A query is an expression that retrieves data from a data source. Queries are usually expressed in a specialized query language, such as SQL for relational databases and XQuery for XML. Therefore, developers have had to learn a new query language for each type of data source or data format that they query. Language-Integrated Query (LINQ) offers a simpler, consistent model for working with data across various kinds of data sources and formats. In a LINQ query, you always work with programming objects.
A LINQ query operation consists of three actions: obtain the data source or sources, create the query, and execute the query.
Data sources that implement the IEnumerable<T> generic interface or the IQueryable<T> generic interface can be queried through LINQ. Instances of the generic ObjectQuery<T> class, which implements the generic IQueryable<T> interface, serve as the data source for LINQ to Entities queries. The ObjectQuery<T> generic class represents a query that returns a collection of zero or more typed objects. You can also let the compiler infer the type of an entity by using the C# keyword
var (Dim in Visual Basic).
In the query, you specify exactly the information that you want to retrieve from the data source. A query can also specify how that information should be sorted, grouped, and shaped before it is returned. In LINQ, a query is stored in a variable. If the query returns a sequence of values, the query variable itself must be a queryable type. This query variable takes no action and returns no data; it only stores the query information. After you create a query you must execute that query to retrieve any data.
LINQ to Entities queries can be composed in two different syntaxes: query expression syntax and method-based query syntax. Query expression syntax is new in C# 3.0 and Visual Basic 9.0, and it consists of a set of clauses written in a declarative syntax similar to Transact-SQL or XQuery. However, the .NET Framework common language runtime (CLR) cannot read the query expression syntax itself. Therefore, at compile time, query expressions are translated to something that the CLR does understand: method calls. These methods are known as the standard query operators. As a developer, you have the option of calling them directly by using method syntax, instead of using query syntax. For more information, see Query Syntax and Method Syntax in LINQ.
Query Expression Syntax
Query expressions are a declarative query syntax. This syntax enables a developer to write queries in a high-level language that is formatted similar to Transact-SQL. By using query expression syntax, you can perform even complex filtering, ordering, and grouping operations on data sources with minimal code. For more information, Basic Query Operations (Visual Basic). For examples that demonstrate how to use the query expression syntax, see the following topics:
Method-Based Query Syntax
Another way to compose LINQ to Entities queries is by using method-based queries. The method-based query syntax is a sequence of direct method calls to LINQ operator methods, passing lambda expressions as the parameters. For more information, see Lambda Expressions. For examples that demonstrate how to use method-based syntax, see the following topics: