C# Aggregate Method

Aggregate applies a method to each element. It applies a function to each successive element. With this extension method, we act upon the aggregate of all previous elements. This makes certain methods, such as sum, possible.

Example. There are two example calls to the Aggregate method here. The first call uses a Func that specifies that the accumulation should be added to the next element. This simply results in the sum of all the elements in the array.


However: The second call is more interesting. It multiplies the accumulation with the next element.

Based on:

.NET 4.5

C# program that calls Aggregate method

using System;
using System.Linq;

class Program
    static void Main()
	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

	result = array.Aggregate((a, b) => b * a);
	// 1 * 2 = 2
	// 2 * 3 = 6
	// 6 * 4 = 24
	// 24 * 5 = 120



Func. The Aggregate method receives a higher-order procedure of type Func. The Func receives two arguments and returns a third. This is the accumulator function. Funcs can be used in many programs.


Extensions. Aggregate is the first method listed in Visual Studio on many collections (like arrays). This is confusing for those new to the C# language. Visual Studio is presenting extension methods.

So: The list of methods, starting with Aggregate, are methods from a namespace called System.Linq.

Extension Method

They are not specific to arrays or Lists. Instead, these extension methods act upon any collection that implements the IEnumerable interface. We can use these methods to make certain tasks simpler.


However: In my experience, Aggregate is one of the least useful extensions. Try using ToArray or ToList to gain familiarity.


Summary. Aggregate is a declarative way of applying an accumulator function to a collection of elements. This means you can multiply or add all elements together. More complex accumulator functions can be used.