CSharp-Coder

Learn C# collections performance: List vs Dictionary vs HashSet vs Queue with Big-O costs, benchmarks, and code examples. Pick the right collection today. Choosing the right C# collections is one of the highest-leverage performance decisions you can make in .NET. A List<T> that works fine with 100 items can quietly turn into a bottleneck at 100,000 items, while swapping it for a HashSet<T> or Dictionary<TKey, TValue> can cut lookup time from milliseconds to nanoseconds. This guide compares List vs Dictionary vs HashSet vs Queue in C#, explains the Big-O cost of every common operation, shows runnable code, and gives you a decision checklist you can apply to real projects. Why C# Collections Performance Matters All four collections live in System.Collections.Generic and all of them implement IEnumerable<T> , so from a LINQ perspective they look interchangeable. They are not. Each one is backed by a different data structure, and that structure dictates...