C# LINQ入门教程：微软官方指南

LINQ教程

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C# LINQ 教程是一本由 Microsoft Press 出版的专业书籍，它是在2007年发布的一份重要的技术资料。本书旨在介绍 LINQ（Language Integrated Query），这是 Microsoft .NET 框架中的一个关键特性，它允许开发人员在 C# 中使用统一的查询语法处理各种数据源，包括集合、数据库、XML 和其他数据结构。该教程适合希望深入理解和掌握 LINQ 的开发者，无论是初学者还是经验丰富的软件工程师。书中作者包括 Paolo Pialorsi 和 Marco Russo，他们的专业知识确保了内容的专业性和实用性。教程基于2007年的预览版本，并且随着技术的更新和改进，可能包含了一些后续的代码更新。这本教程不仅仅是一份技术指南，也反映了当时微软对 LINQ 技术的最新理解和实践。通过学习本书，读者可以了解到以下核心知识点： 1. **语言集成查询（LINQ）基础**：学习如何在 C# 中使用 LINQ 查询语法，如 Lambda 表达式、方法调用表达式和查询操作符，以及它们如何与集合和数组交互。 2. **数据源支持**：了解如何通过 LINQ 无缝访问多种数据源，如内存中的列表、数组、数据库（SQL Server、ADO.NET）、XML 和 WCF 服务。 3. **高级查询功能**：涵盖更复杂的查询操作，如分组、聚合、排序、子查询和连接，以及 LINQ to Objects 和 LINQ to SQL 的区别。 4. **性能优化**：讨论如何编写高效查询并理解 LINQ 在执行计划和性能方面的考虑。 5. **实际应用示例**：书中提供了丰富的代码示例，展示了如何在实际项目中使用 LINQ 解决问题，包括数据分析、业务逻辑和用户界面设计。 6. **最佳实践和注意事项**：作者会分享关于 LINQ 设计模式、代码组织以及避免常见陷阱的建议。 7. **MSDN 和 IntelliSense**：强调了使用 Microsoft 的官方文档和工具（如 Visual Studio 的 IntelliSense 功能）来获取更多帮助和学习资源。 8. **版本兼容性**：由于是基于 May 2007 的预发布版本，可能会涉及到不同 .NET Framework 版本之间的变化和更新，读者需关注当时的最新版本信息。 C# LINQ 教程是一本不可或缺的技术参考书，对于希望在 C# 开发中提升数据处理能力的开发者来说，它是一个深入理解并熟练运用 LINQ 的宝贵资源。通过阅读和实践教程中的内容，开发者将能够更好地利用 LINQ 提升代码的简洁性和可读性，提高开发效率。

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8 Introducing Microsoft LINQ

LINQ requires you to describe your data in terms of entities that are also types in the lan-

guage. When you build a LINQ query, it is always a set of operations on instances of some

classes. These objects might be the real container of data, or they might be a simple descrip-

tion (in terms of metadata) of the external entity you are going to manipulate. A query can be

sent to a database through an SQL command only if it is applied to a set of types that map

tables and relationships contained in the database. After you have defined entity classes, you

can use both approaches we described (joins and entity relationship navigation). The conver-

sion of all these operations in SQL commands is the responsibility of the LINQ engine.

Note

You can create entity classes by using code-generation tools such as SQLMetal or

the LINQ to SQL Designer in Microsoft Visual Studio.

In Listing 1-3, you can see an example of a Product class that maps a relational table named

Products, with five columns that correspond to public data members.

Listing 1-3 Class declaration mapped on a database table

[Table("Products")]

public class Product {

[Column(IsPrimaryKey=true)] public int IdProduct;

[Column(Name="UnitPrice")] public decimal Price;

[Column()] public string ProductName;

[Column()] public bool Taxable;

[Column()] public decimal Tax;

}

When you work on entities that describe external data (such as database tables), you can

create instances of these kinds of classes and manipulate in-memory objects just as if data

from all tables were loaded in memory. These changes are submitted to the database through

SQL commands when you call the SubmitChanges method, as you can see in Listing 1-4.

