When should I use ConcurrentDictionary and Dictionary?

"Use ConcurrentDictionary if you use your dictionary in a lot in code" is kind of vague advice. I don't blame you for the confusion.

ConcurrentDictionary is primarily for use in an environment where you're 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 TryAdd, TryGetValue, TryUpdate, and TryRemove.

For example, consider a typical pattern you might see for working with a normal Dictionary class.

// 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 addressed in "Dictionary locking vs. ConcurrentDictionary".

The biggest reason to use ConcurrentDictionary over the normal Dictionary is thread safety. If your application will get multiple threads using the same dictionary at the same time, you need the thread-safe ConcurrentDictionary, this is particularly true when these threads are writing to or building the dictionary.

The downside to using ConcurrentDictionary without the multi-threading is overhead. All those functions that allow it to be thread-safe will still be there, all the locks and checks will still happen, taking processing time and using extra memory.