Based on May 2007 Pre-release Code— with Updates Online! INTRODUCING MICROSOFT ® LINQ Paolo Pialorsi Marco Russo PUBLISHED BY Microsoft Press A Division of Microsoft Corporation One Microsoft Way Redmond, Washington 98052-6399 Copyright © 2007 by Microsoft Corporation All rights reserved No part of the contents of this book may be reproduced or transmitted in any form or by any means without the written permission of the publisher Library of Congress Control Number: 2007924645 Printed and bound in the United States of America QWT Distributed in Canada by H.B Fenn and Company Ltd A CIP catalogue record for this book is available from the British Library Microsoft Press books are available through booksellers and distributors worldwide For further information about international editions, contact your local Microsoft Corporation office or contact Microsoft Press International directly at fax (425) 936-7329 Visit our Web site at www.microsoft.com/mspress Send comments to mspinput@microsoft.com Microsoft, Microsoft Press, Excel, IntelliSense, MSDN, SharePoint, SQL Server, Visual Basic, Visual Studio, and Windows are either registered trademarks or trademarks of Microsoft Corporation in the United States and/or other countries Other product and company names mentioned herein may be the trademarks of their respective owners The example companies, organizations, products, domain names, e-mail addresses, logos, people, places, and events depicted herein are fictitious No association with any real company, organization, product, domain name, e-mail address, logo, person, place, or event is intended or should be inferred 7KLVERRNH[SUHVVHVWKHDXWKRU¶VYLHZVDQGRSLQLRQV7KHLQIRUPDWLRQFRQWDLQHGLQWKLVERRNLVSURYLGHG without any express, statutory, or implied warranties Neither the authors, Microsoft Corporation, nor its resellers, or distributors will be held liable for any damages caused or alleged to be caused either directly or indirectly by this book Acquisitions Editor: Ben Ryan Developmental Editor: Devon Musgrave Project Editor: Valerie Woolley Editorial and Production: John Pierce and Interactive Composition Corporation Body Part No X13-68390 Table of Contents Acknowledgments .vii Introduction ix LINQ Introduction What Is LINQ? How LINQ Works Relational Model vs Hierarchical/Graph Model XML Manipulation Language Integration 11 Declarative Programming 12 Type Checking 14 Transparency Across Different Type Systems 14 LINQ Flavors 14 LINQ to Objects 15 LINQ to ADO.NET 15 LINQ to XML 16 Present State and Future Direction of LINQ 17 Summary 18 C# Language Features 19 C# 2.0 Revisited 19 Generics 19 Delegates 21 Anonymous Methods 23 Enumerators and Yield 25 C# 3.0 Features 29 Local Type Inference 29 Lambda Expressions 31 Extension Methods 36 Object Initialization Expressions 42 What you think of this book? We want to hear from you! Microsoft is interested in hearing your feedback about this publication so we can continually improve our books and learning resources for you To participate in a brief online survey, please visit: www.microsoft.com/learning/booksurvey/ iii iv Table of Contents Anonymous Types 46 Query Expressions 48 Summary 49 Visual Basic 9.0 Language Features 51 Visual Basic 9.0 and Nullable Types 51 Visual Basic 9.0 Features Corresponding to C# 3.0 52 Local Type Inference 52 Extension Methods 54 Object Initialization Expressions 56 Anonymous Types 58 Query Expressions 59 Lambda Expressions 61 Closures 61 Visual Basic 9.0 Features Without C#3.0 Counterparts 62 XML Support 62 Relaxed Delegates 68 C# 3.0 Features Without Visual Basic 9.0 Counterparts 69 The yield Keyword 69 Anonymous Methods 69 Summary 70 LINQ Syntax Fundamentals 71 LINQ Queries 71 Query Syntax 72 Full Query Syntax 74 Query Operators 76 The Where Operator 77 Projection Operators 78 Ordering Operators 80 Grouping Operators 84 Join Operators 86 Set Operators 90 Aggregate Operators 95 Generation Operators 103 Quantifiers Operators 105 Partitioning Operators 107 Table of Contents v Element Operators 109 Other Operators 113 Deferred Query Evaluation and Extension Methods Resolution 115 Deferred Query Evaluation 115 Extension Methods Resolution 116 Conversion Operators 117 AsEnumerable 117 ToArray and ToList 118 ToDictionary 119 ToLookup 120 OfType and Cast 122 Summary 122 LINQ to ADO.NET 123 LINQ to SQL 123 Entities in LINQ to SQL 124 Data Modeling 126 Data Querying 139 Data Update 151 Binding Metadata 157 Differences Between NET and SQL Type Systems 159 LINQ to DataSet 159 Using LINQ to Load a DataSet 160 Using LINQ to Query a DataSet 160 Using LINQ to Query a Typed DataSet 162 Accessing Untyped DataSet Data 162 LINQ to Entities 163 Summary 166 LINQ to XML 167 Introducing LINQ to XML 168 