Enumerable.OrderBy() / OrderByDescending()

using System.Linq;

// Sort in ascending order.
IOrderedEnumerable<T> asc  = source.OrderBy(x => key);

// Sort in descending order.
IOrderedEnumerable<T> desc = source.OrderByDescending(x => key);

// Add a second and third sort key (chain after OrderBy).
IOrderedEnumerable<T> multi = source.OrderBy(x => key1)
                                     .ThenBy(x => key2)
                                     .ThenByDescending(x => key3);

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

List<int> numbers = new List<int> { 5, 1, 8, 3, 9, 2 };

// Sort in ascending order.
IEnumerable<int> asc = numbers.OrderBy(n => n);
Console.WriteLine(string.Join(", ", asc)); // 1, 2, 3, 5, 8, 9

// Sort in descending order.
IEnumerable<int> desc = numbers.OrderByDescending(n => n);
Console.WriteLine(string.Join(", ", desc)); // 9, 8, 5, 3, 2, 1

// Sort a list of objects by a property.
var people = new List<(string Name, int Age)>
{
    ("Tanaka", 30),
    ("Suzuki", 25),
    ("Sato",   30),
    ("Yamada", 20),
};

// Sort by age in ascending order.
var byAge = people.OrderBy(p => p.Age);
foreach (var p in byAge)
    Console.WriteLine($"{p.Name}: {p.Age}");

// Sort by age ascending; break ties by name ascending.
var byAgeAndName = people.OrderBy(p => p.Age)
                          .ThenBy(p => p.Name);
foreach (var p in byAgeAndName)
    Console.WriteLine($"{p.Name}: {p.Age}");

// Sort strings by length descending; break ties alphabetically.
List<string> fruits = new List<string> { "fig", "apple", "banana", "kiwi", "plum" };
var sorted = fruits.OrderByDescending(f => f.Length).ThenBy(f => f);
Console.WriteLine(string.Join(", ", sorted)); // banana, apple, kiwi, plum, fig

The OrderBy() family of methods performs a stable sort. This means elements that share the same key value retain their original relative order.

Chaining another OrderBy() call after an existing OrderBy() discards the previous sort. Always use ThenBy() when you need to sort by multiple keys. Also, sorting returns a new sequence and does not modify the original collection.

To retrieve the first or last element after sorting, see Enumerable.First() / Last(). To sort within groups, combine this method with Enumerable.GroupBy().