using System;

using System.Immutable;

​

​

namespace System.Immutable {

  using System.Collections;

​

  public interface IImmutable { };

​

  public static partial class ExtensionMethods {

    public static bool ImmutableEquals<TSrc>(this TSrc inst, object obj, Func<TSrc, bool> f) where TSrc : IEquatable<TSrc>, IImmutable {

      if (object.ReferenceEquals(inst, obj)) return true; // same reference ->  equal

      if (obj is null) return false; // this is not null but obj is -> not equal

      if (!(obj is TSrc o)) return false; // obj cannot be cast to this type -> not equal

      if (obj.GetType() != inst.GetType()) return false; // obj is more derived than this -> not equal

      if (inst.GetHashCode() != o.GetHashCode()) return false; // optimization, hash codes are different -> not equal

      return f(o);

    }

​

    public static int ImmutableHash<TSrc>(this TSrc inst, ref int? hashCache, IStructuralEquatable tuple) where TSrc : IEquatable<TSrc>, IImmutable {

      if (hashCache is null) {

          Console.WriteLine("  " + typeof(TSrc)+".GetHashCode");

          hashCache = tuple.GetHashCode();

      }

      return hashCache.Value;

    }

  }

}

​

​

class Base: IEquatable<Base>, IImmutable {

  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)");     

    return this.ImmutableEquals(obj, o => X.Equals(o.X) && Y.Equals(o.Y));

  }

​

  int? hashCache;

  public override int GetHashCode() {

    Console.WriteLine(" Base.GetHashCode");

    return this.ImmutableHash(ref hashCache, Tuple.Create(X, Y));

  }

​

  // boilerplate

  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); }

}

​

class Derived : Base, IEquatable<Derived>, IImmutable {

  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)");  

    return this.ImmutableEquals(obj, o => base.Equals(o as object) && Z.Equals(o.Z) && K.Equals(o.K));

  }

​

  int? hashCache;

  public override int GetHashCode() {

    Console.WriteLine(" Derived.GetHashCode");

    return this.ImmutableHash(ref hashCache, Tuple.Create(base.GetHashCode(), Z, K));

  }

​

  // boilerplate

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

}

​

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");

    }

}