Simplifies LINQ left joins

LeftJoinExtension.cs

using System.Collections.Generic;

namespace System.Linq
{
    public static class LeftJoinExtension
    {
        public static IEnumerable<TResult> LeftJoin<TLeft, TRight, TKey, TResult>(
            this IEnumerable<TLeft> left, 
            IEnumerable<TRight> right, 
            Func<TLeft, TKey> leftKeySelector, 
            Func<TRight, TKey> rightKeySelector,
            Func<TKey, TLeft, TRight, TResult> resultSelector)
        {
            return left
                .GroupJoin(right,
                           leftKeySelector,
                           rightKeySelector,
                           (l, r) => new {Left = l, Right = r})
                .SelectMany(lr => lr.Right.DefaultIfEmpty(),
                            (l, r) => resultSelector(leftKeySelector(l.Left), l.Left, r));
        }
    }
}

LeftJoinUsage.cs

        public void ExtensionTest()
        {
            // Set up the left-side (always there) data
            var people = new List<Person>
                             {
                                 new Person {Id = 1, Name = "Larry Fine"},
                                 new Person {Id = 2, Name = "Joe Armstrong"},
                                 new Person {Id = 3, Name = "Moe Howard"},
                                 new Person {Id = 4, Name = "Curly Howard"},
                                 new Person {Id = 5, Name = "Bertrand Meyer"},
                             };

            // Set up the right-side (optional) data
            var languages = new List<Language>
                                {
                                    new Language {InventorId = 2, Name = "Erlang"},
                                    new Language {InventorId = 5, Name = "Eiffel"}
                                };

            // Nice and simple...
            var result = people.LeftJoin(languages, a => a.Id, b => b.InventorId,
                (k, a, b) => new
                                 {
                                    Person = a.Name , 
                                    Language = (b == null) ? "(Stooge)" : b.Name
                                 });

            // Kick out the results
            foreach (var item in result)
            {
                Console.WriteLine("{0} - {1}", item.Person, item.Language);
            }
        }

        private class Person
        {
            public int Id { get; set; }
            public string Name { get; set; }
        }

        private class Language
        {
            public int InventorId { get; set; }
            public string Name { get; set; }
        }

Output.txt

Larry Fine - (Stooge)
Joe Armstrong - Erlang
Moe Howard - (Stooge)
Curly Howard - (Stooge)
Bertrand Meyer - Eiffel