LINQ: .NET Language-Integrated Query
Language Integrated Query
Language Integrated Query (LINQ, pronounced "link") is a Microsoft .NET Framework component that adds native data querying capabilities to .NET languages using a syntax reminiscent of SQL. Many of the concepts that LINQ has introduced were originally tested in Microsoft's Cω research project. LINQ was released as a part of .NET Framework 3.5 on November 19, 2007.
LINQ defines a set of query operators that can be used to query, project and filter data in arrays, enumerable classes, XML, relational database, and third party data sources. While it allows any data source to be queried, it requires that the data be encapsulated as objects. So, if the data source does not natively store data as objects, the data must be mapped to the object domain. Queries written using the query operators are executed either by the LINQ query processing engine or, via an extension mechanism, handed over to LINQ providers which either implement a separate query processing engine or translate to a different format to be executed on a separate data store (such as on a database server as SQL queries). The results of a query are returned as a collection of in-memory objects that can be enumerated.
Language-Integrated Query (LINQ) is a groundbreaking innovation in Visual Studio 2008 and the .NET Framework version 3.5 that bridges the gap between the world of objects and the world of data.
Traditionally, queries against data are expressed as simple strings without type checking at compile time or IntelliSense support. Furthermore, you have to learn a different query language for each type of data source: SQL databases, XML documents, various Web services, and so on. LINQ makes a query a first-class language construct in C# and Visual Basic. You write queries against strongly typed collections of objects by using language keywords and familiar operators. The following illustration shows a partially-completed LINQ query against a SQL Server database in C# with full type checking and IntelliSense support.
In Visual Studio you can write LINQ queries in Visual Basic or C# with SQL Server databases, XML documents, ADO.NET Datasets, and any collection of objects that supports IEnumerable or the generic IEnumerable<(Of <(T>)>) interface. LINQ support for the ADO.NET Entity Framework is also planned, and LINQ providers are being written by third parties for many Web services and other database implementations.
You can use LINQ queries in new projects, or alongside non-LINQ queries in existing projects. The only requirement is that the project target version 3.5 of the .NET Framework.
Language Extensions
While LINQ is primarily implemented as a library for .NET Framework 2.0, it also defines a set of language extensions that can be optionally implemented by languages to make queries a first class language construct and provide syntactic sugar for writing queries. These language extensions have initially been implemented in C# 3.0, VB 9.0 and Chrome, with other languages like F# and Nemerle having announced preliminary support. The language extensions include:[2]
· Query syntax: Languages are free to choose a query syntax, which it will recognize natively. These language keywords must be translated by the compiler to appropriate LINQ method calls. The languages can implement operator reordering and other optimizations at the keyword level.
· Implicitly typed variables: This enhancement allows variables to be declared without specifying their types. The languages C# 3.0 and Chrome declare them with the var keyword. In VB9.0, the use of the Dim keyword without type declaration accomplishes the same declaration. Such objects are still strongly typed; for these objects the compiler uses type inference to infer the type of the variables. This allows the result of the queries to be specified and their result defined without declaring the type of the intermediate variables.
· Anonymous types: Anonymous types allow classes, which contain only data member declarations, to be inferred by the compiler. This is useful for the Select and Join operators, whose result types may differ from the types of the original objects. The compiler uses type inference to determine the fields contained in the classes and generates accessors and mutators for these fields.
· Object Initializer: Object initializers allow an object to be created and initialized in a single scope, this allows creation of delegates that extract fields from an object, create a new object and assign the extracted data to the fields of the new object in a single statement, as is required for Select and Join operators.
· Lambda expressions: Lambda expressions are used to create delegates inline with other code. This allows the predicates and extraction functions to be written inline with the queries.
LINQ Providers
LINQ also defines another interface, IQueryable<T>, which defines the same interfaces to the Standard Query Operators as IEnumerable<T>. However, the concrete implementation of the interface, instead of evaluating the query, converts the query expression, with all the operators and predicates, into an expression tree.[3] The Expression tree preserves the high level structure of the query and can be examined at runtime. The type of the source collection defines which implementation will run - if the collection type implements IEnumerable<T>, it executes the local LINQ query execution engine and if it implements the IQueryable<T> implementation, it invokes the expression tree-based implementation. An extension method is also defined for IEnumerable<T> collections to be wrapped inside an IQueryable<T> collection, to force the latter implementation.
The expression trees are at the core of LINQ extensibility mechanism, by which LINQ can be adapted for any data source. The expression trees are handed over to LINQ Providers, which are data source-specific implementations that adapt the LINQ queries to be used with the data source. The LINQ Providers analyze the expression trees representing the query ("query trees") and generate a DynamicMethod (which are methods generated at runtime) by using the reflection APIs to emit CIL code. These methods are executed when the query is run.[3] LINQ comes with LINQ Providers for in-memory object collections, SQL Server databases, ADO.NET datasets and XML documents. These different providers define the different flavors of LINQ:
LINQ to Objects
The LINQ to Objects provider is used for querying in-memory collections, using the local query execution engine of LINQ. The code generated by this provider refer the implementations of the standard query operators as defined in the Sequence class and allows IEnumerable<T> collections to be queried locally.
LINQ to XML
The LINQ to XML provider converts an XML document to a collection of XElement objects, which are then queried against using the local execution engine that is provided as a part of the implementation of the standard query operator.[4]
LINQ to SQL
The LINQ to SQL provider allows LINQ to be used to query SQL Server databases as well as SQL Server Compact databases. Since SQL Server data resides on a remote server, and because it already includes a querying engine, LINQ to SQL does not use the query engine of LINQ. Instead, it converts a LINQ query to SQL query which is then sent to SQL Server for processing.[5] However, since SQL Server stores the data as relational data and LINQ works with data encapsulated in objects, the two representations must be mapped to one another. For this reason, LINQ to SQL also defines the mapping framework. The mapping is done by defining classes which corresponds to the tables in database, and containing all or a subset of the columns in the table as data members.
The correspondence, along with other relational model attributes such as primary keys are specified using LINQ to SQL-defined attributes.
LINQ to DataSets
The LINQ to SQL provider works only with Microsoft SQL Server databases; to support any generic database, LINQ also includes the LINQ to DataSets, which uses ADO.NET to handle the communication with the database. Once the data is in ADO.NET Datasets, LINQ to Datasets execute queries against these datasets.
Other providers
The LINQ providers can be implemented by third parties for various data sources as well. Several database server specific providers are available from the database vendors. Some of the popular providers include:
· Data Services: LINQ to ADO.NET Data Services
· Entity Framework: LINQ to Entities[
· DbLinq: LINQ to MySQL, PostgreSQL, and Oracle
· Google search: LINQ to Google
· Windows Search: LINQ to System Search