LINQ

LINQ是微软.NET Framework的一个组件,为.NET语言提供了内置的数据查询能力。它通过查询表达式扩展了语言,这些表达式类似于SQL语句,可用于方便地从数组、可枚举类、XML文档等提取和处理数据。

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Language Integrated Query

From Wikipedia, the free encyclopedia
Language Integrated Query
Influenced bySQLHaskell

Language Integrated Query (LINQ, pronounced "link") is a Microsoft .NET Framework component that adds native dataquerying capabilities to .NET languages, although ports exist for Java[1]PHPJavaScript and ActionScript.

LINQ extends the language by so-called query expressions, which are akin to SQL statements, and can be used to conveniently extract and process data from arrays, enumerable classesXML documents, relational databases, and third party data sources. Other uses, which utilize query expressions as a general framework for readably composing arbitrary computations, include the construction of event handlers[2] or monadicparsers.[3]

LINQ also defines a set of method names (called standard query operators, or standard sequence operators), along with translation rules used by the compiler to translate query expressions into expressions using these method names, lambda expressions and anonymous types.

Many of the concepts that LINQ has introduced were originally tested in Microsoft's  research project. LINQ was released as a major part of .NET Framework 3.5 on November 19, 2007.

Contents

   [hide

[edit]Architecture of LINQ in the .NET Framework

[edit]Standard Query Operators

In what follows, the descriptions of the operators are based on the application of working with collections.

The set of query operators defined by LINQ is exposed to the user as the Standard Query Operator (SQO) API. The query operators supported by the API are:[4]

Select

The Select operator performs a projection on the collection to select interesting aspects of the elements. The user supplies an arbitrary function, as adelegate or lambda expression, which projects the data members.

Where

The Where operator allows the definition of a set of predicate rules that are evaluated for each object in the collection, while objects that do not match the rule are filtered away. The predicate is supplied to the operator as a delegate.

SelectMany

For a user-provided mapping from collection elements to collections, semantically two steps are performed. First, every element is mapped to its corresponding collection. Second, the result of the first step is flattened by one level. Note: Select and Filter are both implementable in terms of SelectMany, as long as singleton and empty collections are available. The translation rules mentioned above still make it mandatory for a LINQ provider to provide the other two operators.

Sum / Min / Max / Average

These operators optionally take a lambda that retrieves a certain numeric value from each element in the collection and uses it to find the sum, minimum, maximum or average values of all the elements in the collection, respectively. Overloaded versions take no lambda and act as if the identity is given as the lambda.

Aggregate

A generalized Sum / Min / Max. This operator takes a lambda that specifies how two values are combined to form an intermediate or the final result. Optionally, a starting value can be supplied, enabling the result type of the aggregation to be arbitrary. Furthermore, a finalization function, taking the aggregation result to yet another value, can be supplied.

Join / GroupJoin
The Join operator performs an  inner join on two collections, based on matching keys for objects in each collection. It takes two functions as delegates, one for each collection, that it executes on each object in the collection to extract the key from the object. It also takes another delegate via which the user specifies which data elements, from the two matched elements, should be used to create the resultant object. The GroupJoin operator performs a  group join. Like the Select operator, the results of a join are instantiations of a different class, with all the data members of both the types of the source objects, or a subset of them.
Take / TakeWhile
The Take operator selects the first n objects from a collection, while the TakeWhile operator, which takes a predicate, selects those objects that match the predicate.
Skip / SkipWhile
The Skip and SkipWhile operators are complements of Take and TakeWhile - they skip the first n objects from a collection, or those objects that match a predicate (for the case of SkipWhile).
OfType
The OfType operator is used to select the elements of a certain type.
Concat
The Concat operator  concatenates two collections.
OrderBy / ThenBy
The OrderBy operator is used to specify the primary sort ordering of the elements in a collection according to some key. The default ordering is in ascending order, to reverse the order, the  OrderByDescending operator is to be used. ThenBy and  ThenByDescending specifies subsequent ordering of the elements. The function to extract the key value from the object is specified by the user as a delegate.
Reverse
The Reverse operator reverses a collection.
GroupBy
The GroupBy operator takes a delegate that extracts a key value and returns a collection of  IGrouping<Key, Values> objects, for each distinct key value. The IGrouping objects 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  unionintersection and  difference operation on two sequences, respectively.
SequenceEqual
The SequenceEqual operator determines whether all elements in two collections are equal and in the same order.
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 that matches the predicate. An exception is thrown, if none or more than one element 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 elements 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 API also specifies certain operators that convert a collection into another type:[4]

