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Collections · easy

2. LINQ (Language Integrated Query)

Functional declarative querying over collections.

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LINQ integrates declarative queries directly into C#. Query operators (`Where`, `Select`, `GroupBy`, `Aggregate`) operate lazily over `IEnumerable<T>` via deferred execution.

csharp
var activeUsers = users
    .Where(u => u.IsActive)
    .Select(u => new { u.Id, u.Name })
    .ToList();

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Check your understanding

1. What is deferred execution in LINQ?
The query is not executed until the results are actually iterated over (e.g. via foreach or ToList())The query runs on a background threadThe query executes immediately on declarationQueries are cached permanently
2. What is the difference between IEnumerable<T> and IQueryable<T>?
IEnumerable evaluates in memory; IQueryable builds expression trees translated to SQL by ORMsIEnumerable is async onlyIQueryable is for arrays onlyThey are identical

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