Is it OK to concurrently read a Dictionary?

Reading the fine manual yields:

Dictionary<TKey, TValue> Class

Thread Safety

A Dictionary can support multiple readers concurrently, as long as the collection is not modified. Even so, enumerating through a collection is intrinsically not a thread-safe procedure. In the rare case where an enumeration contends with write accesses, the collection must be locked during the entire enumeration. To allow the collection to be accessed by multiple threads for reading and writing, you must implement your own synchronization.

For modifications and write operations to the dictionary, ConcurrentDictionary uses fine-grained locking to ensure thread safety. (Read operations on the dictionary are performed in a lock-free manner.)

As for:

how does the ConcurrentDictionary serve the R/W by multiple threads?

Reading the fine manual yields:

ConcurrentDictionary<TKey, TValue> Class

Remarks

For modifications and write operations to the dictionary, ConcurrentDictionary uses fine-grained locking to ensure thread safety. (Read operations on the dictionary are performed in a lock-free manner.)