Equality and polymorphism

Question

With two immutable classes Base and Derived (which derives from Base) I want to define Equality so that

• equality is always polymorphic - that is ((Bas

Answer 1

Well I guess there are two parts to you problem:

1. executing equals at nested level
2. restricting to the same type

Would this work? https://dotnetfiddle.net/eVLiMZ (I had to use some older syntax as it didn't compile in dotnetfiddle otherwise)

using System;


public class Program
{
    public class Base
    {
        public string Name { get; set; }
        public string VarName { get; set; }

        public override bool Equals(object o)
        {
            return object.ReferenceEquals(this, o) 
                || o.GetType()==this.GetType() && ThisEquals(o);
        }

        protected virtual bool ThisEquals(object o)
        {
            Base b = o as Base;
            return b != null
                && (Name == b.Name);
        }

        public override string ToString()
        {
            return string.Format("[{0}@{1} Name:{2}]", GetType(), VarName, Name);
        }

        public override int GetHashCode()
        {
            return Name.GetHashCode();
        }
    }

    public class Derived : Base
    {
        public int Age { get; set; }

        protected override bool ThisEquals(object o)
        {
            var d = o as Derived;
            return base.ThisEquals(o)
                && d != null
                && (d.Age == Age);
        }

        public override string ToString()
        {
            return string.Format("[{0}@{1} Name:{2} Age:{3}]", GetType(), VarName, Name, Age);
        }

        public override int GetHashCode()
        {
            return base.GetHashCode() ^ Age.GetHashCode();
        }
    }

    public static void Main()
    {
        var b1 = new Base { Name = "anna", VarName = "b1" };
        var b2 = new Base { Name = "leo", VarName = "b2" };
        var b3 = new Base { Name = "anna", VarName = "b3" };
        var d1 = new Derived { Name = "anna", Age = 21, VarName = "d1" };
        var d2 = new Derived { Name = "anna", Age = 12, VarName = "d2" };
        var d3 = new Derived { Name = "anna", Age = 21, VarName = "d3" };

        var all = new object [] { b1, b2, b3, d1, d2, d3 };

        foreach(var a in all) 
        {
            foreach(var b in all)
            {
                Console.WriteLine("{0}.Equals({1}) => {2}", a, b, a.Equals(b));
            }
        }
    }
}