Switch vs SelectMany + TakeUntil - system.reactive

Question

I'm writing an operator that effectively does the same as Switch plus some other stuff. I always understood Switch to effectively be a neat wrapper around SelectMany combined with TakeUntil. However whilst implementing my new extension I've noticed something different about the order of creation/disposal of inner sequences. I'm wondering how I can replicate the order Switch works in since it seems likely we'd always want to dispose the existing inner before creating a new one.
I've written 2 tests to illustrate what I mean.
StandardSwitch will give the following output:
Disposing Inner followed by Creating New Inner
HandCrankedSwitch will give:
Creating New Inner,
Disposing Inner
See example code below:
[TestMethod]
public void StandardSwitch()
{
var outer = Observable.Interval(TimeSpan.FromSeconds(1))
.Do(x => Console.WriteLine("New Outer Value is {0}", x));
var wait = new CountdownEvent(8);
outer
.Select(CreateInner)
.Switch()
.Subscribe(x =>
{
Console.WriteLine("Subscriber Got {0}", x);
wait.Signal();
});
wait.Wait();
Console.WriteLine("Done");
}
[TestMethod]
public void HandCrankedSwitch()
{
var outer = Observable.Interval(TimeSpan.FromSeconds(1))
.Do(x => Console.WriteLine("New Outer Value is {0}", x));
var wait = new CountdownEvent(8);
outer.Publish(x => x.Select(CreateInner).SelectMany(xs => xs.TakeUntil(x)))
.Subscribe(x =>
{
Console.WriteLine("Subscriber Got {0}", x);
wait.Signal();
});
wait.Wait();
Console.WriteLine("Done");
}
internal IObservable<long> CreateInner(long notUsed)
{
return Observable.Create<long>(obs =>
{
Console.WriteLine("Creating New Inner");
var compDis = new CompositeDisposable {Disposable.Create(() => Console.WriteLine("Disposing Inner"))};
var dis = Observable.Interval(TimeSpan.FromSeconds(0.4))
.Do(x => Console.WriteLine("New Inner Value is {0}", x))
.Subscribe(obs);
compDis.Add(dis);
return compDis;
});
}