Non-intersecting LeftDurationSelector in Reactive Extensions Join

Question

I want to create an observable that emits when the right observable emits a value and takes the last left observable value with it, i.e., as described in the following marble diagram (taken from combining-sequences)

reading through the documentation in the above link suggests that this can be achieved by ensuring a non-intersecting left duration selector and a right duration selector that returns immediately. So I tried this:

leftObservable.Join(rightObservable, _ => leftObservable.Publish().RefCount(), _ => Observable.Empty<Unit>(), (l,r) => (l,r))

but that doesn't seem to emit anything. What am I doing wrong?

Update

I think this may be down to the observables that are being fed into the Join statement, i.e., they originate from a switch, e.g.,

var left1 = Observable.Interval(TimeSpan.FromSeconds(5));
var left2 = Observable.Interval(TimeSpan.FromSeconds(7));

var left = left1.Select(x => left2).Switch();
var right = Observable.Interval(TimeSpan.FromSeconds(1));

var test = left.Join(right, _ => left.Publish().RefCount(), _ => Observable.Empty<Unit>(), (l, r) => (l, r))
    .Distinct(x => x.r)
    .Subscribe(x =>
    {
        Debug.WriteLine($"left: {x.l}, right: {x.r}");
    });

If I replace left with left in the join statement, things work as expected. So I must be misunderstanding how switch works in this context?

Answer 1

    static async Task Main()
    {

        var left = Observable.Interval(TimeSpan.FromSeconds(0.3));
        var right = Observable.Timer(TimeSpan.FromSeconds(1),TimeSpan.FromSeconds(1)).Select(i => (char)('A'+i)).Take(5);

        right.WithLatestFrom(left).Subscribe(i => Console.WriteLine(i));

        Console.ReadLine();
    }

gives the output:

(A, 2)
(B, 5)
(C, 8)
(D, 12)
(E, 15)

Looks like what you're expecting?