  • AsEnumerable: converts the collection to IEnumerable<T> type.
  • AsQueryable: converts the collection to IQueryable<T> type.
  • ToArray: converts the collection to an array.
  • ToList: converts the collection to IList<T> type.
  • ToDictionary: converts the collection to IDictionary<K, T> type, indexed by the key K.
  • ToLookup: converts the collection to ILookup<K, T> type, indexed by the key K.
  • Cast: converts a non-generic IEnumerable collection to one of IEnumerable<T> by casting each element to type T. Throws an exception for incompatible types.
  • OfType: converts a non-generic IEnumerable collection to one of IEnumerable<T>. Only elements of type T are included.

[edit]Language extensions

While LINQ is primarily implemented as a library for .NET Framework 3.5, it also defines optional language extensions that 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 Oxygene, with other languages like F# and Nemerle having announced preliminary support. The language extensions include:[5]

  • Query syntax: A language is free to choose a query syntax that it will recognize natively. These language keywords must be translated by the compiler to appropriate LINQ method calls.
  • Implicitly typed variables: This enhancement allows variables to be declared without specifying their types. The languages C# 3.0 and Oxygene declare them with the var keyword. In VB9.0, the Dim keyword without type declaration accomplishes the same. Such objects are still strongly typed; for these objects the compiler infers the types of variables via type inference, which allows the results of the queries to be specified and defined without declaring the type of the intermediate variables.
  • Anonymous types: Anonymous types allow classes that 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, as required for Select and Join operators.
  • Lambda expressions: Lambda expressions denote delegates or expression trees, allowing predicates and extraction functions to be written inline with queries.

For example, in the query to select all the objects in a collection with SomeProperty less than 10,

var results =  from c in SomeCollection
               where c.SomeProperty < 10
               select new {c.SomeProperty, c.OtherProperty};
 
foreach (var result in results)
{
        Console.WriteLine(result);
}

the types of variables resultc and results all are inferred by the compiler in accordance to the signatures of the methods eventually used. The basis for choosing the methods is formed by the query expression-free translation result

var results =
     SomeCollection
        .Where(c => c.SomeProperty < 10)
        .Select(c => new {c.SomeProperty, c.OtherProperty});
 
foreach (var result in results)
{
     Console.WriteLine(result.ToString());
}

[edit]LINQ Providers

The C# 3.0 specification defines a so-called Query Expression Pattern along with translation rules from a LINQ expression to an expression in a subset of C# 3.0 without LINQ expressions. The translation thus defined is actually un-typed, which, in addition to lambda expressions being interpretable as either delegates or expression trees, allows for a great degree of flexibility for libraries wishing to expose parts of their interface as LINQ expression clauses. For example,LINQ to Objects works on IEnumerable<T>s and with delegates, whereas LINQ to SQL makes use of the expression trees.

The expression trees are at the core of the LINQ extensibility mechanism, by which LINQ can be adapted for many data sources. 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. If they choose so, the LINQ Providers analyze the expression trees contained in a query in order to generate essential pieces needed for the execution of a query. This can be SQL fragments or any other completely different representation of code as further manipulatable data. 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:

[edit]LINQ to Objects

The LINQ to Objects provider is used for in-memory collections, using the local query execution engine of LINQ. The code generated by this provider refers to the implementation of the standard query operators as defined on the Sequence pattern and allows IEnumerable<T> 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.[6]

[edit]LINQ to XML (formerly called XLINQ)

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.[7]

