C# Aggregate() method

The Aggregate() method in C# applies an accumulator function over a sequence of elements. It processes each element in the collection and combines them into a single result using a specified function. This method is part of LINQ and provides a powerful way to perform custom aggregation operations.

Syntax

The Aggregate() method has three main overloads −

// Simple aggregation without seed
TSource Aggregate<TSource>(Func<TSource, TSource, TSource> func)

// Aggregation with seed value
TAccumulate Aggregate<TSource, TAccumulate>(TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func)

// Aggregation with seed and result selector
TResult Aggregate<TSource, TAccumulate, TResult>(TAccumulate seed, Func<TAccumulate, TSource, TAccumulate> func, Func<TAccumulate, TResult> resultSelector)

Parameters

• seed − The initial accumulator value (optional).

• func − An accumulator function to apply on each element.

• resultSelector − A function to transform the final accumulator value (optional).

Using Aggregate() with Seed Value

This example finds the longest string from an array, comparing against a seed value −

using System;
using System.Linq;

class Demo {
    static void Main() {
        string[] arr = { "DemoOne", "DemoTwo", "DemoThree", "DemoFour"};
        string res = arr.AsQueryable().Aggregate("DemoFive", (longest, next) => next.Length > longest.Length ? next : longest, str => str.ToLower());
        Console.WriteLine("The string with more number of characters: {0}", res);
    }
}

The output of the above code is −

The string with more number of characters: demothree

Using Aggregate() for Mathematical Operations

The Aggregate() method is commonly used for mathematical calculations like sum, product, or finding maximum values −

using System;
using System.Linq;

class MathOperations {
    static void Main() {
        int[] numbers = { 1, 2, 3, 4, 5 };
        
        // Calculate product of all numbers
        int product = numbers.Aggregate((acc, num) => acc * num);
        Console.WriteLine("Product: " + product);
        
        // Calculate sum with seed value
        int sum = numbers.Aggregate(10, (acc, num) => acc + num);
        Console.WriteLine("Sum with seed 10: " + sum);
        
        // Find maximum value
        int max = numbers.Aggregate((acc, num) => num > acc ? num : acc);
        Console.WriteLine("Maximum: " + max);
    }
}

The output of the above code is −

Product: 120
Sum with seed 10: 25
Maximum: 5

Using Aggregate() for String Concatenation

You can use Aggregate() to build strings by concatenating elements with custom formatting −

using System;
using System.Linq;

class StringAggregation {
    static void Main() {
        string[] words = { "Hello", "World", "from", "C#" };
        
        // Simple concatenation
        string sentence = words.Aggregate((acc, word) => acc + " " + word);
        Console.WriteLine("Sentence: " + sentence);
        
        // With seed and result selector
        string formatted = words.Aggregate("Result:", (acc, word) => acc + " [" + word + "]", result => result.ToUpper());
        Console.WriteLine("Formatted: " + formatted);
    }
}

The output of the above code is −

Sentence: Hello World from C#
Formatted: RESULT: [HELLO] [WORLD] [FROM] [C#]

Conclusion

The Aggregate() method provides flexible aggregation capabilities in C# LINQ. It allows you to apply custom logic to combine elements in a sequence, with optional seed values and result transformation functions for complex scenarios.