C# in Depth

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Query expressionsand LINQ to ObjectsThis chapter covers■■■■■■■Streaming sequences of dataDeferred executionStandard query operatorsQuery expression translationRange variables and transparent identifiersProjecting, filtering, and sortingJoining and groupingYou may well be tired of all the hyperbole around LINQ by now. We’ve seen someexamples in chapters 1 and 3, and you’ve almost certainly read some examples andarticles on the Web. This is where we separate myth from reality:■■■LINQ isn’t going to turn the most complicated query into a one-liner.LINQ isn’t going to mean you never need to look at raw SQL again.LINQ isn’t going to magically imbue you with architectural genius.275Licensed to Rhona Hadida
276 CHAPTER 11 Query expressions and LINQ to ObjectsGiven all that, LINQ is still going to change how most of us think about code. It’s not asilver bullet, but it’s a very powerful tool to have in your development armory. We’llexplore two distinct aspects of LINQ: the framework support, and the compiler translationof query expressions. The latter can look odd to start with, but I’m sure you’lllearn to love them.Query expressions are effectively “preprocessed” by the compiler into “normal”C# 3, which is then compiled in a perfectly ordinary way. This is a neat way of integratingqueries into the language without changing its semantics all over the place:it’s syntactic sugar in its purest form. Most of this chapter is a list of the preprocessingtranslations performed by the compiler, as well as the effects achieved when theresult uses the Enumerable extension methods.You won’t see any SQL or XML here—all that awaits us in chapter 12. However, withthis chapter as a foundation you should be able to understand what the more excitingLINQ providers do when we meet them. Call me a spoilsport, but I want to take awaysome of their magic. Even without the air of mystery, LINQ is still very cool.First let’s consider what LINQ is in the first place, and how we’re going to explore it.11.1 Introducing LINQA topic as large as LINQ needs a certain amount of background before we’re ready tosee it in action. In this section we’ll look at what LINQ is (as far as we can discern it), afew of the core principles behind it, and the data model we’re going to use for all theexamples in this chapter and the next. I know you’re likely to be itching to get intothe code, so I’ll keep it fairly brief. Let’s start with an issue you’d think would be quitestraightforward: what counts as LINQ?11.1.1 What’s in a name?LINQ has suffered from the same problem that .NET had early in its life: it’s a termthat has never been precisely defined. We know what it stands for: Language INtegratedQuery—but that doesn’t actually help much. LINQ is fundamentally about datamanipulation: it’s a means of accessing data from different sources in a unified manner.It allows you to express logic about that manipulation in the language of yourchoice—C# in our case—even when the logic needs to be executed in an entirely differentenvironment. It also attempts to eliminate (or at least vastly reduce) the impedancemismatch 1 —the difficulties introduced when you need to integrate two environmentswith very different data models, such as the object-oriented model of .NET and therelational data model of SQL.All of the C# 3 features we’ve seen so far (with the exception of partial methodsand automatic properties) contribute to the LINQ story, so how many of these do youneed to use before you can consider yourself to be using LINQ? Do you need to beusing query expressions, which are the final genuine language feature in C# 3? If you use1http://en.wikipedia.org/wiki/Object-Relational_impedance_mismatch explains this for the specific objectrelationalcase, but impedance mismatches are present in other situations, such as XML representations andweb services.Licensed to Rhona Hadida
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C# in DepthLicensed to Rhona Hadida
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C# in DepthJON SKEETMANNINGGreenwic
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Licensed to Rhona Hadida
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Licensed to Rhona Hadida
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xCONTENTS1.3 The .NET platform 24Di
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xiiCONTENTS5.4 Inline delegate acti
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xivCONTENTS9.4 Changes to type infe
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xviCONTENTS13.2 Delegation as the n
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xviiiFOREWORDOf the latter kind, th
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xxPREFACEdidn’t start using C#2 u
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xxiiACKNOWLEDGMENTSThe aforemention
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xxivABOUT THIS BOOKwant to become e
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xxviABOUT THIS BOOKSQL statements.
