While .NET provides powerful thread-safe collections in the System.Collections.Concurrent namespace—such as ConcurrentDictionary<TKey, TValue> and ConcurrentQueue<T> —it notably lacks a built-in ConcurrentList<T> .

If you have ever needed to read and write to a list from multiple background threads simultaneously, you quickly realize that standard collections like List<T> fall apart under concurrent pressure.

In this article, we will build a custom, production-grade ConcurrentList<T> from scratch in C# using ReaderWriterLockSlim , explore why dynamic lists are chosen over fixed arrays, test it with a multithreaded stress test, and analyze the performance costs involved.

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