using System;

using System.Immutable;

​

​

namespace System.Immutable {

  public interface IImmutableEquatable<T> : IEquatable<T> { };

​

  public static partial class ExtensionMethods {

    public static bool Equals<T>(this T inst, object obj, Func<T, bool> thisEquals) where T : IImmutableEquatable<T> =>

      object.ReferenceEquals(inst, obj) // same reference ->  equal

      || !(obj is null) // this is not null but obj is -> not equal

      && obj.GetType() == inst.GetType() // obj is more derived than this -> not equal

      && inst.GetHashCode() == obj.GetHashCode() // optimization, hash codes are different -> not equal

      && obj is T o // obj cannot be cast to this type -> not equal

      && thisEquals(o);

​

    public static int GetHashCode<T>(this T inst, ref int? hashCache, Func<int> thisHashCode) where T : IImmutableEquatable<T> {

      if (hashCache is null) hashCache = thisHashCode();

      return hashCache.Value;

    }

  }

}

​

​

class Base: IImmutableEquatable<Base> {

  public readonly int X;

  readonly int Y;

​

  public Base(int X, int Y) { 

    this.X = X; 

    this.Y = Y; 

  }

  public bool ThisEquals(Base o) { 

    Console.WriteLine("  Base _Equals");

    return X.Equals(o.X) && Y.Equals(o.Y);

  }

  public int ThisHashCode() {

    Console.WriteLine("  Base _GetHashCode");

    return Tuple.Create(X, Y).GetHashCode();

  }

​

  // boilerplate

  public override bool Equals(object obj) => this.Equals(obj, ThisEquals);

  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); }

  protected int? hashCache;

  public override int GetHashCode() => this.GetHashCode(ref hashCache, ThisHashCode);

}

​

class Derived : Base, IImmutableEquatable<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 bool ThisEquals(Derived o) {

    Console.WriteLine("  Derived _Equals");

    return base.ThisEquals(o) && Z.Equals(o.Z) && K.Equals(o.K);

  }

​

  public new int ThisHashCode() {

    Console.WriteLine("  Derived _GetHashCode");

    return Tuple.Create(base.ThisHashCode(), Z, K).GetHashCode();

  }

  // boilerplate

  public override bool Equals(object obj) => this.Equals(obj, ThisEquals);

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

  public override int GetHashCode() => this.GetHashCode(ref hashCache, ThisHashCode);

}

​

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, b5=null;

        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");

​

        Console.WriteLine("b5==d1"); Console.WriteLine("   "+ (b5==d1)); 

        Console.WriteLine("b5==d4"); Console.WriteLine("   "+ (b5==d4)); 

    }

}