How to Use LINQ Sum in C# for Simple and Complex Collections

Use LINQ Sum in C# to add all numeric values in a collection easily by calling Sum() on the collection or using a selector function for complex types. It returns the total sum as a single number.

Syntax

The Sum() method can be used in two main ways:

• Simple collection: collection.Sum() sums all numeric elements.
• Complex collection: collection.Sum(item => item.Property) sums a specific numeric property from each item.

This method works on collections like arrays, lists, or any IEnumerable<T> where T is numeric or you provide a selector.

csharp

var numbers = new List<int> {1, 2, 3, 4};
int total = numbers.Sum();

var people = new List<Person> {
    new Person { Age = 20 },
    new Person { Age = 30 }
};
int ageSum = people.Sum(p => p.Age);

Example

This example shows how to sum simple numbers and sum a property from objects in a list using LINQ Sum().

csharp

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

class Person
{
    public int Age { get; set; }
}

class Program
{
    static void Main()
    {
        var numbers = new List<int> { 5, 10, 15 };
        int totalNumbers = numbers.Sum();
        Console.WriteLine($"Sum of numbers: {totalNumbers}");

        var people = new List<Person>
        {
            new Person { Age = 25 },
            new Person { Age = 35 },
            new Person { Age = 40 }
        };
        int totalAge = people.Sum(p => p.Age);
        Console.WriteLine($"Sum of ages: {totalAge}");
    }
}

Output

Sum of numbers: 30
Sum of ages: 100

Common Pitfalls

Common mistakes when using Sum() include:

• Calling Sum() on an empty collection without a default value can cause an exception for nullable types.
• Using Sum() on non-numeric types without a selector causes compile errors.
• For complex objects, forgetting to provide a selector function to specify which property to sum.

Always ensure the collection contains numeric values or use a selector to extract numeric properties.

csharp

var emptyList = new List<int>();
// int sumEmpty = emptyList.Sum(); // This returns 0 safely for int

var people = new List<Person>();
// int sumAge = people.Sum(); // Compile error: no parameterless Sum for Person

// Correct way:
int sumAge = people.Sum(p => p.Age);

Quick Reference

Usage	Description	Example
Sum simple numbers	Adds all numbers in a numeric collection	numbers.Sum()
Sum property values	Adds a numeric property from objects	people.Sum(p => p.Age)
Empty collection	Returns 0 for value types	emptyList.Sum()
Nullable types	Returns null if all elements are null	nullableList.Sum()

Key Takeaways

Use Sum() to add all numeric values in a collection quickly.

For collections of objects, provide a selector function to sum a specific numeric property.

Calling Sum() on empty collections returns 0 for value types safely.

Avoid using Sum() on non-numeric collections without a selector to prevent errors.

LINQ Sum works on any IEnumerable<T> with numeric types or selectors.