ConcurrentDictionary.GetOrAdd() facts

In any combination of any number of threads running for any values with the same key:

  • It is guaranteed that only one item will be added
    • and this will be the only call to which true is returned.
  • It is not guaranteed that the delegate overload will only execute the delegate once
    • because the class uses optimistic concurrency control.
    • The implicit assumption is that it’s rare for multiple concurrent calls to occur in reality, so more efficient to compute anyway, then replace or throw away as appropriate.
    • If the delegate is expensive or has unwanted side effects, a Lazy<> can be used instead.