How to create a lookup and use it for quick searches?

Short answer: How to create a lookup and use it for quick searches? var lookup = employees.ToLookup(e => e.Department); var itEmployees = lookup["IT"]; foreach (var emp in itEmployees) Console.WriteLine(emp.Name); Explanation: Lookup is like a dictionary but allows multiple values per key.

Real-world example (ShopNest)

ShopNest uses LINQ to query orders: filter by date, project to a summary DTO, and order by total—readable C# that becomes SQL under EF Core.

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.