C# in Depth

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Filtering and ordering a sequence293Listing 11.8Sorting by the severity of a bug, from high to low priorityUser tim = SampleData.Users.TesterTim;var query = from bug in SampleData.AllDefectswhere bug.Status != Status.Closedwhere bug.AssignedTo == timorderby bug.Severity descendingselect bug;foreach (var bug in query){Console.WriteLine("{0}: {1}", bug.Severity, bug.Summary);}The output of listing 11.8 shows that we’ve sorted the results appropriately:Showstopper: Webcam makes me look baldMajor: Subtitles only work in WelshMajor: Play button points the wrong wayMinor: Network is saturated when playing WAV fileTrivial: Installation is slowHowever, you can see that we’ve got two major defects. Which order should those betackled in? Currently no clear ordering is involved. Let’s change the query so thatafter sorting by severity in descending order, we sort by “last modified time” in ascendingorder. This means that Tim will test the bugs that have been fixed a long time agobefore the ones that were fixed recently. This just requires an extra expression in theorderby clause, as shown in listing 11.9.Listing 11.9Ordering by severity and then last modified timeUser tim = SampleData.Users.TesterTim;var query = from bug in SampleData.AllDefectswhere bug.Status != Status.Closedwhere bug.AssignedTo == timorderby bug.Severity descending, bug.LastModifiedselect bug;foreach (var bug in query){Console.WriteLine("{0}: {1} ({2:d})",bug.Severity, bug.Summary, bug.LastModified);}The results of listing 11.9 are shown here. Note how the order of the two majordefects has been reversed.Showstopper: Webcam makes me look bald (08/27/2007)Major: Play button points the wrong way (08/17/2007)Major: Subtitles only work in Welsh (08/23/2007)Minor: Network is saturated when playing WAV file (08/31/2007)Trivial: Installation is slow (08/15/2007)Licensed to Rhona Hadida
294 CHAPTER 11 Query expressions and LINQ to ObjectsSo, that’s what the query expression looks like—but what does the compiler do? It simplycalls the OrderBy and ThenBy methods (or OrderByDescending/ThenBy-Descending for descending orders). Our query expression is translated intoSampleData.AllDefects.Where (bug => bug.Status != Status.Closed).Where (bug => bug.AssignedTo == tim).OrderByDescending (bug => bug.Severity).ThenBy (bug => bug.LastModified)Now that we’ve seen an example, let’s look at the general syntax of orderby clauses.They’re basically the contextual keyword orderby followed by one or more orderings.An ordering is just an expression (which can use range variables) optionally followed bydescending, which has the obvious meaning. The translation for the first ordering is acall to OrderBy or OrderByDescending, and any other orderings are translated using acall to ThenBy or ThenByDescending, as shown in our example.The difference between OrderBy and ThenBy is quite simple: OrderBy assumes ithas primary control over the ordering, whereas ThenBy understands that it’s subservientto one or more previous orderings. For LINQ to Objects, ThenBy is only defined asan extension method for IOrderedEnumerable, which is the type returned byOrderBy (and by ThenBy itself, to allow further chaining).It’s very important to note that although you can use multiple orderbyclauses, each one will start with its own OrderBy or OrderByDescendingclause, which means the last one will effectively “win.” There may be somereason for including multiple orderby clauses, but it would be veryunusual. You should almost always use a single clause containing multipleorderings instead.As noted in chapter 10, applying an ordering requires all the data tobe loaded (at least for LINQ to Objects)—you can’t order an infinitesequence, for example. Hopefully the reason for this is obvious—youdon’t know whether you’ll see something that should come at the start of the resultingsequence until you’ve seen all the elements, for example.We’re about halfway through learning about query expressions, and you may be surprisedthat we haven’t seen any joins yet. Obviously they’re important in LINQ just asthey’re important in SQL, but they’re also complicated. I promise we’ll get to them indue course, but in order to introduce just one new concept at a time, we’ll detour vialet clauses first. That way we can learn about transparent identifiers before we hit joins.Warning:only use oneorderbyclause!11.4 Let clauses and transparent identifiersMost of the rest of the operators we still need to look at involve transparent identifiers.Just like range variables, you can get along perfectly well without understanding transparentidentifiers, if you only want to have a fairly shallow grasp of query expressions.If you’ve bought this book, I hope you want to know C# 3 at a deeper level, which will(among other things) enable you to look compilation errors in the face and knowwhat they’re talking about.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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Anonymous types225new { Name = "Hol
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Anonymous types227That certainly wo
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Summary229typed local variables—a
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Lambda expressions and expression t
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Lambda expressions as delegates233L
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Simple examples using List and even
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Simple examples using List and even
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Expression trees239code to run on y
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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
Page 300 and 301: Extension methods in .NET 3.5269Now
Page 302 and 303: Usage ideas and guidelines271extens
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Page 306 and 307: Query expressionsand LINQ to Object
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
Page 314 and 315: Simple beginnings: selecting elemen
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Page 328 and 329: Joins29711.5 JoinsEach part of the
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Page 332 and 333: Joins30111.5.2 Group joins with joi
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Page 338 and 339: Groupings and continuations307In ju
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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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C# in Depth

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