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Tip #009

Sum per group with AggregateBy

.NET 9

Code card: Sum per group with AggregateBy. Program output: Cat: 35 / Dog: 259

About this tip

AggregateBy groups and aggregates in one pass. No GroupBy + Select + Sum pipeline. Total weight per species without GroupBy().Select(Sum): .NET 9 AggregateBy groups and folds in one pass. It is available since .NET 9. The key line is var totals = pets.AggregateBy( in aggregateby.cs. Run it and the output reads Cat: 35, Dog: 259.

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Code

aggregateby.cs
Pet[] pets = [
    new(Species.Cat, 20), new(Species.Dog, 60),
    new(Species.Dog, 199), new(Species.Cat, 15),
];


var totals = pets.AggregateBy(
    p => p.Species, 0, (sum, p) => sum + p.Weight);


foreach (var (species, weight) in totals)
    Console.WriteLine($"{species}: {weight}");


record Pet(Species Species, int Weight = 0);
enum Species { Unknown, Cat, Dog }

Output

Output
$ dotnet run aggregateby.cs
Cat: 35
Dog: 259