C# 13 (.NET 9) introduces three new LINQ methods: Index, CountBy, and AggregateBy. Index adds a 0-based position to each element in a collection. CountBy groups elements by a key and returns the count per group, optionally accepting a custom IEqualityComparer. AggregateBy groups elements by a key and applies an aggregation function with a seed value, similar to the existing Aggregate method but per group. All three methods simplify common data-processing patterns that previously required more verbose LINQ chains.
