LINQ Ordering & Grouping

Syntax

collection.OrderBy(keySelector).GroupBy(keySelector)

Examples

List<int> numbers = new List<int> { 5, 1, 4, 2, 3 };

var ascending = numbers.OrderBy(n => n).ToList();
Console.WriteLine(string.Join(", ", ascending));  // 1, 2, 3, 4, 5

var descending = numbers.OrderByDescending(n => n).ToList();
Console.WriteLine(string.Join(", ", descending));  // 5, 4, 3, 2, 1

record Person(string Department, string Name);

List<Person> people = new List<Person>
{
    new Person("Sales", "Fola"),
    new Person("Engineering", "Zain"),
    new Person("Engineering", "Amir")
};

var sorted = people
    .OrderBy(p => p.Department)
    .ThenBy(p => p.Name)
    .ToList();

foreach (var p in sorted) Console.WriteLine($"{p.Department}: {p.Name}");
// Engineering: Amir, Engineering: Zain, Sales: Fola

record Student(string Name, string Grade);

List<Student> students = new List<Student>
{
    new Student("Fola", "A"),
    new Student("Zain", "B"),
    new Student("Jamal", "A")
};

var byGrade = students.GroupBy(s => s.Grade);

foreach (var group in byGrade)
{
    Console.WriteLine($"Grade {group.Key}: {string.Join(", ", group.Select(s => s.Name))}");
}
// Grade A: Fola, Jamal
// Grade B: Zain

Best practices

• Use ThenBy()/ThenByDescending() for secondary sort criteria instead of trying to encode multiple sort keys into one comparison
• Remember each GroupBy() group is itself an IEnumerable<T> with a Key property - iterate or call .ToList() on it just like any other sequence
• Combine GroupBy() with Select() to project each group into a summary (like a count or average) rather than keeping the full group
• OrderBy() returns a new ordered sequence rather than sorting in place - unlike List<T>.Sort(), the original collection is untouched

At a glance

Purpose

Applications on the .NET platform

File extension

.cs

Runs in

.NET runtime

Usually used with

.NET SDK and libraries

Specifications & further reading

• C# language reference

Related C# documentation