Aggregate

The C# Aggregate() method applies a method to each element (calling the function on successive elements). We specify the function with a Func (often with lambda syntax).

With this extension method, we act upon the aggregate of all previous elements. This makes certain methods, such as Sum, possible.

Add example

The first call uses a Func that specifies that the accumulation should be added to the next element. This results in the sum of all the elements in the array.

using System;
using System.Linq;

int[] array = { 1, 2, 3, 4, 5 };
int result = array.Aggregate((a, b) => b + a);
// 1 + 2 = 3
// 3 + 3 = 6
// 6 + 4 = 10
// 10 + 5 = 15
Console.WriteLine(result);
15

Multiply example

The second call is more interesting. It multiplies the accumulation with the next element. The difference between adding and multiplying is significant.

using System;
using System.Linq;

int[] array = { 1, 2, 3, 4, 5 };
int result = array.Aggregate((a, b) => b * a);
// 1 * 2 = 2
// 2 * 3 = 6
// 6 * 4 = 24
// 24 * 5 = 120
Console.WriteLine(result);
120

Func

The Aggregate method receives a higher-order procedure of type Func. The Func receives 2 arguments and returns a third. This is the accumulator function.

Extensions

Aggregate() is the first method listed in Visual Studio on many collections (like arrays). Visual Studio is presenting extension methods.

Aggregate() is a declarative way of applying an accumulator function to a collection of elements. It can multiply or add all elements together.