.NET 4.0 added the awesome System.Collections.Concurrent namespace, which includes the very useful ConcurrentDictionary class. While the ConcurrentDictionary is thread safe, it can experience problems with adding values during high concurrency...
ConcurrentDictionary.GetOrAdd may invoke the valueFactory multiple times per key.
This behavior will only happens under high load, and even if the valueFactory does get invoked multiple times the dictionary entry will only ever be set once. Normally this is not much of a problem. However, if you are using this Dictionary to store large or expensive objects (such as unmanaged resources or database connections), then the accidental instantiation of multiple of these could be a real problem for your application.
Don't worry, there is a very simple solution to avoid this problem: just create Lazy
