New AggregateBy LINQ Method

2026-01-02 19:04 · 👁 2

🚀 What’s New in C#: The New AggregateBy LINQ Method

If you’ve ever written GroupBy(...).Select(g => ...) just to sum, count, or accumulate values by a key, the new AggregateBy LINQ method is for you.

Introduced in .NET 9 / C# 13, AggregateBy allows you to aggregate directly by key without creating intermediate groupings.

🧠 What is AggregateBy?

Think of it as Aggregate, but grouped by a key.

Instead of producing IGrouping<TKey, TElement> like GroupBy, it:

Uses a key selector

Maintains an accumulator per key

Returns (Key, Accumulator) pairs

This makes it ideal for simple and efficient “group + aggregate” scenarios.

✍️ Before vs After ❌ Traditional approach with GroupBy

var totals = sales
    .GroupBy(s => s.Product)
    .Select(g => new 
    { 
        Product = g.Key, 
        Total = g.Sum(x => x.Quantity) 
    })
    .ToDictionary(x => x.Product, x => x.Total);

✅ Using AggregateBy

var totals = sales
    .AggregateBy(
        keySelector: s => s.Product,
        seed: 0,
        func: (acc, s) => acc + s.Quantity)
    .ToDictionary(x => x.Key, x => x.Value);

👉 Key difference: No IGrouping, no projection step — just direct aggregation per key.

🎯 When AggregateBy Really Shines

Totals or counts per key (sales per product, requests per endpoint, errors per type)

Custom accumulators (build DTOs, track min/max, keep last value, etc.)

Large collections where avoiding extra allocations matters

It’s especially useful when you don’t actually need a group — just the result.

#csharp #dotnet #developerexperience #cleanCode