IOptionsMonitor is singleton but listener registration is not thread safe

[pakrym]

Bad things might happen if two consumers subscribe concurrently.
Also if subscription happens during invocation.
/cc @HaoK

[HaoK]

Yuck, yup not threadsafe

[HaoK]

Discussed with @ajcvickers will fix with either using a SynchronizedCollection or explicit locks

[pakrym]

You can also use event internally it is already synchronized.

[HaoK]

Good idea, this is pretty much the exact usage pattern for events...

[HaoK]

ff42006