using System;

​

class Base: IEquatable<Base> {

  public readonly int X;

  readonly int Y;

​

  public Base(int X, int Y) { 

    this.X = X; 

    this.Y = Y; 

  }

​

  public override bool Equals(object obj) {

    Console.WriteLine(" Base.Equals(object)");   

    if (object.ReferenceEquals(this, obj)) return true;

    if (obj is null || obj.GetType()!=this.GetType()) return false;

​

    return obj is Base o 

      && X.Equals(o.X)

      && Y.Equals(o.Y);

  }

​

  public bool Equals(Base o) {  Console.WriteLine(" Base.Equals(Base)"); return object.Equals(this, o); }

  public static bool operator ==(Base o1, Base o2) { Console.WriteLine(" Base=="); return object.Equals(o1, o2); }

  public static bool operator !=(Base o1, Base o2) { Console.WriteLine(" Base!=");  return !object.Equals(o1, o2); }

​

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

}

​

class Derived : Base, IEquatable<Derived> {

  public readonly int Z;

  readonly int K;

​

  public Derived(int X, int Y, int Z, int K) : base(X, Y) {

    this.Z = Z; 

    this.K = K; 

  }

​

  public override bool Equals(object obj) {

    Console.WriteLine(" Derived.Equals(object)");  

    if (object.ReferenceEquals(this, obj)) return true;

    if (obj is null || obj.GetType()!=this.GetType()) return false;

​

    return obj is Derived o

      && base.Equals(obj) /* ! */

      && Z.Equals(o.Z)

      && K.Equals(o.K);

  }

​

  public bool Equals(Derived o) { Console.WriteLine(" Derived.Equals(Derived)"); return object.Equals(this, o); }

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

}

​

public class Program

{

    public static void Main()

    {

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

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

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

        Console.Write("d1.Equals(d2)"); Console.WriteLine("  " +  d1.Equals(d2)+"\n");

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

        Console.Write("d1.Equals(d3)"); Console.WriteLine("  " +  d1.Equals(d3)+"\n");

​

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

        Console.Write("d1.Equals(d4)"); Console.WriteLine("  " +  d1.Equals(d4)+"\n");

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

        Console.Write("b1.Equals(b2)"); Console.WriteLine("  " +  b1.Equals(b2)+"\n");

​

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

        Console.Write("b1.Equals(b3)"); Console.WriteLine("  " +  b1.Equals(b3)+"\n");

​

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

        Console.Write("b1.Equals(b4)"); Console.WriteLine("  " +  b1.Equals(b4)+"\n");

​

        Base b = new Base(0, 1);

        Console.WriteLine("b==d1"); Console.WriteLine("  "+ (b==b1)); 

        Console.Write("b.Equals(d1)"); Console.WriteLine("  " +  b.Equals(d1)+"\n");

    }

}