What is the difference between Dictionary and ConcurrentDictionary?

Dictionary<TKey, TValue> is not thread-safe. Concurrent access from multiple threads for reading and writing can lead to exceptions or undefined behavior. It is optimized for single-threaded use or when thread management is handled externally.

ConcurrentDictionary<TKey, TValue> is thread-safe. It is designed for concurrent use and provides safe methods for adding, updating, and retrieving items from multiple threads simultaneously without the need for external synchronization. It uses internal locking mechanisms (notably, segmentation), which minimizes conflicts between threads.