How to implement guide with concurrent dictionary in c# when using multiple threads - keynotfoundexception
I'm prototyping a solution and I've been banging my head against this for hours. I'm working with a `KeyNotFoundException` when trying to access values from a `ConcurrentDictionary` while multiple threads are updating it. I'm using .NET 6 and the `ConcurrentDictionary<TKey, TValue>` class to store user session data in a web application. The goal is to update session information from various endpoints simultaneously, but I'm running into issues where sometimes keys seem to be missing. Here’s a minimal example of what I'm doing: ```csharp using System.Collections.Concurrent; using System.Threading.Tasks; public class SessionManager { private ConcurrentDictionary<string, string> _sessions = new ConcurrentDictionary<string, string>(); public void AddOrUpdateSession(string sessionId, string data) { _sessions.AddOrUpdate(sessionId, data, (key, oldValue) => data); } public string GetSessionData(string sessionId) { return _sessions[sessionId]; // This line throws KeyNotFoundException sometimes. } } public class Program { public static async Task Main(string[] args) { SessionManager sessionManager = new SessionManager(); var tasks = new List<Task>(); for (int i = 0; i < 100; i++) { string sessionId = "session" + i; tasks.Add(Task.Run(() => sessionManager.AddOrUpdateSession(sessionId, "data" + i))); } await Task.WhenAll(tasks); // Trying to access data, which sometimes fails for (int i = 0; i < 100; i++) { string sessionId = "session" + i; Console.WriteLine(sessionManager.GetSessionData(sessionId)); } } } ``` I’ve confirmed that all session IDs are unique and should be present by the time I try to access them in `GetSessionData`. The scenario seems to arise under high concurrency. I've also attempted to use `TryGetValue` instead of directly accessing `_sessions[sessionId]` to check for existence, but that doesn't resolve the underlying question either. Is there a better way to handle concurrent updates and access to ensure that I don't run into missing keys? Any insights into debugging this would be appreciated. Any ideas what could be causing this? My development environment is Linux. I've been using C# for about a year now. I'm open to any suggestions.