Aggregate method in C#

The Aggregate method in C# is a powerful LINQ extension method that performs a sequential operation on all elements in a collection. It applies an accumulator function over a sequence, allowing you to perform custom aggregations like mathematical operations, string concatenations, or complex data transformations.

Syntax

Following are the main overloads of the Aggregate method −

// Basic syntax - uses first element as seed
public static TSource Aggregate<TSource>(
    this IEnumerable<TSource> source,
    Func<TSource, TSource, TSource> func
);

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

Parameters

• source − The sequence to aggregate over
• func − An accumulator function to be invoked on each element
• seed − The initial accumulator value (optional)

Using Aggregate for Mathematical Operations

Example - Multiplication

Here is how to multiply all elements in an array using the Aggregate method −

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        int[] arr = { 10, 15, 20 };
        
        // Multiplication using Aggregate
        int result = arr.Aggregate((x, y) => x * y);
        Console.WriteLine("Product: " + result);
        
        // Step-by-step: 10 * 15 = 150, then 150 * 20 = 3000
    }
}

The output of the above code is −

Product: 3000

Example - Sum with Seed Value

Using a seed value allows you to start with an initial accumulator −

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        int[] numbers = { 5, 10, 15 };
        
        // Sum with seed value of 100
        int sum = numbers.Aggregate(100, (acc, x) => acc + x);
        Console.WriteLine("Sum with seed: " + sum);
        
        // Finding maximum value
        int max = numbers.Aggregate((x, y) => x > y ? x : y);
        Console.WriteLine("Maximum: " + max);
    }
}

The output of the above code is −

Sum with seed: 130
Maximum: 15

Using Aggregate for String Operations

Example - String Concatenation

using System;
using System.Linq;

public class Demo {
    public static void Main() {
        string[] words = { "Hello", "World", "C#", "Programming" };
        
        // Concatenate with spaces
        string sentence = words.Aggregate((x, y) => x + " " + y);
        Console.WriteLine("Sentence: " + sentence);
        
        // Create CSV format with seed
        string csv = words.Aggregate("Items:", (acc, word) => acc + " " + word + ",");
        Console.WriteLine("CSV: " + csv.TrimEnd(','));
    }
}

The output of the above code is −

Sentence: Hello World C# Programming
CSV: Items: Hello, World, C#, Programming

Common Use Cases

• Mathematical operations − Product, sum, finding min/max values
• String manipulation − Concatenation, building formatted strings
• Custom aggregations − Complex data transformations and calculations
• Functional programming − Reducing collections to single values

Conclusion

The Aggregate method in C# provides a flexible way to perform sequential operations on collections, from simple mathematical calculations to complex data transformations. It's particularly useful when built-in methods like Sum() or Max() don't meet your specific aggregation needs.