[edit]LINQ to SQL (formerly called DLINQ)

The LINQ to SQL provider allows LINQ to be used to query SQL Server databases, including SQL Server Compact databases. Since SQL Server data may reside on a remote server, and because SQL Server has its own query engine, LINQ to SQL does not use the query engine of LINQ. Instead, it converts a LINQ query to a SQLquery that is then sent to SQL Server for processing.[8] 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 a mapping framework. The mapping is done by defining classes that correspond to the tables in the database, and containing all or a subset of the columns in the table as data members.[9] The correspondence, along with other relational model attributes such as primary keys, are specified using LINQ to SQL-defined attributes. For example,

[Table(Name="Customers")]
public class Customer
{
     [Column(IsPrimaryKey = true)]
     public int CustID;
 
     [Column]
     public string CustName;
}

This class definition maps to a table named Customers and 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 that can be used to create the mapping between the data schemas in the object as well as the 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.[9]

The mapping is implemented by the DataContext that takes a connection string to the server, and can be used to generate a Table<T> where T is the type to which the database table will be mapped. The Table<T> encapsulates the data in the table, and implements the IQueryable<T> interface, 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.[9]

[edit]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.[10]

[edit]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:

[edit]Performance

Casual or beginning users may struggle with subtleties in the LINQ to Objects features and syntax. A naive implementation can suffer from a serious degradation of performance.[27]

LINQ to XML and LINQ to SQL performance compared to ADO.NET depends on the use case.[28]

[edit]PLINQ

Version 4 of the .NET framework includes PLINQ, or Parallel LINQ, a parallel execution engine for LINQ queries. It defines the IParallelEnumerable<T> interface. If the source collection implements this interface, the parallel execution engine is invoked. The PLINQ engine can execute parts of a query concurrently on multiple threads, providing faster results.[29]

[edit]Other language implementations

  • Saffron is an extension to Java incorporating SQL-like relational expressions. Relations can be in-memory collections, database tables, or other data sources. It was developed independently of LINQ in 2001 by Julian Hyde, who later authored the Mondrian OLAP server.
  • jLinq jLinq is a fully extensible Javascript library that allows you to perform LINQ style queries on arrays of object.
  • jlink-node is a fork of jLinq, ported to a node.js module by Nik Martin. An NPM Package also available.
  • JSINQ is Kai Jäger's JavaScript implementation of LINQ to Objects. Also provides a compiler that translates LINQ-style query expressions into JavaScript code.
  • JSLINQ JSLINQ is yet another Javascript library that allows you to perform LINQ style queries on data.
  • Chris Pietschmann's LINQ to JavaScript is a LINQ implementation that extends JavaScript's Array object with LINQ capabilities.
  • Phinq is Tommy Montgomery's PHP implementation of LINQ.
  • PHPLinq is Maarten Balliauw's PHP implementation of LINQ.
  • Quaere is a Java implementation of LINQ.
  • JaQue is a typesafe Java implementation of LINQ.
  • JaQu a Java implementation of LINQ.
  • Querydsl is a typesafe Java implementation of LINQ.
  • SBQL4J is a Java extension with capabilities of LINQ, based on Stack-Based Approach. It provides type safe queries to Java with powerful engine.
  • hxLINQ is a haXe port of Chris Pietschmann's LINQ to JavaScript.
  • asq is a Python implementation of LINQ-to-objects and Parallel LINQ-to-objects (PLINQ).
  • Embarcadero Prism, also known as Delphi Prism, supports LINQ.

