Pass a lambda expression in place of IComparer or IEqualityComparer or any single-method interface?

Question

I happened to have seen some code where this guy passed a lambda expression to a ArrayList.Sort(IComparer here) or a IEnumerable.SequenceEqual(IEnumerable list, IEqualityCom

Answer 1

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;

/// 

/// Utility class for creating  instances 
/// from Lambda expressions.
/// 
public static class EqualityComparerFactory
{
    /// 
Creates the specified .
    /// The type to compare.
    /// The get hash code delegate.
    /// The equals delegate.
    /// An instance of .
    public static IEqualityComparer Create(
        Func getHashCode,
        Func equals)
    {
        if (getHashCode == null)
        {
            throw new ArgumentNullException(nameof(getHashCode));
        }

        if (equals == null)
        {
            throw new ArgumentNullException(nameof(equals));
        }

        return new Comparer(getHashCode, equals);
    }

    private class Comparer : IEqualityComparer
    {
        private readonly Func _getHashCode;
        private readonly Func _equals;

        public Comparer(Func getHashCode, Func 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(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(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(
            a => a.Id.GetHashCode(),
            (a, b) => a.Id==b.Id);
        var intersection = list1.Intersect(list2, comparer).ToList();
    }
}