Maybe it is my experience with databases but I have always thought that whenever you compare something with null it is always false. Obviously there are some exceptions.
Historically I've seen the Hashtable be favored over ConcurrentDictionary with the assumption that is was more efficient allowing for lock free reads. Well they both allow lock free reads so which one is really the better option?
I don't remember where I first heard about the circuit breaker pattern but I had seen it a few times in a few different forms under different names. Over time I have come to believe that the original approach described to me might not be the most appropriate.