Flatten list of lists using SelectMany List<List<int>> numbers = new() { new() { 1, 2 }, new() { 3, 4 }, new() { 5 } };?

Short answer: var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n);

Example code

var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n); var flat = numbers.SelectMany(n => n);

Real-world example (ShopNest)

ShopNest maps entities to DTOs with .Select(o => new OrderSummaryDto { Id = o.Id, Total = o.Total }) so the API does not leak internal fields.

Say this in the interview

1. Define — one clear sentence (the short answer above).
2. Example — relate it to a project like ShopNest or your real work.
3. Trade-off — when you would not use it.