ComparerUtil

using System;
using System.Collections.Generic;
using System.Linq;

namespace EqualityComparerPOC
{
    public static class ComparerUtil
    {
        public static DifferencesResult<T> GetDifferences<T, TKey>(
            IEnumerable<T> originalList,
            IEnumerable<T> changedList,
            Func<T, TKey> keyExtractor)
        {

            DifferencesResult<T> results = new DifferencesResult<T>();

            var keyComparer = new KeyEqualityComparer<T, TKey>(keyExtractor);

            results.Added = changedList.Except(originalList, keyComparer);
            results.Removed = originalList.Except(changedList, keyComparer);

            return results;
        }

        public static DifferencesResult<T> GetDifferences<T, TKey, TDiff>(
            IEnumerable<T> originalList,
            IEnumerable<T> changedList,
            Func<T, TKey> keyExtractor,
            Func<T, TDiff> differenceExtractor)
        {

            DifferencesResult<T> results = new DifferencesResult<T>();

            var keyComparer = new KeyEqualityComparer<T, TKey>(keyExtractor);
            var diffComparer = new KeyEqualityComparer<T, TDiff>(differenceExtractor);

            results.Added = changedList.Except(originalList, keyComparer);
            results.Removed = originalList.Except(changedList, keyComparer);

            results.Changed = changedList.Except(originalList, diffComparer)
                                        .Except(results.Added, keyComparer)
                                        .Except(results.Added, keyComparer);

            return results;
        }

        public class DifferencesResult<T>
        {
            public IEnumerable<T> Added { get; set; }
            public IEnumerable<T> Removed { get; set; }
            public IEnumerable<T> Changed { get; set; }
        }

        /// <summary>
        /// Generice impl of <see cref="IEqualityComparer"/>
        /// </summary>
        /// <example>
        /// var treeComparer = new KeyEqualityComparer<TreeDefinition, int?>((t) => t.TreeID);
        /// </example>
        /// <typeparam name="T">The type of object comparing</typeparam>
        /// <typeparam name="TKey">The type of key comparing</typeparam>
        class KeyEqualityComparer<T, TKey> : IEqualityComparer<T>
        {
            protected readonly Func<T, TKey> keyExtractor;

            public KeyEqualityComparer(Func<T, TKey> keyExtractor)
            {
                this.keyExtractor = keyExtractor;
            }

            public virtual bool Equals(T x, T y)
            {
                return this.keyExtractor(x).Equals(this.keyExtractor(y));
            }

            public int GetHashCode(T obj)
            {
                return this.keyExtractor(obj).GetHashCode();
            }
        }
    }
}