Why should you care?

Interview and production bugs happen when you pick the wrong structure — O(n) lookups in a hot loop instead of a Dictionary.

See it live — copy this example

Run snippets in a .NET console app, LINQPad, or https://dotnetfiddle.net. Write Big O above every solution.

int[] nums = { 3, 1, 4 };
var list = new List<int> { 3, 1, 4 };
list.Add(5);
Console.WriteLine(list.Count);

What happened?

• Arrays have fixed length.
• List grows.
• Count is O(1).
• Choosing List vs array depends on whether size changes.

Practice next

1. Create a console app.
2. Run the snippet.
3. Time adding 100_000 items to List vs resizing logic yourself.
4. Try Span later for advanced work.
5. Print list[0] safely with Count check.

Remember

Structures trade space for time. C# BCL already gives solid defaults. Measure before micro-optimizing.

Hot path lookup

API validates SKUs on every request.

Outcome: You reach for HashSet/Dictionary, not List.Contains in a loop.