Umbraco-CMS/src/Umbraco.Core/DelegateEqualityComparer.cs

using System;
using System.Collections.Generic;

namespace Umbraco.Cms.Core
{
    /// <summary>
    /// A custom equality comparer that excepts a delegate to do the comparison operation
    /// </summary>
    /// <typeparam name="T"></typeparam>
    public class DelegateEqualityComparer<T> : IEqualityComparer<T>
    {
        private readonly Func<T, T, bool> _equals;
        private readonly Func<T, int> _getHashcode;

        #region Implementation of IEqualityComparer<in T>

        public DelegateEqualityComparer(Func<T, T, bool> equals, Func<T, int> getHashcode)
        {
            _getHashcode = getHashcode;
            _equals = equals;
        }

        public static DelegateEqualityComparer<T> CompareMember<TMember>(Func<T, TMember> memberExpression) where TMember : IEquatable<TMember>
        {
            return new DelegateEqualityComparer<T>(
                (x, y) => memberExpression.Invoke(x).Equals((TMember)memberExpression.Invoke(y)),
                x =>
                    {
                        var invoked = memberExpression.Invoke(x);
                        return !ReferenceEquals(invoked, default(TMember)) ? invoked.GetHashCode() : 0;
                    });
        }

        /// <summary>
        /// Determines whether the specified objects are equal.
        /// </summary>
        /// <returns>
        /// true if the specified objects are equal; otherwise, false.
        /// </returns>
        /// <param name="x">The first object of type <paramref name="T"/> to compare.</param><param name="y">The second object of type <paramref name="T"/> to compare.</param>
        public bool Equals(T x, T y)
        {
            return _equals.Invoke(x, y);
        }

        /// <summary>
        /// Returns a hash code for the specified object.
        /// </summary>
        /// <returns>
        /// A hash code for the specified object.
        /// </returns>
        /// <param name="obj">The <see cref="T:System.Object"/> for which a hash code is to be returned.</param><exception cref="T:System.ArgumentNullException">The type of <paramref name="obj"/> is a reference type and <paramref name="obj"/> is null.</exception>
        public int GetHashCode(T obj)
        {
            return _getHashcode.Invoke(obj);
        }

        #endregion
    }
}