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.
IGroupingobjects, for each distinct key value. The
IGroupingobjects can then be used to enumerate all the objects for a particular key value.Distinct: The Distinct operator removes duplicate instances of a key value from a collection. The function to retrieve the key value is to be supplied as a delegate.Union / Intersect / Except: These operators are used to perform a union, intersection and difference operation on two sequences, respectively.EqualAll: The EqualAll operator checks if all elements in two collections are equal.First / FirstOrDefault / Last / LastOrDefault: These operators take a predicate. The First operator returns the first element for which the predicate yields true or throws an exception if nothing matches. The FirstOrDefault operator is like the First operator except that it returns the default value for the element type (usually a null reference) in case nothing matches the predicate. The last operator retrieves the last element to match the predicate, or throws an exception in case nothing matches. The LastOrDefault returns the default element value if nothing matches.Single: The Single operator takes a predicate and returns the element which matches the predicate. An exception is thrown if none or more than one elements match the predicate.ElementAt: The ElementAt operator retrieves the element at a given index in the collection.Any / All / Contains: The Any operator checks if there are any element in the collection matching the predicate. It does not select the element, but returns true for a match. The All operator checks if all elements match the predicate. The Contains operator checks if the collection contains a given value.Count: The Count operator counts the number of elements in the given collection.
The Standard Query Operator also specifies certain operators which converts a collection into other types:
IDictionarytype, indexed by the key K.
ILookuptype, indexed by the key K.
IEnumerablecollection to one of
IEnumerableby casting each element to type
T. Throws an exception for incompatible types.
IEnumerablecollection to one of
IEnumerable. Only elements of type
The query operators are defined in the
IEnumerable interface as generic extension methods, and a concrete implementation is provided in the
Sequence class. As a result, any class which implements the
IEnumerable interface has access to these methods and are queryable. LINQ also defines a set of generic
Func delegates, which define the type of delegates handled by the LINQ query methods. Any function wrapped in a
Func delegate can be used by LINQ. Each of these methods return an
IEnumerable, so the output of one can be used as input to another, resulting in query composability. The functions, however, are lazily evaluated, i.e., the collections are enumerated only when the result is retrieved. The enumeration is halted as soon as a match is found, and the delegates evaluated on it. When a subsequent object in the resultant collection is retrieved, the enumeration of the source collection is continued beyond the element already evaluated. However, grouping operations, like GroupBy and OrderBy, as well as Sum, Min, Max, Average and Aggregate, require data from all elements in collection, and force an eager evaluation. LINQ does not feature a query optimizer and the query operators are evaluated in the order they are invoked. The LINQ methods are compilable in .NET Framework 2.0, as well.
varkeyword. In VB9.0, the use of the
Dimkeyword 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.
For example, in the query to select all the objects in a collection with
SomeProperty less than 10,
the types of variables result, c and results all are inferred by the compiler - assuming
IEnumerable, c will be
SomeClass, results will be
IEnumerable and result will be
SomeOtherClass will be a compiler generated class with only the
OtherProperty properties and their values set from the corresponding clauses of the source objects. The operators are then translated into method calls as:
IQueryable, which defines the same interfaces to the Standard Query Operators as
IEnumerable. 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. 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, it executes the local LINQ query execution engine and if it implements the
IQueryableimplementation, it invokes the expression tree-based implementation. An extension method is also defined for
IEnumerablecollections to be wrapped inside an
IQueryablecollection, 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. 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 collections to be queried locally. Current implementation of LINQ to Objects uses e.g. O(n) linear search for simple lookups, and is not optimised for complex queries.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.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. 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. For example,
Customersand the two data members correspond to two columns. The classes must be defined before LINQ to SQL can be used. Visual Studio 2008 includes a mapping designer which can be used to create the mapping between the data schemas in the object as well as relational domain. It can automatically create the corresponding classes from a database schema, as well as allow manual editing to create a different view by using only a subset of the tables or columns in a table.
DataContextwhich takes a connection string to the server, and can be used to generate a
Tablewhere T is the type that the database table will be mapped to. The
Tableencapsulates the data in the table, and implements the
IQueryableinterface, so that the expression tree is created, which the LINQ to SQL provider handles. It converts the query into T-SQL and retrieves the result set from the database server. Since the processing happens at the database server, local methods, which are not defined as a part of the lambda expressions representing the predicates, cannot be used. However, it can use the stored procedures on the server. Any changes to the result set are tracked and can be submitted back to the database server.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:
IParallelEnumerableinterface. If the source collection implements this interface, the parallel execution engine is invoked. The PLINQ engine executes a query in a distributed manner on a multi-core or multi-processor system.
US Patent Issued to Microsoft on Oct. 19 for "Extending Expression-Based Syntax for Creating Object Instances" (Washington Inventors)
Oct 20, 2010; ALEXANDRIA, Va., Oct. 23 -- United States Patent no. 7,818,719, issued on Oct. 19, was assigned to Microsoft Corp. (Redmond, Wash...