thread safety concerns for generic parameters on concurrent collections

Question

When using System.Collections.Concurrent, what should I be aware of for the contained items?

For instance: ConcurrentDictionary<int,HashSet<string>> vs ConcurrentDictionary<int,ConcurrentBag<string>>

The hashset may be desireable because of its behavior... but its not contained in the safe concurrent collections space. So do I need to worry that multiple threads may write to a contained hashset at once... or will those accesses be managed within the context of the concurrent dictionary in a thread safe manner? MSDN states that instance members of HashSet are not guaranteed to be thread safe.

Please explain the implications and provide guidance. Thank you.

Answer 1

The concurrent collections do no synchronization for access to the contained items. How could they?

For example:

var myValue = myDictionary[myKey];
... I can now do what I want with myValue without the dictionary being aware of it.

In your case you must implement your own synchronization for access to the HashSet.