Waiting for all threads to finish writing in ConcurrentDictionary

Question

There's a concurrent dictionary, which is collecting information from different sources and once in a minute is supposed to get refreshed and pass collected data to another handler.

var currentDictionarySnapshot = _currentDictionary;
_currentDictionary = new ConcurrentDictionary<int, string>();
return new ReadOnlyDictionary<int, string>(currentDictionarySnapshot);

What I need is to make currentDictionarySnapshot wait for all threads that have reference on it to finish writing and create new ReadOnlyDictionary just after. Does ConcurrentDictionary support it natively or how can I provide this if it doesn't?

you may consider using `Task.WhenAll(Task1, Task2,...);` method — chouaib, Feb 20 '15 at 08:57
My writing methods is not asynchronous, and even if it was, it would be messy to distinguish tasks of primary dictionary and snapshot. — Sovent, Feb 20 '15 at 08:59

score 0 · Answer 1 · answered Feb 20 '15 at 08:59

Conceptually you seem to need a ReaderWriterLockSlim. "Readers" would be the threads writing to the dictionary. The "writer" is the thread copying the dictionary.

Note, that the built in RWL's do not scale. They simply lock internally on some data structures for every operation. This destroys the scalability property of the ConcurrentDictionary which scales through lock striping.

score -1 · Accepted Answer · answered Feb 20 '15 at 12:52

-1

Well, solution for my problem I made is creating class with ManualResetEventSlim and adding counter to it. When time to read is coming, I make snapshot of this class like I do it with ConcurrentDictionary and wait for counter become 0.

answered Feb 20 '15 at 12:52

Sovent

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How do you ensure that no further writers come along right after the count has dropped to zero? In any case this is not an answer since it is too vague. – usr Feb 20 '15 at 14:03
I make snapshot and replace the instance of this class by a new one. An old one will remain with his writers and allow them to finish. – Sovent Feb 21 '15 at 09:53
Snapshotting by copying is not instant. You'll get a dictionary that is "smeared" over time. I any case this does not answer the question which is how to wait for writes to finish. Even though you are the OP your answer is held to the same standards as other answers. – usr Feb 21 '15 at 10:21
It's not snapshotting by copying, I create another reference to dictionary and make readonly dictionary from it. And I make old variable refer to a new, empty dictionary,so all new writers come to a new one . I can't paste whole code of my solution because it's restricted by my employer. But my answer would help me with my problem and I think, this is the point. – Sovent Feb 22 '15 at 12:38
Creating another reference does not prevent writers to write to the old dict. They might still have the old ref. You need to answer what was asked. It does not matter that this is the ultimate approach you chose. – usr Feb 22 '15 at 13:09

Waiting for all threads to finish writing in ConcurrentDictionary

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