微软官方LINQ全集：英文版详解

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微软LINQ全集是一本由微软出版社出版的专业技术书籍，专注于Microsoft LINQ（Language Integrated Query）的相关知识。LINQ是.NET框架的一部分，它允许开发人员在C#、VB.NET等.NET语言中以一种声明式的方式编写查询，从而简化数据访问和操作。本书深入探讨了LINQ的核心概念、语法、API以及如何在各种Microsoft产品如SQL Server、Visual Studio和Windows平台上有效应用。书中包含了丰富的内容，包括但不限于： 1. **基础知识**：介绍LINQ的基本原理，如查询上下文、查询表达式、方法语法和扩展方法，让读者对查询操作有清晰的理解。 2. **与.NET框架集成**：详细讲解如何在.NET框架的各个组件（如LINQ to SQL、Entity Framework、LINQ to Objects等）中使用LINQ，以及不同场景下的最佳实践。 3. **SQL Server支持**：展示了如何通过LINQ to SQL和T-SQL查询来操作关系数据库，包括如何设计和管理LINQ to SQL映射文件，以及优化查询性能。 4. **Visual Studio支持**：介绍了Visual Studio中的IntelliSense和MSDN文档如何辅助开发者编写和调试LINQ查询，以及Visual Studio工具集如何增强LINQ体验。 5. **范例和实战**：书中包含大量实例，涵盖了从简单到复杂的不同场景，帮助读者掌握实际编程技巧。 6. **版权和法律信息**：提醒读者关于版权的规定，未经许可不得复制或传播书中的内容，同时也提供了国际版的购买和获取途径。 7. **联系方式**：为读者提供了反馈渠道，以便于分享意见和问题，体现了Microsoft Press致力于为读者提供支持和持续改进的态度。微软LINQ全集是针对.NET开发者的一本必备参考书，它不仅深入浅出地介绍了LINQ技术，还提供了丰富的实践指导，有助于提升开发者的数据处理能力和代码组织能力。无论是初学者还是经验丰富的程序员，都能从中获益匪浅。

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.

Chapter 1 LINQ Introduction 13

LINQ enables a more declarative style of coding for C# and Visual Basic. A LINQ query

describes operations on data through a declarative construct instead of an iterative one. LINQ

allows the intentions of programmers to be made more explicit, and this knowledge of pro-

grammer intent is fundamental to obtaining a higher level of services from the underlying

framework. For example, think about parallelization. An SQL query can be split into several

concurrent operations, simply because it does not place any constraint on the kind of table

scan algorithm applied. A C# foreach loop is harder to split into several loops over different

parts of an array that could be executed in parallel by different processors.

More Info

Parallel LINQ (PLINQ) is a research project on the parallelism that can be

obtained by writing code with LINQ.

Declarative programming can take advantage of services offered by compilers and frame-

works, and in general it is easier to read and maintain. This single “feature” can be the most

important one because it boosts programmers’ productivity. For example, suppose that you

want to get a list of all static methods available in the current application domain that return

an IEnumerable<T> interface. You can use LINQ to write a query over Reflection:

var query =

from assembly in AppDomain.CurrentDomain.GetAssemblies()

from type in assembly.GetTypes()

from method in type.GetMethods()

where method.IsStatic

&& method.ReturnType.GetInterface( "IEnumerable`1" ) != null

orderby method.DeclaringType.Name, method.Name

group method by new { Class = method.DeclaringType.Name,

Method = method.Name };

The equivalent C# code that handles data is longer to write, harder to read, and probably more

error prone. You can see a possible version that is not particularly optimized in Listing 1-10.

Listing 1-10 C# code equivalent to a LINQ query over Reflection

List<String> results = new List<string>();

foreach( var assembly in AppDomain.CurrentDomain.GetAssemblies()) {

foreach( var type in assembly.GetTypes() ) {

foreach( var method in type.GetMethods()) {

if (method.IsStatic &&

method.ReturnType.GetInterface("IEnumerable`1") != null) {

string fullName = String.Format( "{0}.{1}",

method.DeclaringType.Name,

method.Name );

if (results.IndexOf( fullName ) < 0) {

results.Add( fullName );

}

results.Sort();

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