C# - Difficulty with Concurrent Dictionary Locking and Performance in Multi-threaded Environment
I'm working with a performance scenario when using `ConcurrentDictionary` in a multi-threaded environment. Specifically, I'm trying to store and update user sessions in a web application using ASP.NET Core 6.0, but I'm noticing important delays under high load. My current implementation involves a simple `ConcurrentDictionary<string, SessionData>` where `SessionData` is a class containing session details. The question arises when multiple threads attempt to access the dictionary simultaneously. Here's a snippet of my code: ```csharp public class SessionManager { private ConcurrentDictionary<string, SessionData> _sessions = new ConcurrentDictionary<string, SessionData>(); public void AddOrUpdateSession(string sessionId, SessionData data) { _sessions.AddOrUpdate(sessionId, data, (key, oldValue) => data); } public SessionData GetSession(string sessionId) { _sessions.TryGetValue(sessionId, out var sessionData); return sessionData; } } ``` When I run load tests, I sometimes receive a `System.InvalidOperationException` indicating that the collection was modified during enumeration, which I assume is related to the way sessions are being accessed while being modified. I've considered locking the dictionary with a `lock` statement around accesses but Iām concerned about the impact on performance. I've tried optimizing by reducing the number of updates to the session data, but it hasn't resolved the scenario. Also, using `AsParallel()` with LINQ queries on the dictionary doesn't seem to help, as it results in even more exceptions. Is there a best practice for handling this kind of scenario in C#? How can I improve the performance while ensuring thread safety for `ConcurrentDictionary`? Any insights or alternative data structures you recommend would be greatly appreciated. I'm working with C# in a Docker container on Windows 10.