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Language-Integrated Query (LINQ) is the name for a set of technologies based on the integration of query capabilities directly into the C# language. 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. With LINQ, a query is a first-class language construct, just like classes, methods, and events.
When you write queries, the most visible "language-integrated" part of LINQ is the query expression. Query expressions are written in a declarative query syntax. By using query syntax, you perform filtering, ordering, and grouping operations on data sources with a minimum of code. You use the same query expression patterns to query and transform data from any type of data source.
The following example shows a complete query operation. The complete operation includes creating a data source, defining the query expression, and executing the query in a foreach
statement.
// Specify the data source.
int[] scores = [97, 92, 81, 60];
// Define the query expression.
IEnumerable<int> scoreQuery =
from score in scores
where score > 80
select score;
// Execute the query.
foreach (var i in scoreQuery)
{
Console.Write(i + " ");
}
// Output: 97 92 81
You might need to add a using
directive, using System.Linq;
, for the preceding example to compile. The most recent versions of .NET make use of implicit usings to add this directive as a global using. Older versions require you to add it in your source.
foreach
statement.There are two ways you enable LINQ querying of in-memory data. If the data is of a type that implements IEnumerable<T>, you query the data by using LINQ to Objects. If it doesn't make sense to enable enumeration by implementing the IEnumerable<T> interface, you define LINQ standard query operator methods, either in that type or as extension methods for that type. Custom implementations of the standard query operators should use deferred execution to return the results.
The best option for enabling LINQ querying of a remote data source is to implement the IQueryable<T> interface.
LINQ providers that implement IQueryable<T> can vary widely in their complexity.
A less complex IQueryable
provider might access a single method from a Web service. This type of provider is very specific because it expects specific information in the queries that it handles. It has a closed type system, perhaps exposing a single result type. Most of the execution of the query occurs locally, for example by using the Enumerable implementations of the standard query operators. A less complex provider might examine only one method call expression in the expression tree that represents the query, and let the remaining logic of the query be handled elsewhere.
An IQueryable
provider of medium complexity might target a data source that has a partially expressive query language. If it targets a Web service, it might access more than one method of the Web service and select which method to call based on the information that the query seeks. A provider of medium complexity would have a richer type system than a simple provider, but it would still be a fixed type system. For example, the provider might expose types that have one-to-many relationships that can be traversed, but it wouldn't provide mapping technology for user-defined types.
A complex IQueryable
provider, such as the Entity Framework Core provider, might translate complete LINQ queries to an expressive query language, such as SQL. A complex provider is more general because it can handle a wider variety of questions in the query. It also has an open type system and therefore must contain extensive infrastructure to map user-defined types. Developing a complex provider requires a significant amount of effort.
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Events
17 Mar, 9 pm - 21 Mar, 10 am
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