System.Linq

Summary

The System.Linq namespace contains the core types and methods for Language Integrated Query (LINQ). LINQ enables you to write queries directly in C# or Visual Basic against a variety of data sources, including in-memory collections, databases, XML documents, and more.

Key features include:

• Query Operators: A rich set of methods (like Where, Select, OrderBy, GroupBy) that perform common data manipulation tasks.
• Deferred Execution: Queries are not executed until the results are actually needed, which can improve performance.
• Expression Trees: LINQ to SQL and other LINQ providers translate queries into expression trees that can be compiled and executed against external data sources.

Classes

• Enumerable

  Contains the static extension methods for querying collections that implement IEnumerable<T>.

• Queryable

  Contains the static extension methods for querying objects that implement IQueryable<T>. Used primarily by LINQ providers.

Interfaces

• IGrouping<TKey, TElement>

  Represents a group of elements that have the same key.

• ILookup<TKey, TElement>

  Represents a collection of elements that are indexed by a specified key.

Extension Methods (Enumerable)

The System.Linq.Enumerable class provides a wide range of extension methods for querying sequences.

• Where<TSource>(IEnumerable<TSource>, Func<TSource, bool>)

  Filters a sequence of values based on a predicate.

  var evenNumbers = numbers.Where(n => n % 2 == 0);

• Select<TSource, TResult>(IEnumerable<TSource>, Func<TSource, TResult>)

  Projects each element of a sequence into a new form.

  var names = users.Select(u => u.Name);

• OrderBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>)

  Sorts the elements of a sequence in ascending order.

  var sortedProducts = products.OrderBy(p => p.Price);

• GroupBy<TSource, TKey>(IEnumerable<TSource>, Func<TSource, TKey>)

  Groups the elements of a sequence based on a specified key.

  var groupedByCity = people.GroupBy(p => p.City);