LINQ Basics: Where & Select

Syntax

collection.Where(condition).Select(transform)

Examples

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10 };

var evens = numbers.Where(n => n % 2 == 0).ToList();
Console.WriteLine(string.Join(", ", evens));  // 2, 4, 6, 8, 10

List<string> names = new List<string> { "fola", "zain", "jamal" };

var capitalized = names.Select(n => n.ToUpper()).ToList();
Console.WriteLine(string.Join(", ", capitalized));  // FOLA, ZAIN, JAMAL

var lengths = names.Select(n => n.Length).ToList();
Console.WriteLine(string.Join(", ", lengths));  // 4, 4, 5

List<string> words = new List<string> { "apple", "hi", "banana", "ok", "cherry" };

var result = words
    .Where(w => w.Length > 2)
    .Select(w => w.ToUpper())
    .OrderBy(w => w)
    .ToList();

Console.WriteLine(string.Join(", ", result));  // APPLE, BANANA, CHERRY

List<int> numbers = new List<int> { 1, 2, 3, 4, 5, 6 };

var evens = from n in numbers
            where n % 2 == 0
            select n;

Console.WriteLine(string.Join(", ", evens));  // 2, 4, 6

Best practices

• Chain Where() before Select() when doing both, so filtering happens on the smaller, reduced set first for better performance
• Remember LINQ queries are lazy - nothing actually executes until you iterate the result or call ToList()/ToArray()/Count(), etc.
• Prefer method syntax (.Where().Select()) for most code - reach for query syntax (from...where...select) mainly when it reads more naturally, like with joins
• Materialize a query with .ToList() when you need to use the result multiple times, to avoid re-running the query (and its source) each time

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