Outer join extension

OuterJoinExtensions.cs

public static class OuterJoinExtensions
{
  public static IEnumerable<TResult> LeftOuterJoin( this IEnumerable<TLeft> left, IEnumerable<TRight> right, Func<TLeft, TLefTKey> leftKey, Func<TRight,TRightKey> rightKey, Func<TLeft,TRight,TResult> resultSelector )
  {
    return left.GroupJoin( right, leftKey, rightKey, (key,matches) => { Key = key, Matches = matches })
      .SelectMany( x => x.Matches.DefaultIfEmpty(), resultSelector);
  }
}