Enumerable.Where()

using System.Linq;

// Use a lambda expression to filter elements by a condition.
IEnumerable<T> result = source.Where(x => condition);

// Indexed overload (gives access to both the element and its position).
IEnumerable<T> result = source.Where((x, i) => condition);

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

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

// Extract only even numbers.
IEnumerable<int> evens = numbers.Where(n => n % 2 == 0);
Console.WriteLine(string.Join(", ", evens)); // 2, 4, 6, 8, 10

// Extract elements greater than 5.
IEnumerable<int> bigOnes = numbers.Where(n => n > 5);
Console.WriteLine(string.Join(", ", bigOnes)); // 6, 7, 8, 9, 10

// Combine multiple conditions (even and greater than 5).
IEnumerable<int> filtered = numbers.Where(n => n % 2 == 0 && n > 5);
Console.WriteLine(string.Join(", ", filtered)); // 6, 8, 10

// Filter a list of strings by a specific condition.
List<string> fruits = new List<string> { "apple", "banana", "cherry", "apricot" };
IEnumerable<string> aFruits = fruits.Where(f => f.StartsWith("a"));
Console.WriteLine(string.Join(", ", aFruits)); // apple, apricot

// Use the indexed overload to get only elements at even indices.
IEnumerable<int> evenIdx = numbers.Where((n, i) => i % 2 == 0);
Console.WriteLine(string.Join(", ", evenIdx)); // 1, 3, 5, 7, 9

// The same result using query syntax (SQL-like).
IEnumerable<int> query = from n in numbers
                          where n % 2 == 0
                          select n;
Console.WriteLine(string.Join(", ", query)); // 2, 4, 6, 8, 10

Where() uses lazy evaluation, so the sequence is not materialized when the method is called. Processing occurs only when the sequence is actually enumerated — for example, by ToList() or a foreach loop.

Where() only filters; it never modifies the original sequence. To use the filtered result as a list, chain a call to ToList().

You can also chain multiple Where() calls to combine conditions. When you need to transform elements while filtering, it is common to combine Where() with Enumerable.Select().