PropertyComparer.cs

using System;
using System.Collections;
using System.Collections.Generic;
using System.Reflection;

namespace ConsoleApplication
{
    class PropertyComparer : IEnumerable
    {
        private readonly Dictionary<string, Func<object, object, bool>> _except = new Dictionary<string, Func<object, object, bool>>();

        private static IEnumerable<Tuple<PropertyInfo, PropertyInfo>> PropertiesInCommon(object left, object right)
        {
            var leftType = left.GetType();
            var rightType = right.GetType();
            foreach (var leftProp in leftType.GetProperties())
            {
                var rightProp = rightType.GetProperty(leftProp.Name);
                if (rightProp != null)
                {
                    yield return Tuple.Create(leftProp, rightProp);
                }
            }
        }

        private Func<object, object, bool> GetComparer(string name)
        {
            Func<object, object, bool> result;
            if (_except.TryGetValue(name, out result))
            {
                return result;
            }

            return Equals;
        }

        public void Add<T>(string name, Func<T, T, bool> comparer) => _except[name] = (l, r) => comparer((T) l, (T)r);

        public void Add<T>(string name, IComparer<T> comparer) => Add<T>(name, (l, r) => comparer.Compare(l, r) == 0);

        public bool ConsideredEqual(object left, object right)
        {
            foreach (var prop in PropertiesInCommon(left, right))
            {
                var leftVal = prop.Item1.GetValue(left);
                var rightVal = prop.Item2.GetValue(right);

                var comparer = GetComparer(prop.Item1.Name);
                if (!comparer(leftVal, rightVal))
                {
                    return false;
                }
            }

            return true;
        }

        // don't use this. it's just to convince C# to give use nice syntax
        IEnumerator IEnumerable.GetEnumerator()
        {
            return ((IEnumerable) _except).GetEnumerator();
        }
    }

    class Program
    {
        class Cat
        {
            public string Name { get; set; }
        }

        class Dog
        {
            public string Name { get; set; }
        }

        static void Main(string[] args)
        {
            var c = new Cat {Name = "Fido"};
            var d = new Dog {Name = "Fido"};

            var comparer = new PropertyComparer();
            Console.WriteLine(comparer.ConsideredEqual(c, d));

            d.Name = "fiDO";

            Console.WriteLine(comparer.ConsideredEqual(c, d));

            var amendedComparer = new PropertyComparer {{"Name", StringComparer.OrdinalIgnoreCase}};
            Console.WriteLine(amendedComparer.ConsideredEqual(c, d));
        }
    }
}