C# LINQ Aggregate — Complete Guide

In this tutorial, you'll learn about C# LINQ Aggregate. We cover key concepts, practical examples, and best practices.

You need to combine all elements of a sequence into a single value — concatenating strings, multiplying numbers, or building a comma-separated list. You write a loop with an accumulator variable. The LINQ Aggregate method encapsulates this pattern.

Wrong

var numbers = new[] { 1, 2, 3, 4, 5 };
var product = 1;
foreach (var n in numbers)
{
    product *= n;
}

Output: 120 — works, but uses mutable state and multiple lines.

Right

var numbers = new[] { 1, 2, 3, 4, 5 };
var product = numbers.Aggregate(1, (acc, n) => acc * n);

Output: 120. The seed 1 is the initial accumulator. The lambda (acc, n) => acc * n runs for each element.

Without a seed, the first element becomes the initial accumulator:

var product = numbers.Aggregate((acc, n) => acc * n);
// Same result: 120

Practical example — building a CSV line:

var words = new[] { "apple", "banana", "cherry" };
var csv = words.Aggregate((acc, w) => $"{acc},{w}");
// "apple,banana,cherry"

Prevention

• Use Aggregate for custom accumulations not covered by Sum, Count, Min, Max.
• Use Aggregate with a seed for type-safe accumulation (seed can differ from element type).
• Use Aggregate without seed when the result is the same type as elements.
• Use string.Join instead of Aggregate for string concatenation — it is more efficient.
• Use Aggregate with IQueryable for SQL translation (supported by EF Core for simple operations).
• Avoid Aggregate when readability suffers — a foreach loop is sometimes clearer.

Common Mistakes with linq aggregate

1. Mixing let bindings with <- bindings in do notation, producing type errors
2. Overlapping type class instances that cause GHC to reject the program with ambiguous dispatch errors
3. Non-exhaustive pattern matches that compile with warnings then crash at runtime

These mistakes appear frequently in real-world CSHARP code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Aggregate is used in DodaZIP for computing checksums across archive entries. For more LINQ, visit DodaTech.