linq

LINQ, or Language-Integrated Query, is a feature of the .NET Framework that embeds query capabilities directly into programming languages such as C# and VB.NET. It enables developers to write queries against a variety of data sources using a consistent, strongly typed syntax, reducing the need for separate query languages and boilerplate data access code.

LINQ supports two syntax styles: query syntax, which resembles SQL with from, where, select, and orderby clauses, and method syntax, which uses fluent extension methods like Where, Select, and OrderBy. Queries operate on sequences, represented by IEnumerable<T> for in-memory collections or IQueryable<T> for remote data sources. Execution is typically deferred until the results are enumerated, allowing query composition and potential optimization by providers.

A core feature of LINQ is the standard query operators, implemented as extension methods in the System.Linq namespace. These operators enable filtering, projection, sorting, grouping, joining, and aggregation in a uniform manner across data sources. LINQ also relies on providers to translate queries into source-specific operations. Common providers include LINQ to Objects for in-memory data, LINQ to Entities (used with Entity Framework) for relational databases, LINQ to XML for XML data, and LINQ to DataSet. Expression trees underpin IQueryable providers, capturing the query for translation and execution by the data source.

History and impact: LINQ was introduced with .NET Framework 3.5 in 2007 as part of C# 3.0 and VB.NET upgrades. It popularized typed, declarative querying across diverse data sources and reduced boilerplate code. While powerful, performance and behavior depend on the provider and query translation, and some operators may have limited support or require careful construction to avoid inefficiencies.