ConcurrentDictionary, .NET 4.0’s New Thread-Safe Hashtable

The ConcurrentDictionary is a cornerstone in .NET 4.0’s greatly increased emphasis on parallel and concurrent programming. But before we delve into it, we offer a review of the problems in previous versions of .NET.

The first version of the hash table in .NET was System.Collections.Hashtable. While it wasn’t thread-safe, you could in theory get a thread-safe wrapper simply by calling Hashtable.Synchronized. Unfortunately this wrapper wasn’t actually thread-safe because of the way it was used.

Say, for example, you wanted to check if a key exists in the collection. If not, then you want to perform a non-repeatable operation, the result of which would be stored there. Even though both ContainsKey and set_Item are independently thread-safe, there is no way to directly compose them. Instead you have to take a lock on the SyncRoot, negating the whole reason you asked for a synchronized version in the first place.

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