C# LINQ inner and left outer join

Left outer join extension method is based on How to: Perform Left Outer Joins article.

[Test]
public void Test()
{
  var list1 = new List<int>() { 1, 2, 3, 4, 5}.Select(x=>new {a=x, b=x*x});
  var list2 = new List<int>() { 1, 2, 3, 4 }.Select(x => new { a = x, b = x * x * x });
 
  var innerJoin = list1.Join(list2, x => x.a, y => y.a, (x, y) => new {a = x.a, b = x.b, c = y.b});
 
  innerJoin.ForEach(Console.WriteLine);
 
  var leftOuterJoin = list1.LeftOuterJoin(list2, x => x.a, y => y.a, (x, y) => new { a = x.a, b = x.b, c = (y != null) ? y.b : (int?)null });
 
  leftOuterJoin.ForEach(Console.WriteLine);
}
public static class Extensions
{
  public static IEnumerable<TResult> LeftOuterJoin<TOuter, TInner, TKey, TResult>(
    this IEnumerable<TOuter> outer, 
    IEnumerable<TInner> inner, 
    Func<TOuter, TKey> outerKeySelector, 
    Func<TInner, TKey> innerKeySelector, 
    Func<TOuter, TInner, TResult> resultSelector)
  {
    return 
      from outerItem in outer
      join innerItem in inner on outerKeySelector(outerItem) equals innerKeySelector(innerItem) into gj
      from sub in gj.DefaultIfEmpty()
      select resultSelector(outerItem, sub);
  }
}