LINQ to XML API 169 XElement 170 XDocument 173 XAttribute 173 XNode 173 XName and XNamespace 175 Chapter LINQ to XML 195 corresponding to the XPath expression provided as an argument, and the latter returns the full list of elements matching the expression Internally, they both use the XPathEvaluator class The following example selects all the customer elements of customers located in Italy: var result = xmlCustomers.XPathSelectElements( "/customer[@country = 'Italy']"); This method is sometimes useful for defining LINQ queries that work on a subset of nodes of the source XML graph—in case we need to filter customer elements by the country attribute value An example is shown in Listing 6-41 Listing 6-41 Sample LINQ query over the result of the XPathSelectElements extension method var ordersOfItalianCustomersFromXml = from c in xmlCustomers.XPathSelectElements( "/customer[@country = 'Italy']") let xName = (String)c.Element("name") let xCity = (String)c.Attribute("city") join o in orders on xName equals o.Name orderby xName select new { Name = xName, City = xCity, IdProduct = o.IdProduct, Quantity = o.Quantity }; Remember that XPath rules are checked at run time, not at compile time Therefore, be careful when defining them within LINQ queries Consider also that like many other LINQ extension methods, the XPath methods we have just evaluated also support deferred query evaluation Once again, keep in mind that every time we use queries defined using these methods, the result is refreshed by rescanning the source XML graph, unless you copy it yourself by using any of the extension methods provided by LINQ queries for doing that (such as ToList, ToArray, ToDictionary, or ToLookup) Summary In this chapter, you saw how to define XML content using the LINQ to XML API—by means of both functional construction and Visual Basic 9.0 XML literals You also saw how to query, traverse, and transform XML nodes graphs using LINQ to XML query extensions Finally, you evaluated the support offered by System.Xml.Linq to XPath, XSD, and XSLT INTRODUCING MICROSOFT LINQ ® Your first look at building data-rich applications using Microsoft LINQ Get a head start using Microsoft Language Integrated Query (LINQ) Two experienced developers give you advance insights and expert guidance for using this unified model for data access You will learn how to write queries in your native programming language—Microsoft Visual Basic® or Visual C#®—using next-generation querying facilities in the Microsoft NET Framework Discover how to: > Query and manipulate different data domains with a unified syntax > Work with local type inference, lambda expressions, and extension methods > Use XML literals, late binding over XML, and relaxed delegates About the Authors: Paolo Pialorsi is a consultant, trainer, and author specializing in developing Web-based and distributed applications He is a founder of DevLeap, a company focused on producing content for the professional developer community Paolo is the author of three books on XML and Web services, and a regular speaker at industry conferences Marco Russo is a founder of DevLeap and a consultant and trainer who specializes in the Microsoft NET Framework and Microsoft SQL Server™ He is an active contributor to developer communities, an avid blogger on Microsoft technologies, and the author of two books on the C# programming language and the common language runtime > Query items using methods and generics in the System.Linq namespace > Apply deferred query evaluation and extension methods resolution > Query relational structures and physical models with LINQ to ADO.NET > Convert queries into native SQL with LINQ to SQL > Define and manage XML content by using LINQ to XML Resource Roadmap Developer Step by Step Hands-on tutorial covering fundamental techniques and features O Practice files on CD O Prepares and informs new-to-topic programmers O Developer Reference Expert coverage of core topics Extensive, pragmatic coding examples O Builds professional-level proficiency with a Microsoft technology O O Get Visual Basic and Visual C# code samples on the Web For system requirements, see the Introduction Advanced Topics O O O Deep coverage of advanced techniques and capabilities Extensive, adaptable coding examples Promotes full mastery of a Microsoft technology For more information, see inside back cover Part No X13-68400 microsoft.com/mspress ISBN-13: 978-0-7356-2391-0 ISBN-10: 0-7356-2391-0 90000 U.S.A $34.99 [Recommended] 780735 623910 Programming/ Microsoft Visual Studio/LINQ ... ix LINQ Introduction What Is LINQ? How LINQ Works ... 14 LINQ Flavors 14 LINQ to Objects 15 LINQ to ADO.NET ... 122 LINQ to ADO.NET 123 LINQ to SQL 123 Entities in LINQ to SQL