implementing Concurrent Data Access in .NET 7 Using a Singleton Service
I've been banging my head against this for hours. I'm working with a question with concurrent data access while using a singleton service in my .NET 7 Web API. The service is responsible for maintaining an in-memory cache of user sessions, but I keep working with `InvalidOperationException: The collection was modified; enumeration operation may not execute.` when multiple requests hit the API at the same time. Here's a simplified version of my service: ```csharp public class SessionService { private readonly List<string> _sessions = new List<string>(); private readonly object _lock = new object(); public void AddSession(string sessionId) { lock (_lock) { _sessions.Add(sessionId); } } public IEnumerable<string> GetAllSessions() { lock (_lock) { return _sessions.ToList(); // scenario arises here } } } ``` While the `AddSession` method seems to work fine with the lock, I noticed that when multiple requests come in, the `GetAllSessions` method causes the exception when trying to enumerate over `_sessions`. I suspect that calling `ToList()` while other threads are modifying `_sessions` might be the culprit, but I don't know how to refactor this safely. I've tried using `ConcurrentBag<string>` instead of `List<string>`, but my application logic requires maintaining the order of session IDs. How can I resolve this scenario while still adhering to best practices for concurrency in .NET? Any insights or recommendations on the appropriate collection type or locking mechanism would be greatly appreciated! I've been using C# for about a year now. Has anyone dealt with something similar?