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

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.

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: http://msdn.microsoft.com/en-us/library/hh737198%28VS.110%29.aspx">Comparer<T>.Create</a>

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

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();
    }
}

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));

public class Comparer2<T, TKey> : IComparer<T>, IEqualityComparer<T>
{
    private readonly Expression<Func<T, TKey>> _KeyExpr;
    private readonly Func<T, TKey> _CompiledFunc
    // Constructor
    public Comparer2(Expression<Func<T, TKey>> getKey)
    {
        _KeyExpr = getKey;
        _CompiledFunc = _KeyExpr.Compile();
    } 
     
    public int Compare(T obj1, T obj2)
    {
        return Comparer<TKey>.Default.Compare(_CompiledFunc(obj1), _CompiledFunc(obj2));
    }

    public bool Equals(T obj1, T obj2)
    { 
        return EqualityComparer<TKey>.Default.Equals(_CompiledFunc(obj1), _CompiledFunc(obj2));
    }

    public int GetHashCode(T obj)
    {
         return EqualityComparer<TKey>.Default.GetHashCode(_CompiledFunc(obj));
    }
}

use it like this

ArrayList.Sort(new Comparer2<Product, string>(p => p.Name));

You can't pass it directly however you could do so by defining a LambdaComparer class that excepts a Func<T,T,int> and then uses that in it's CompareTo.

It is not quite as concise but you could make it shorter through some creative extension methods on Func.

These methods don't have overloads that accept a delegate instead of an interface, but:

• You can normally return a simpler sort key through the delegate you pass to Enumerable.OrderBy
• Likewise, you could call Enumerable.Select before calling Enumerable.SequenceEqual
• It should be straightforward to write a wrapper that implements IEqualityComparer<T> in terms of Func<T, T, bool>
• F# lets you implement this sort of interface in terms of a lambda :)

I vote for the dreaming theory.

You can't pass a function where an object is expected: derivatives of System.Delegate (which is what lambdas are) don't implement those interfaces.

What you probably saw is a use of the of the Converter<TInput, TOutput> delegate, which can be modeled by a lambda. Array.ConvertAll uses an instance of this delegate.

In case if you need this function for use with lambda and possibly two different element types:

static class IEnumerableExtensions
{
    public static bool SequenceEqual<T1, T2>(this IEnumerable<T1> first, IEnumerable<T2> second, Func<T1, T2, bool> comparer)
    {
        if (first == null)
            throw new NullReferenceException("first");

        if (second == null)
            throw new NullReferenceException("second");

        using (IEnumerator<T1> e1 = first.GetEnumerator())
        using (IEnumerator<T2> e2 = second.GetEnumerator())
        {
            while (e1.MoveNext())
            {
                if (!(e2.MoveNext() && comparer(e1.Current, e2.Current)))
                    return false;
            }

            if (e2.MoveNext())
                return false;
        }

        return true;
    }
}