I'm always confused on which one of these to pick. As I see it I use
"Use ConcurrentDictionary if you use your dictionary a lot in code" is kind of vague advice. I don't blame you for the confusion.
ConcurrentDictionary is primary for use in an environment where you'd updating the dictionary from multiple threads (or async tasks). You can use a standard
Dictionary from as much code as you like if it's from a single thread ;)
If you look at the methods on a ConcurrentDictionary, you'll spot some interesting methods like
For example, consider a typical pattern you might see for working with a normal
// There are better ways to do this... but we need an example ;) if (!dictionary.ContainsKey(id)) dictionary.Add(id, value);
This has an issue in that between the check for whether it contains a key and calling
Add a different thread could call
Add with that same
id. When this thread calls
Add, it'll throw an exception. The method
TryAdd handles that for you and will return a true/false telling you whether it added it (or whether that key was already in the dictionary).
So unless you're working in a multi-threaded section of code, you probably can just use the standard
Dictionary class. That being said, you could theoretically have locks to prevent concurrent access to a dictionary; that question is already address in "Dictionary locking vs. ConcurrentDictionary".