Collections and LINQ

Query a collection

LINQ (System.Linq) adds methods like Where, Select and OrderBy to any collection. They read like a pipeline and usually do not change the original list.

A small pipeline

Keep the even numbers, square them, then print. Nothing runs until ToList() or a loop forces the query to execute.

var nums = new List<int> { 1, 2, 3, 4, 5, 6 };
var result = nums.Where(n => n % 2 == 0)
                 .Select(n => n * n)
                 .ToList();
Console.WriteLine(string.Join(", ", result));

Output:

4, 16, 36

Deferred execution surprise

A LINQ query is re-evaluated each time you enumerate it. If it hits a database or is expensive, call ToList() once and reuse the list.

Quick check: Which LINQ method keeps only items matching a condition?

• Select
• OrderBy
• GroupBy
• Where