The name of your business rule implies, that it runs in parallel. If the dictionary is used in parallel as well (by using globals?), you will run in problems, because the standard dictionary doesn’t support parallel processing.
For parallel processing, you have to us the ConcurrentDictionary (an import System.Collections.Concurrent line is needed).
ConcurrentDictionary has a TryAdd and TryUpdate method to avoid problems from running in parallel.
I hope this helps, for a better understanding of your problem and a more concrete help, we need to know more about the business rule.