Listing 1-4 Database update calling the SubmitChanges method

var taxableProducts =

from p in db.Products

where p.Taxable == true

select p;

foreach( Product product in taxableProducts ) {

RecalculateTaxes( product );

}

db.SubmitChanges();

The Product class in the preceding example represents a row in the Products table of an

external database. When SubmitChanges is called, all changed objects generate an SQL

command to update the corresponding rows in the table.

Chapter 1 LINQ Introduction 9

More Info Class entities that match tables and relationships in the database are further

described in Chapter 5, “LINQ to ADO.NET.”

XML Manipulation

LINQ has a different set of classes and extensions to support the manipulation of XML data.

We will create some examples using the following scenario. Imagine that your customers are

able to send orders using XML files like the ORDERS.XML file shown in Listing 1-5.

Listing 1-5 A fragment of an XML file of orders

<?xml version="1.0" encoding="utf-8" ?>

</orders>

Using standard Microsoft .NET 2.0 System.Xml classes, you can load the file using a DOM

approach or you can parse its contents using an XmlReader implementation. Regardless of the

solution you choose, you must always consider nodes, node types, XML namespaces, and

whatever else is related to the XML world. Many developers do not like working with XML

because it requires the knowledge of another domain of data structures and uses syntax

of its own.

If you need to extract all the products ordered with their quantities, you can parse the orders

file using an XmlReader to accomplish this, as shown in Listing 1-6.

Listing 1-6 Reading the XML file of orders using an XmlReader

String nsUri = "http://schemas.devleap.com/Orders";

XmlReader xmlOrders = XmlReader.Create( "Orders.xml" );

List<Order> orders = new List<Order>();

Order order = null;

while (xmlOrders.Read()) {

switch (xmlOrders.NodeType) {

case XmlNodeType.Element:

if ((xmlOrders.Name == "order") &&

(xmlOrders.NamespaceURI == nsUri)) {

order = new Order();

order.CustomerID = xmlOrders.GetAttribute( "idCustomer" );

order.Product = new Product();

order.Product.IdProduct =

Int32.Parse( xmlOrders.GetAttribute( "idProduct" ) );

order.Product.Price =

Decimal.Parse( xmlOrders.GetAttribute( "price" ) );

12 Introducing Microsoft LINQ

instead of writing this code:

var query =

Customers

.Where( c => c.Country == "Italy" );

.OrderBy( c => c.Name )

.Select( c => new { c.Name, c.City } );

Many people call this simplification syntax sugaring because it is just a simpler way to write

code that defines a query over data. However, there is more to it than that. Many language con-

structs and syntaxes are necessary to support what seems to be just a few lines of code that

query data. Under the cover of this simple query expression are local type inference, exten-

sion methods, lambda expressions, object initialization expressions, and anonymous types.

All of these features are useful by themselves, but if you look at the overall picture you should

see that there is a big step in two directions: one moving to a more declarative style of coding,

and one lowering the impedance mismatch between data and code.

Declarative Programming

What are the differences between an SQL query and an equivalent C# 2.0 or Visual Basic 8.0

program that filters data contained in native storage (such as a table for SQL or an array for

C# or Visual Basic)?

In SQL, you can write the following:

SELECT * FROM Customers WHERE Country = 'Italy'

In C#, you would probably write this:

public List<Customer> ItalianCustomers( Customer customers[] )

{

List<Customer> result = new List<Customer>();

foreach( Customer c in customers ) {

if (c.Country == "Italy") result.Add( c );

}

return result;

}

C# code takes longer to write and read. But the most important consideration is expressivity.

In SQL, you describe what you want. In C#, you describe how to obtain the expected result. In

SQL, the selection of the best algorithm to implement the how (which is more explicitly dealt

with in C#) is the responsibility of the query engine. This is because the SQL query engine has

more freedom to apply optimizations than a C# compiler, which has many more constraints

on the how.

Note

The previous C# code sample could be written in .NET 2.0 using a Find predicate,

but for many developers the required anonymous delegate syntax is not very easy to use.

However, the samples are merely illustrative of the different programming paradigms.

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