What are the default Schedulers for each observable operator?

Question

If you do not use the overload which takes a scheduler as an argument, Rx will pick a default scheduler by using the principle of least concurrency. This means that the scheduler which introduces the least amount of concurrency that satisfies the needs of the operator is chosen. For example, for operators returning an observable with a finite and small number of messages, Rx calls Immediate. For operators returning a potentially large or infinite number of messages, CurrentThread is called. For operators which use timers, ThreadPool is used.

I would like to actually have a reference sheet for which observable operators use which default Scheduler, but I can't find one anywhere. What are the default Schedulers for each observable operator?

I'm actually surprised there isn't a list somewhere...if I get some time to dump into it, I'll walk through the rx codebase and see if I can compile a list, but no, I'm not aware of any floating 'round the nets... — JerKimball, Mar 13 '13 at 16:50
Yeah I hunted around too, to no avail. Some of them are obvious but some really aren't! — Alex, Mar 13 '13 at 17:34

score 77 · Accepted Answer · edited Jan 19 '17 at 09:31

77

Wow, that was not trivial to find...

Deep within the bowels of the System.Reactive.Concurrency namespace, there is an internal static class called SchedulerDefaults, which is declared as:

internal static class SchedulerDefaults
{
    internal static IScheduler AsyncConversions 
    { get { return DefaultScheduler.Instance; }}

    internal static IScheduler ConstantTimeOperations 
    { get { return ImmediateScheduler.Instance; }}

    internal static IScheduler Iteration 
    { get { return CurrentThreadScheduler.Instance; }}

    internal static IScheduler TailRecursion 
    { get { return ImmediateScheduler.Instance; }}

    internal static IScheduler TimeBasedOperations 
    { get { return DefaultScheduler.Instance; }}
}

AsyncConversions is used by:

Start, ToAsync, FromAsyncPattern

ConstantTimeOperations is used by:

Empty, GetSchedulerForCurrentContext, Return, StartWith, Throw

Iteration is used by:

Generate, Range, Repeat, TakeLast, ToObservable, and the ReplaySubject<T>

TailRecursion is used by:

Run

TimeBasedOperations is used by:

Buffer, Delay, DelaySubscription, Generate, Interval, Sample, Skip, SkipLast
SkipUntil, Take, TakeLast, TakeLastBuffer, TakeUntil, Throttle, TimeInterval,
Timeout, Timer, Timestamp, Window

edited Jan 19 '17 at 09:31

Alex

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answered Mar 13 '13 at 21:07

JerKimball

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Ideally you would pass in your own scheduler when ever you use any of these method so that you can unit test your code. But it is always good to know which one is the best to use, so this list is great for that! – Lee Campbell Mar 22 '13 at 18:17
@LeeCampbell I was personally astounded at how deep I had to dig to find it...sheesh. Agree 100% on "use your own damned scheduler", tho. – JerKimball Mar 22 '13 at 18:20
Thanks. What about `Where` and `Select` and `Do`? – Colonel Panic Dec 01 '14 at 10:06
@ColonelPanic - pretty sure `Where` and `Select` aren't run through any scheduler - not sure about `Do`. – JerKimball Dec 01 '14 at 16:46
`Do` certainly isn't either. – Alastair Maw Dec 09 '14 at 13:21

What are the default Schedulers for each observable operator?

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