LeftJoin using a generator

LeftJoin2.cs

// Based on https://gist.github.com/reidev275/f765c506aea0e3e3432da591b2c30c15#file-leftJoin-cs

public struct Join<A, B>
{
    public A First { get; set; }
    public B Second { get; set; }
}

public static IEnumerable<Join<A, B>> LeftJoin2<A, B, C>(this IEnumerable<A> from, IEnumerable<B> join,
    Func<A, C> onFrom, Func<B, C> onJoin) where C : IComparable<C>
{
    var fromS = from.OrderBy(onFrom);
    using (var fromE = fromS.GetEnumerator())
    {
        if (!fromE.MoveNext())
        {
            yield break;
        }

        var joinS = join.OrderBy(onJoin);
        using (var joinE = joinS.GetEnumerator())
        {
            if (!joinE.MoveNext())
            {
                do
                {
                    yield return new Join<A, B> {First = fromE.Current, Second = default(B)};
                } while (fromE.MoveNext());

                yield break;
            }

            var match = false;
            while (true)
            {
                var key = onFrom(fromE.Current);
                var b = onJoin(joinE.Current);
                var comp = key.CompareTo(b);
                switch (comp)
                {
                    case -1:
                        if (!match) yield return new Join<A, B> {First = fromE.Current, Second = default(B)};
                        if (!fromE.MoveNext())
                        {
                            yield break;
                        }

                        match = false;
                        break;

                    case 1:
                        if (!joinE.MoveNext())
                        {
                            while (fromE.MoveNext())
                            {
                                yield return new Join<A, B> {First = fromE.Current, Second = default(B)};
                            }

                            yield break;
                        }

                        match = false;
                        break;

                    default:
                        yield return new Join<A, B> {First = fromE.Current, Second = joinE.Current};
                        match = true;
                        if (!joinE.MoveNext())
                        {
                            while (fromE.MoveNext())
                            {
                                yield return new Join<A, B> {First = fromE.Current, Second = default(B)};
                            }

                            yield break;
                        }

                        break;
                }
            }
        }
    }
}