If I have a ConcurrentDictionary object ConcurrentDictionary<int, Dictionary<string, string>>() dict;, is the nested Dictionary locked when operations are being performed on the outer ConcurrentDictionary?
Scenario: an outer ConcurrentDictionary outerDict is performing an
outerDict.Add(42, new Dictionary<string, string>())
on one thread, and, on another thread (simultaneously), an inner Dictionary is performing an
outerDict[30].Add("hello", "world").
Is concurrency applied to the modification of both the outer ConcurrentDictionary and the nested Dictionary in the above scenario, or are both operations performed at the same time?
CodePudding user response：
Of course not, they're different dictionaries with different access rules.
Your example however is fine, because you're accessing different dictionaries from different threads. If you were to do this instead:
outerDict[30]["a"] = "b"; // in thread 1
outerDict[30]["g"] = "h"; // in thread 2
You'd quickly run into issues.
CodePudding user response：
There are 4 operations here:
- creating a new object. Doing that has no concurrency impact, since nobody else has a reference to the object at that time.
- adding the object to the concurrent dictionary. No problem, because it's a concurrent dictionary.
- accessing the 30th entry in the concurrent dictionary. No problem, because it's a concurrent dictionary
- adding a new value to that 30th dictionary: problematic, if another thread has a reference to the 30th dictionary as well.