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, IEqualityComparer here) where an IComparer or an IEqualityComparer was expected.

I can't be sure if I saw it though, or I am just dreaming. And I can't seem to find an extension on any of these collections that accepts a Func<> or a delegate in their method signatures.

Is there such an overload/extension method? Or, if not, is it possible to muck around like this and pass an algorithm (read delegate) where a single-method interface is expected?

Update Thanks, everyone. That's what I thought. I must've been dreaming. I know how to write a conversion. I just wasn't sure if I'd seen something like that or just thought I'd seen it.

Yet another update Look, here, I found one such instance. I wasn't dreaming after all. Look at what this guy is doing here. What gives?

And here's another update: Ok, I get it. The guy's using the Comparison<T> overload. Nice. Nice, but totally prone to mislead you. Nice, though. Thanks.

Answer 1

I'm not much sure what useful it really is, as I think for most cases in the Base Library expecting an IComparer there's an overload that expects a Comparison... but just for the record:

in .Net 4.5 they've added a method to obtain an IComparer from a Comparison: Comparer.Create

so you can pass your lambda to it and obtain an IComparer.

Answer 2

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();
    }
}

Answer 3

You can provide a lambda for a Array.Sort method, as it requires a method that accepts two objects of type T and returns an integer. As such, you could provide a lambda of the following definition (a, b) => a.CompareTo(b). An example to do a descending sort of an integer array:

int[] array = { 1, 8, 19, 4 };

// descending sort 
Array.Sort(array, (a, b) => -1 * a.CompareTo(b));