Namespaces

LINQ

System.Linq Namespace

The System.Linq namespace provides the core functionality for Language Integrated Query (LINQ), a powerful feature of the .NET Framework that adds rich querying capabilities directly into the .NET language.

Overview

LINQ allows you to write queries against collections of objects, XML documents, databases, and other data sources in a uniform way. It offers a declarative syntax that makes queries more readable and concise compared to traditional loop-based data manipulation.

LINQ Providers

LINQ itself is a specification. The actual execution of queries depends on the LINQ provider that understands how to translate LINQ queries into operations on a specific data source. Common LINQ providers include:

  • LINQ to Objects: For querying in-memory collections like arrays and lists.
  • LINQ to SQL: For querying relational databases using SQL Server.
  • LINQ to XML: For querying XML documents.
  • Entity Framework: A more modern ORM that supports LINQ queries against relational databases.

LINQ Query Syntax

Query syntax is a set of keywords that resemble SQL query structure, making it intuitive for developers familiar with database querying.

var query = from number in numbers where number % 2 == 0 orderby number descending select number * 2;

LINQ Method Syntax

Method syntax uses extension methods defined on the IEnumerable<T> and IQueryable<T> interfaces. It's often used when query syntax is not sufficient or for chaining operations.

var query = numbers.Where(n => n % 2 == 0) .OrderByDescending(n => n) .Select(n => n * 2);

Key LINQ Extension Methods

Where<TSource>(this IEnumerable<TSource> source, Func<TSource, bool> predicate)

Filters a sequence of values based on a predicate.

Parameters:
  • source: An IEnumerable<T> to filter.
  • predicate: A function to test each element of the sequence for a condition.

Example:

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

Select<TSource, TResult>(this IEnumerable<TSource> source, Func<TSource, TResult> selector)

Projects each element of a sequence into a new form.

Parameters:
  • source: An IEnumerable<T> to project.
  • selector: A transform function to apply to each element.

Example:

var doubledNumbers = numbers.Select(n => n * 2);

OrderBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

Sorts the elements of a sequence in ascending order.

Parameters:
  • source: An IEnumerable<T> whose elements to sort.
  • keySelector: A function to extract a key from each element.

Example:

var sortedNumbers = numbers.OrderBy(n => n);

GroupBy<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector)

Groups the elements of a sequence according to a specified key selector function.

Parameters:
  • source: An IEnumerable<T> whose elements to group.
  • keySelector: A function to extract the keys that group elements.

Example:

var groupedByFirstDigit = numbers.GroupBy(n => n.ToString().Substring(0, 1));

ToList<TSource>(this IEnumerable<TSource> source)

Converts an IEnumerable<T> to a List<T>.

Example:

List<int> numberList = numbers.ToList();

FirstOrDefault<TSource>(this IEnumerable<TSource> source)

Returns the first element of a sequence, or a default value if the sequence contains no elements.

Example:

int firstOrDef = numbers.FirstOrDefault();

This documentation provides a high-level overview of LINQ within the System.Linq namespace. For detailed API specifications and advanced topics, please refer to the official Microsoft documentation.