GenericEqualityComparer

GenericEqualityComparer.cs

using Microsoft.VisualStudio.TestTools.UnitTesting;
using System;
using System.Collections.Generic;
using System.Linq.Expressions;

namespace Utils
{
    public class GenericEqualityComparer<T> : EqualityComparer<T>
    {

        public GenericEqualityComparer(Func<T, int> getHashCode)
        {
            this.getHashCode = getHashCode;
        }

        public GenericEqualityComparer() : this(x => x.GetHashCode()) { }

        public GenericEqualityComparer(Expression<Func<T, Object>> expression) : this(Build(expression)) { }

        private static Func<T, int> Build(Expression<Func<T, Object>> expression)
        {
            var execute = expression.Compile();
            return x =>
            {
                var result = execute(x);
                if (result == null) throw new ArgumentNullException();
                return result.GetHashCode();
            };
        }

        private Func<T, int> getHashCode;

        public override bool Equals(T x, T y)
        {
            if (x == null && y == null) return true;
            if (x == null || y == null) return false;
            return GetHashCode(x).Equals(GetHashCode(y));
        }

        public override int GetHashCode(T obj)
        {
            if (obj == null) throw new ArgumentNullException();
            return getHashCode(obj);
        }
    }

    [TestClass]
    public class GenericEqualityComparerTest
    {

        private class Product
        {
            public string Name { get; set; }
            public string ProductNo { get; set; }
        }

        [TestMethod]
        public void TestComparer()
        {

            var a = new Product { Name = "Test", ProductNo = "123", };
            var b = new Product { Name = "Test", ProductNo = "123", };
            var c = new Product { Name = "Test", ProductNo = "456", };

            // Default, same as a.Equals(b) with null check;
            var comparerA = new GenericEqualityComparer<Product>();
            Assert.IsFalse(comparerA.Equals(a, b));
            Assert.IsFalse(comparerA.Equals(b, c));

            // Compare by single property
            var comparerB = new GenericEqualityComparer<Product>(x => x.Name);
            Assert.IsTrue(comparerB.Equals(a, b));
            Assert.IsTrue(comparerB.Equals(b, c));

            // Compare by multiple properties
            var comparerC = new GenericEqualityComparer<Product>(x => new { x.Name, x.ProductNo });
            Assert.IsTrue(comparerC.Equals(a, b));
            Assert.IsFalse(comparerC.Equals(b, c));

            // Custom implementation
            var comparerD = new GenericEqualityComparer<Product>(x => x.Name.GetHashCode());
            Assert.IsTrue(comparerD.Equals(a, b));
            Assert.IsTrue(comparerD.Equals(b, c));

        }
    }
}