[edit]See also

[edit]References

<think>嗯,用户想了解如何在C#或VB.NET中使用LINQ技术。首先,我需要回忆一下LINQ的基本概念和语法结构。LINQ全称是Language Integrated Query,允许在.NET语言中进行数据查询,支持多种数据源,比如集合、数据库、XML等。用户可能希望知道基本语法和常见操作,比如查询表达式和方法语法。 接下来,用户可能关心如何在C#和VB.NET中分别实现LINQ查询。C#使用类似SQL的语法,而VB.NET的语法更接近自然语言。例如,C#中的from...where...select结构,而VB.NET使用From...Where...Select。需要分别举例说明,并展示两种语言的差异。 然后,用户可能想了解LINQ的实际应用场景,比如操作集合、数据库(通过Entity Framework或LINQ to SQL)、XML处理等。需要提供具体的代码示例,例如筛选集合中的元素、分组、排序,以及连接数据库查询的例子。 另外,用户可能对LINQ的方法语法和查询表达式语法的区别感兴趣。比如,方法语法使用Lambda表达式,而查询表达式更声明式。需要说明两者的选择依据,比如复杂查询时可能更适合用查询表达式,而简单操作用方法语法更简洁。 还需要注意用户提到的引用内容,特别是引用[2]中提到的VB.NET匿名类在Group by多列时需要指定Key的问题。这可能涉及到在LINQ中使用匿名类型进行分组时的注意事项,需要指出在VB.NET中必须将所有分组字段标记为Key,否则无法正确分组。同时,引用[3]提到VB.NET的名称变化和特性,比如My命名空间、泛型等,但LINQ部分需要重点突出。 此外,用户可能想知道LINQ的性能优化、延迟执行与立即执行的区别,以及如何调试LINQ查询。需要简要提及这些方面,并给出相关示例,例如使用.ToList()强制立即执行,或者使用日志记录中间结果。 最后,生成相关问题时要覆盖LINQ的基本概念、语法差异、应用场景、性能优化和常见问题处理。确保问题能够引导用户进一步深入学习,比如如何结合Entity Framework使用LINQ,或者如何处理复杂的分组和连接操作。</think>### LINQ技术详解(C#/VB.NET) LINQ(Language Integrated Query)是.NET框架中实现数据查询的编程模型,支持强类型查询和智能提示。其核心语法分为**查询表达式**和**方法语法**两种形式。 #### 一、基础语法对比 1. **C#实现** ```csharp // 查询表达式 var result = from p in products where p.Price > 100 select p.Name; // 方法语法(Lambda表达式) var result = products.Where(p => p.Price > 100).Select(p => p.Name); ``` 2. **VB.NET实现** ```vbnet ' 查询表达式 Dim result = From p In Products Where p.Price > 100 Select p.Name ' 方法语法 Dim result = Products.Where(Function(p) p.Price > 100).Select(Function(p) p.Name) ``` #### 二、关键应用场景 1. **集合操作**(筛选/分组/排序) ```csharp // 多条件分组(注意VB.NET需指定Key属性[^2]) var groupedData = from item in orders group item by new { item.CustomerID, item.Year } into g select new { g.Key, Total = g.Sum(x => x.Amount) }; ``` 2. **数据库交互**(Entity Framework) ```csharp using (var context = new NorthwindContext()) { var query = from c in context.Customers where c.City == "London" select new { c.Name, c.Phone }; } ``` 3. **XML处理** ```csharp XElement xmlDoc = XElement.Load("data.xml"); var prices = from item in xmlDoc.Elements("Product") where (double)item.Element("Price") > 50 select item.Element("Name").Value; ``` #### 三、技术特性对比 | 特性 | C#实现 | VB.NET实现 | |-----------------|-------------------------------|----------------------------------| | 匿名类型初始化 | `new { Name = "A", Value = 1 }` | `New With {.Name = "A", .Value = 1}` | | 分组多列处理 | 自动识别组合键 | 必须声明所有Key字段[^2] | | 空值处理 | 使用`??`操作符 | 使用`If()`运算符 | #### 四、优化建议 1. **延迟执行机制**:LINQ默认采用延迟加载,使用`.ToList()`或`.ToArray()`可强制立即执行 2. **复合查询**:优先使用`join...on`替代多个`Where`子句 3. **索引利用**:对`List<T>`使用`AsParallel()`启用并行查询 ```vbnet ' VB.NET中的特殊语法[^3] Dim query = Aggregate book In Books Where book.Price > 50 Into Average(book.Price) ```
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