TakeUntil not working as documented?

Question

From the docs for the TakeUntil operator (emphasis mine):

The TakeUntil subscribes and begins mirroring the source Observable. It also monitors a second Observable that you provide. If this second Observable emits an item or sends a termination notification, the Observable returned by TakeUntil stops mirroring the source Observable and terminates.

If this is true, then why does this block?:

Observable.Never<Unit>()
    .TakeUntil(Observable.Empty<Unit>())
    .Wait();

Take from a non-terminating sequence until an empty sequence emits an item. That sounds like it should block indefinitely. — juharr
Why would you think this would throw an exception instead of waiting forever? — Matthew Whited
@juharr: You're right in that no items are emitted, but Empty() should send a termination notification right away, should it not? — lbergnehr
@seldon OK, I see what you mean now. TakeUntil should see that the second sequence has terminated and thus return a terminated sequence and the Wait shouldn't block, but I don't see any reason for an exception. — juharr
fwiw, the (old) MSDN Documentation is different than the main Rx site's and only states that TakeUntil "Returns the values from the source observable sequence until the other observable sequence produces a value." without considering terminated observables. — Preston Guillot

supertopi supertopi · Accepted Answer · 2016-02-18T10:18:13

Preston Guillot is on point in the comments section:

The (old) MSDN Documentation is different than the main Rx site's, and only states that TakeUntil "Returns the values from the source observable sequence until the other observable sequence produces a value." without considering terminated observables.

Let's take a look at the source code for Observable.TakeUntil, specifically class O which represents the "terminator" Observable. We can see that parent.OnCompleted notification is sent on O.OnNext and O.OnError.

So the reason why your code is blocking, is that Observable.Empty (which acts as the "terminator") emits only an OnCompleted notification.

TakeUntil not working as documented?

1 Answers