If you’ve ever written code that “works fine” in local testing… and then randomly breaks in production under load, there’s a good chance you met this villain:
Shared mutable state + multiple threads.
A normal Dictionary<TKey, TValue> is fast, but it’s not safe when multiple threads read/write at the same time. You can get:
exceptions (“Collection was modified…”)
missing updates
corrupted internal state
values that mysteriously disappear
weird bugs that you cannot reproduce easily
That’s why .NET provides ConcurrentDictionary<TKey, TValue>: a dictionary designed for high-concurrency scenarios.
This blog explains it in a clear way with examples you can actually use.
Dictionary assumes one thread is mutating it at a time. Even if multiple threads are only reading, it’s usually fine—but once writes happen, all bets are off.
