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Enumerable.Aggregate<TSource> Method (IEnumerable<TSource>, Func<TSource, TSource, TSource>)

.NET Framework 4

Applies an accumulator function over a sequence.

Namespace:  System.Linq
Assembly:  System.Core (in System.Core.dll)
C#
C++
F#
VB
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public static TSource Aggregate<TSource>(
	this IEnumerable<TSource> source,
	Func<TSource, TSource, TSource> func
)

Type Parameters

TSource

The type of the elements of source.

Parameters

source
Type: System.Collections.Generic.IEnumerable<TSource>
An IEnumerable<T> to aggregate over.
func
Type: System.Func<TSource, TSource, TSource>
An accumulator function to be invoked on each element.

Return Value

Type: TSource
The final accumulator value.

Usage Note

In Visual Basic and C#, you can call this method as an instance method on any object of type IEnumerable<TSource>. When you use instance method syntax to call this method, omit the first parameter. For more information, see Extension Methods (Visual Basic) or Extension Methods (C# Programming Guide).
Exception Condition
ArgumentNullException

source or func is null.
InvalidOperationException

source contains no elements.

The Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) method makes it simple to perform a calculation over a sequence of values. This method works by calling func one time for each element in source. Each time func is called, Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) passes both the element from the sequence and an aggregated value (as the first argument to func). The first element of source is used as the initial aggregate value. The result of func replaces the previous aggregated value. Aggregate<TSource>(IEnumerable<TSource>, Func<TSource, TSource, TSource>) returns the final result of func.

To simplify common aggregation operations, the standard query operators also include a general purpose count method, Count, and four numeric aggregation methods, namely Min, Max, Sum, and Average.

The following code example demonstrates how to use Aggregate to build a sentence from an array of strings.

C#
VB
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            string sentence = "the quick brown fox jumps over the lazy dog";

            // Split the string into individual words.
            string[] words = sentence.Split(' ');

            // Prepend each word to the beginning of the 
            // new sentence to reverse the word order.
            string reversed = words.Aggregate((workingSentence, next) =>
                                                  next + " " + workingSentence);

            Console.WriteLine(reversed);

            // This code produces the following output:
            //
            // dog lazy the over jumps fox brown quick the

.NET Framework

Supported in: 4, 3.5

.NET Framework Client Profile

Supported in: 4, 3.5 SP1

Portable Class Library

Supported in: Portable Class Library

Windows 7, Windows Vista SP1 or later, Windows XP SP3, Windows Server 2008 (Server Core not supported), Windows Server 2008 R2 (Server Core supported with SP1 or later), Windows Server 2003 SP2

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.
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Clarity...
Func<TSource, TSource, TSource>, for clarity, is...

Func<TOldAcumulatorValue, TNextElement, TNewAcumulatorValue>

Note, the last generic parameter in Func<> is the return type (it should have been the first generic parameter, but no sense crying over spilled milk).
History
  • 3/28/2012
  • G1
Dont' Use Below Example
The example below is bad. For one thing, it's an example of a different overload of the Aggregate function, so it's in the wrong section. It also unnecessarily creates new Person objects all over the place. It would be much better to just replace the line using the Aggregate function with: $0$0 $0 $0int aggAgeWithBias = People.Aggregate(-6, (runningTotal, next) => runningTotal + next.Value.Age);$0
History
A more enlightening example.
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace ExampleAggregate
{
    public class Person
    {
        public string Name;
        public int Age;
        public Person(string n, int a)
        {
            Name = n;
            Age  = a;
        }
    }

    class Program
    {
        static void Main(string[] args)
        {
            SortedDictionary<int, Person> People = new SortedDictionary<int, Person>
            { {0,new Person("Anna", 50)},
              {8,new Person("Allan",53)},
              {4,new Person("Nina", 63)}
            };
            int aggAgeWithBias= People.Aggregate(new Person("", -6), (runningTotal, next) => new Person("", runningTotal.Age + next.Value.Age),result => result.Age);
            Console.WriteLine("Aggregate Age: " + aggAgeWithBias.ToString());
        }
    }
}

Output:
Aggregate Age: 160

The 1st parameter is the initial element used for the 1st running total, which then contains the intermediate results generated by the calculation performed on each element of the collection presented to the operation as operand 'next'.

The 'SortedDictionary' is used as too many examples in MSDN just use List<>.
History
A better example is required.
An example closer to reality would be of more help.
e.g. Produce an aggregate age from a collection of 'People'.
History
Make it easier to read/follow
maybe the following might be a lot easier to read...

string reversed = words.Aggregate((resultingSentence, currentWord) => currentWord + " " + resultingSentence);



History