什么时候应该使用 ConcurrentDictionary 和 Dictionary？

ConcurrentDictionary is useful when you need to access a dictionary across multiple threads (i.e. multithreading). Vanilla Dictionary objects do not possess this capability and therefore should only be used in a single-threaded manner.

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.

"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".

A ConcurrentDictionary is useful when you want a high-performance dictionary that can be safely accessed by multiple threads concurrently. Compared to a standard Dictionary protected with a lock, it is more efficient under heavy usage because of its granular locking implementation. Instead of all threads competing for a single lock, the ConcurrentDictionary maintains multiple locks internally, minimizing this way the contention, and limiting the possibility of becoming a bottleneck.

Despite these nice characteristics, the number of scenarios where using a ConcurrentDictionary is the best option is actually quite small. There are two reasons for that:

1. The thread-safety guaranties offered by the ConcurrentDictionary are limited to the protection of its internal state. That's it. If you want to do anything slightly non-trivial, like for example updating the dictionary and another variable as an atomic operation, you are out of luck. This is not a supported scenario for a ConcurrentDictionary. Even protecting the elements it contains (in case they are mutable objects) is not supported. If you try to update one of its values using the AddOrUpdate method, the dictionary will be protected but the value will not. The Update in this context means replace the existing value with another one, not modify the existing value.

2. Whenever you find tempting to use a ConcurrentDictionary, there are usually better alternatives available. Alternatives that do not involve shared state, which is what a ConcurrentDictionary essentially is. No matter how efficient is its locking scheme, it will have a hard time beating an architecture where there is no shared state at all, and each thread does its own thing without interfering with the other threads. Commonly used libraries that follow this principle are the PLINQ and the TPL Dataflow library. Below is a PLINQ example:

Dictionary<string, Product> dictionary = productIDs
.AsParallel()
.Select(id => GetProduct(id))
.ToDictionary(product => product.Barcode);

Instead of creating a dictionary beforehand, and then having multiple threads filling it concurrently with values, you can trust PLINQ to produce a dictionary utilizing more efficient strategies, involving partitioning of the initial workload, and assigning each partition to a different worker thread. A single thread will eventually aggregate the partial results, and fill the dictionary.

The accepted answer above is correct. However, it is worth mentioning explicitly if a dictionary is not being modified i.e. it is only ever read from, regardless of number of threads, then Dictionary<TKey,TValue> is preferred because no synchronization is required.

e.g. caching config in a Dictionary<TKey,TValue>, that is populated only once at startup and used throughout the application for the life of the application.

When to use a thread-safe collection : ConcurrentDictionary vs. Dictionary

If you are only reading key or values, the Dictionary<TKey,TValue> is faster because no synchronization is required if the dictionary is not being modified by any threads.