System.Linq.Aggregate - .NET API Documentation

Summary

Applies an accumulator function over a sequence. The specified seed value is used as the initial accumulator value.

Method Signature

public static TSource Aggregate<TSource>(this IEnumerable<TSource> source, Func<TSource, TSource> func);

public static TAccumulate Aggregate<TSource, TAccumulate>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func);

public static TResult Aggregate<TSource, TAccumulate, TResult>(this IEnumerable<TSource> source, TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector);

Parameters

source: An IEnumerable<TSource> to aggregate over.
func: A function to iterate over each element of the sequence.
seed: The initial accumulator value.
resultSelector: A transform function to apply to the final accumulator value.

Returns

TSource: The transformed accumulator value.
TAccumulate: The transformed accumulator value.
TResult: The transformed accumulator value.

Remarks

The Aggregate method performs the following operations:

Initializes the accumulator to the seed value.
For each element in the sequence, applies the func delegate to the current accumulator value and the element, and then updates the accumulator with the result.
After iterating through all elements, if resultSelector is provided, it applies this delegate to the final accumulator value to produce the result. Otherwise, the final accumulator value is returned.

If the source sequence is empty, Aggregate returns the seed value. If func is null, an ArgumentNullException is thrown.

Example


using System;
using System.Collections.Generic;
using System.Linq;

public class Example
{
    public static void Main(string[] args)
    {
        List<int> numbers = new List<int> { 1, 2, 3, 4, 5 };

        // Example 1: Sum of numbers
        int sum = numbers.Aggregate(0, (current, next) => current + next);
        Console.WriteLine($"Sum: {sum}"); // Output: Sum: 15

        // Example 2: Concatenate numbers into a string
        string concatenated = numbers.Aggregate("", (current, next) => current + next.ToString());
        Console.WriteLine($"Concatenated: {concatenated}"); // Output: Concatenated: 12345

        // Example 3: Using resultSelector to find the average
        double average = numbers.Aggregate(0, (current, next) => current + next, total => (double)total / numbers.Count);
        Console.WriteLine($"Average: {average}"); // Output: Average: 3

        // Example 4: More complex aggregation (e.g., building a string with separators)
        string csvString = numbers.Aggregate("Numbers: ", (current, next) => current + next + ", ", result => result.TrimEnd(','));
        Console.WriteLine($"CSV String: {csvString}"); // Output: CSV String: Numbers: 1, 2, 3, 4, 5
    }
}

.NET API

Aggregate Method