I was also googling the web for a solution, but i didn't found any satisfying one. So i've created a generic EqualityComparerFactory:
using System;
using System.Collections.Generic;
/// <summary>
/// Utility class for creating <see cref="IEqualityComparer{T}"/> instances
/// from Lambda expressions.
/// </summary>
public static class EqualityComparerFactory
{
/// <summary>Creates the specified <see cref="IEqualityComparer{T}" />.</summary>
/// <typeparam name="T">The type to compare.</typeparam>
/// <param name="getHashCode">The get hash code delegate.</param>
/// <param name="equals">The equals delegate.</param>
/// <returns>An instance of <see cref="IEqualityComparer{T}" />.</returns>
public static IEqualityComparer<T> Create<T>(
Func<T, int> getHashCode,
Func<T, T, bool> equals)
{
if (getHashCode == null)
{
throw new ArgumentNullException(nameof(getHashCode));
}
if (equals == null)
{
throw new ArgumentNullException(nameof(equals));
}
return new Comparer<T>(getHashCode, equals);
}
private class Comparer<T> : IEqualityComparer<T>
{
private readonly Func<T, int> _getHashCode;
private readonly Func<T, T, bool> _equals;
public Comparer(Func<T, int> getHashCode, Func<T, T, bool> equals)
{
_getHashCode = getHashCode;
_equals = equals;
}
public bool Equals(T x, T y) => _equals(x, y);
public int GetHashCode(T obj) => _getHashCode(obj);
}
}
The idea is, that the CreateComparer method takes two arguments: a delegate to GetHashCode(T) and a delegate to Equals(T,T)
Example:
class Person
{
public int Id { get; set; }
public string LastName { get; set; }
public string FirstName { get; set; }
}
class Program
{
static void Main(string[] args)
{
var list1 = new List<Person>(new[]{
new Person { Id = 1, FirstName = "Walter", LastName = "White" },
new Person { Id = 2, FirstName = "Jesse", LastName = "Pinkman" },
new Person { Id = 3, FirstName = "Skyler", LastName = "White" },
new Person { Id = 4, FirstName = "Hank", LastName = "Schrader" },
});
var list2 = new List<Person>(new[]{
new Person { Id = 1, FirstName = "Walter", LastName = "White" },
new Person { Id = 4, FirstName = "Hank", LastName = "Schrader" },
});
// We're comparing based on the Id property
var comparer = EqualityComparerFactory.Create<Person>(
a => a.Id.GetHashCode(),
(a, b) => a.Id==b.Id);
var intersection = list1.Intersect(list2, comparer).ToList();
}
}
