While the .NET ConcurrentDictionary<T, V> is thread-safe, not all of its methods are atomic. Microsoft points out (http://msdn.microsoft.com/en-us/library/dd997369.aspx) that the GetOrAdd and AddOrUpdate methods that take in a delegate invoke the delegate outside of the locking mechanism used by the dictionary.
Because the locking is not employed when invoking the delegate, it could get executed more than once. I wrote some sample code and posted it onto a new StackOverflow community called CodeReview to get some feedback from the community on an approach to use the delegate overloads while maintaining atomicity. I found that by writing an extension method for the delegate methods I could use the .NET Lazy<T> class as a wrapper for the value in the dictionary. Because the Lazy<T> class is only evaluated once, the new methods are atomic.
Pinned below is the CodeReview post.