C# - How to Handle Concurrency Issues with ConcurrentDictionary When Adding Multiple Values?
I've spent hours debugging this and I've looked through the documentation and I'm still confused about I'm currently using a `ConcurrentDictionary<int, List<string>>` to manage a collection of items grouped by an integer key... The issue arises when I try to add multiple values to the same key concurrently from different threads. I expected `ConcurrentDictionary` to handle this gracefully, but I'm finding that values are sometimes being lost or not added as expected. Here’s a simplified version of what I'm doing: ```csharp ConcurrentDictionary<int, List<string>> myDictionary = new ConcurrentDictionary<int, List<string>>(); void AddItem(int key, string value) { myDictionary.AddOrUpdate(key, new List<string> { value }, (k, oldList) => { oldList.Add(value); return oldList; }); } ``` When I run this code in a loop where multiple threads call `AddItem`, I see that sometimes the `oldList` does not contain all expected items. It seems like it’s possible for the addition of new values to occur in a manner where some entries might be dropped or not reflected properly. I tried using `ToList()` on `oldList` before adding, hoping it would create a snapshot of the list state, but that led to more complications with concurrent modifications. Is there a better way to ensure that all items are added correctly for the same key when multiple threads are trying to update the dictionary simultaneously? I'm using .NET 6.0 and have ensured that I’m following the recommended patterns for concurrency, but this specific case seems tricky. Any insights or best practices would be greatly appreciated! I'm working on a service that needs to handle this. What am I doing wrong? Is this even possible? This is part of a larger microservice I'm building. Any ideas what could be causing this?