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about the cover illustrationThe cap
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xxxCOMMENTS FROM THE TECH REVIEWThi
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Chapter 1 presents a bird’s-eye v
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4 CHAPTER 1 The changing face of C#
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10 CHAPTER 1 The changing face of C
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Core foundations:building on C# 1Th
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34 CHAPTER 2 Core foundations: buil
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C# 2 and 3: new features on a solid
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C# 2 and 3: new features on a solid
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Summary59Listing 2.7 shows a number
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Part 2C#2:solving the issuesof C#1I
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Parameterizedtyping with genericsTh
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Simple generics for everyday use65o
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Simple generics for everyday use67I
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Simple generics for everyday use69t
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Simple generics for everyday use71N
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Simple generics for everyday use73}
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Beyond the basics75parameters. You
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Beyond the basics77NOTEC# vs. CLI s
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Beyond the basics79combined togethe
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Beyond the basics813.3.3 Implementi
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Beyond the basics83derive from a pa
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Advanced generics85implement IEquat
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Advanced generics87within another,
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Advanced generics89ArrayListobject[
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Advanced generics91}{}return new Co
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Advanced generics93Listing 3.10Usin
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Advanced generics95REFLECTING GENER
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Generic collection classes in .NET
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Limitations of generics in C# and o
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Summary111My personal opinion is th
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What do you do when you just don’
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System.Nullable and System.Nullable
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C# 2’s syntactic sugar for nullab
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Novel uses of nullable types131part
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Novel uses of nullable types133else
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Novel uses of nullable types135}{}i
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Fast-tracked delegatesThis chapter
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Saying goodbye to awkward delegate
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Covariance and contravariance141met
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Covariance and contravariance143som
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Inline delegate actions with anonym
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Capturing variables in anonymous me
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Implementingiterators the easy wayT
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C# 1: the pain of handwritten itera
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C# 2: simple iterators with yield s
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Real-life example: iterating over r
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Pseudo-synchronous code with the Co
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Summary181(without using a dedicate
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Concluding C# 2:the final featuresT
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Partial types185C# 2 allows more th
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Partial types187code generator can
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Partial types189Listing 7.2A partia
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Static classes191}public static str
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Namespace aliases193same syntax is
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Namespace aliases195Listing 7.6usin
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Pragma directives197}}Console.Write
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Fixed-size buffers in unsafe code19
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Exposing internal members to select
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Exposing internal members to select
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Part 3C# 3—revolutionizinghow we
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Cutting fluffwith a smart compilerT
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Automatically implemented propertie
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Implicit typing of local variables2
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Implicit typing of local variables2
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Simplified initialization215simple
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Simplified initialization217The par
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Simplified initialization219CREATIN
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Simplified initialization221Home =
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Implicitly typed arrays2238.4 Impli
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Page 260 and 261: Summary229typed local variables—a
Page 262 and 263: Lambda expressions and expression t
Page 264 and 265: Lambda expressions as delegates233L
Page 266 and 267: Simple examples using List and even
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Page 270 and 271: Expression trees239code to run on y
Page 272 and 273: Expression trees241So, what’s the
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Page 284 and 285: Summary253find the better conversio
Page 286 and 287: Extension methodsThis chapter cover
Page 288 and 289: Life before extension methods257Sys
Page 290 and 291: Extension method syntax259extension
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Page 298 and 299: Extension methods in .NET 3.5267as
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Page 302 and 303: Usage ideas and guidelines271extens
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Page 308 and 309: Introducing LINQ277the extension me
Page 310 and 311: Introducing LINQ279Figure 11.1 show
Page 312 and 313: Introducing LINQ281flexible. In par
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LINQ to SQL325using (var context =
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Translations using IQueryable and I
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LINQ to DataSet335where defect.Fiel
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LINQ to DataSet337Creating and popu
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LINQ to XML339■ Create a document
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LINQ to XML341You can nest query ex
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LINQ to XML343need the string form,
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LINQ beyond .NET 3.5345Listing 12.1
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LINQ beyond .NET 3.5347LINQ TO ACTI
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LINQ beyond .NET 3.5349All of the L
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Summary351you look back at all the
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The changing nature of language pre
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Delegation as the new inheritance35
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Life in a parallel universe357Array
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appendix:LINQ standardquery operato
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Conversion361Cast and OfType conver
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Equality operations363A.4 Element o
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Joins365A.7 GroupingThere are two g
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Quantifiers367Table A.9Partitioning
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Sorting369For the examples of these
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indexSymbols!= 82with generics 76#p
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INDEX 373C# (continued)influences 1
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INDEX 375confusion 150, 222over cap
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INDEX 377Document Object Model (DOM
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INDEX 379fully qualified type name
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INDEX 381interfaces (continued)no c
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INDEX 383List`1 93lists 278local va
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INDEX 385Nullable (continued)Value
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INDEX 387RRails 22random access 102
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INDEX 389Standard Query Operators (
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INDEX 391type parameters (continued
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