Language Integrated Query (LINQ, pronounced "link") is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages.
LINQ extends the language by the addition of query expressions, which are akin to SQL statements, and can be used to conveniently extract and process data from arrays, enumerable classes, XML documents, relational databases, and third-party data sources. Other uses, which utilize query expressions as a general framework for readably composing arbitrary computations, include the construction of event handlers[1] or monadic parsers.[2]
LINQ also defines a set of method names (called standard query operators, or standard sequence operators), along with translation rules used by the compiler to translate fluent-style query expressions into expressions using these method names, lambda expressions and anonymous types.
Many of the concepts that LINQ has introduced were originally tested in Microsoft's Cω research project. LINQ was released as a major part of .NET Framework 3.5 on November 19, 2007.
https://en.wikipedia.org/wiki/Language_Integrated_Query