When we use the term language integrated query to indicate that query is an integrated feature of the developer’s primary programming languages (e.g., C#, Visual Basic). Language integrated query allows query expressions to benefit from the rich metadata, compile-time syntax checking, static typing and IntelliSense that was previously available only to imperative code. Language integrated query also allows a single general purpose declarative query facility to be applied to all in-memory information, not just information from external sources.
.NET Language Integrated Query defines a set of general purpose standard query operators that allow traversal, filter, and projection operations to be expressed in a direct yet declarative way in any .NET-based programming language. The standard query operators allow queries to be applied to any IEnumerable<T>-based information source.
LINQ allows third parties to augment the set of standard query operators with new domain-specific operators that are appropriate for the target domain or technology. More importantly, third parties are also free to replace the standard query operators with their own implementations that provide additional services such as remote evaluation, query translation, optimization, etc. By adhering to the conventions of the LINQ pattern, such implementations enjoy the same language integration and tool support as the standard query operators.
The extensibility of the query architecture is used in the LINQ project itself to provide implementations that work over both XML and SQL data. The query operators over XML (XLinq) use an efficient, easy-to-use in-memory XML facility to provide XPath/XQuery functionality in the host programming language.
The query operators over relational data (DLinq) build on the integration of SQL-based schema definitions into the CLR type system. This integration provides strong typing over relational data while retaining the expressive power of the relational model and the performance of query evaluation directly in the underlying store.
