new MyDerived(0, 1, 2, 3), d4 = null;

        MyClass b1 = d1, b2 = d2, b3 = d3, b4 = d4;

        Console.WriteLine("  d1==d2 => "+ (d1==d2));

        Console.WriteLine("  d1.Equals(d2) => " + (d1.Equals(d2)));

        Console.WriteLine("  d1==d3 => " + (d1==d3));

        Console.WriteLine("  d1.Equals(d3) => " + (d1.Equals(d3)));

        Console.WriteLine("  d1==d4 => " + (d1 == d4));

        Console.WriteLine("  d1.Equals(d4) => " + (d1.Equals(d4)));

        Console.WriteLine("  b1==b2 => "+ (b1==b2));

        Console.WriteLine("  b1.Equals(b2) => " + (b1.Equals(b2)));

        Console.WriteLine("  b1==b3 => " + (b1==b3));

        Console.WriteLine("  b1.Equals(b3) => " + (b1.Equals(b3)));

        Console.WriteLine("  b1==b4 => " + (b1 == b4));

        Console.WriteLine("  b1.Equals(b4) => " + (b1.Equals(b4)));

    }

}