Summary

Conclusions

LINQ lets you describe queries over sequences declaratively: the operators Where, Select, OrderBy, GroupBy, Join, and others are extension methods for IEnumerable<T> that accept lambda expressions, and query syntax compiles into the same calls. Most operators use deferred execution: a query processes data during iteration, sees changes to the source, and runs again on each iteration; ToList and ToArray capture the result. Element operators react differently to an empty sequence and duplicates, aggregate operators calculate totals, GroupBy and joins build reports from several sources, and the new .NET 9–10 operators (CountBy, AggregateBy, Index, LeftJoin) shorten typical queries. Internally, LINQ is built on iterators, so your own operators are created the same way.

Self-check questions

1. What is LINQ? Which interface is LINQ to Objects built on?
2. How do method syntax and query syntax differ?
3. What are Where, Select, and SelectMany used for?
4. What is an anonymous type? What are its limitations?
5. How do you sort a sequence by several keys?
6. What is deferred execution? Which operators execute immediately?
7. How do First, FirstOrDefault, and Single differ?
8. How does MaxBy differ from Max?
9. How does GroupBy work? What is IGrouping<TKey, TElement>?
10. What do CountBy and AggregateBy return?
11. How does Join differ from GroupJoin and LeftJoin?
12. How do you implement pagination with LINQ?
13. How do you implement your own LINQ operator?
14. Why is Any() better than Count() > 0 for checking whether there are elements?

Useful links

• LINQ: https://learn.microsoft.com/dotnet/csharp/linq/
• LINQ queries: https://learn.microsoft.com/dotnet/csharp/linq/get-started/introduction-to-linq-queries
• Standard query operators: https://learn.microsoft.com/dotnet/csharp/linq/standard-query-operators/
• The Enumerable class: https://learn.microsoft.com/dotnet/api/system.linq.enumerable
• What's new in .NET 10 libraries: https://learn.microsoft.com/dotnet/core/whats-new/dotnet-10/libraries