Is this non-locked TryGetValue() dictionary access thread-safe?
a) This is not thread-safe, as the underlying Dictionary itself is not thread safe. If another thread is calling Add at the same time, undefined behavior can occur.
b) This is effectively an attempt at double-checked locking.
I would recommend using the ConcurrentDictionary class instead, as it's designed for this scenario. Another option would be to use a ReaderWriterLockSlim (or ReaderWriterLock), if you're not targetting .NET 4.0.
Question a: Is it thread-safe? If yes, why?
Not only is it not thread safe; it will also throw with NullReferenceException if accessed while another thread is reorganizing the hash buckets. The lock statement is wicked fast, don't avoid it.
Question b: How is this double-TryGetValue() pattern called?
It's called a 'bug' by most people ;)
Sadly, no.
I carry around a custom HashMap that has this property.
The defect is in the rehash() function.