Concurrent collections is it safe to modify from different threads

Question

Let say I have:

MyCollection = new ConcurrentDictionary<string, int>();

I now it is safe to add and remove items from MyCollection . But What about modifying items. For example is it safe to do:

MyCollection["key"] = 1;   // thread 1

and

MyCollection["key"] = 2;   // thread 2

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Example 2:

MyCollection2 = new ConcurrentDictionary<string, List<int>>();

Is it safe to do?

MyCollection2["key"].Add(1);  // Thread1

and

MyCollection2["key"].Add(2);  // Thread2

where Thread1 and Thread2 are executing at the same time. Do I have to create a lock when modifying items?

Answer 1

Whether or not it is safe to modify the item is entirely independent on whether or not it is in a ConcurrentDictionary. That's no different than just having a list that you mutate from two different threads. In the case of List, it's not safe; if you use a type that is designed to be mutated from multiple different threads, such as a ConcurrentDictionary<string, ConcurrentQueue<int>> then that would be fine.

ConcurrentQueue is only ensuring that calls to methods of that class are observed to be atomic; it is making no other guarantees surrounding thread safety.

Answer 2

The first is perfectly safe. The ConcurrentDictionary is thread safe, so getting/setting values on different threads at the same time won't cause issues.

Your second is not safe. Just because a ConcurrentDictionary contains a reference to an instance of type X does not make type X thread safe. Neither does it block while accessing the value of any particular key.

Remember,

MyCollection2["key"].Add(1);

is the same as

var list = MyCollection2["key"];
list.Add(1);

It is clear that, in this case, all thread-safe code (ConcurrentDictionary access) has completed, but non-thread safe code (List<T> access) has not.