1511: Selecting an Anonymous Type Following the groupby Clause in C#.NET

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Listing 1511: Selecting an Anonymous Type Following the groupby Clause
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using System; using SystemCollectionsGeneric;
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15: LINQ with Query Expressions
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using SystemLinq; // private static void GroupKeywords1() { IEnumerable<IGrouping<bool, string>> keywordGroups = from word in Keywords group word by wordContains('*'); var selection = from groups in keywordGroups select new { IsContextualKeyword = groupsKey, Items = groups }; foreach (var wordGroup in selection) { ConsoleWriteLine(EnvironmentNewLine + "{0}:", wordGroupIsContextualKeyword "Contextual Keywords" : "Keywords"); foreach (var keyword in wordGroupItems) { ConsoleWrite(" " + keywordReplace("*", null)); } } } //
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OUTPUT 157:
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Keywords: abstract as base bool break byte case catch char checked class const continue decimal default delegate do double else enum event explicit extern false finally fixed float for foreach goto if implicit in int interface internal is lock long namespace new null object operator out override params private protected public readonly ref return sbyte sealed short sizeof stackalloc static string struct switch this throw true try typeof uint ulong unchecked unsafe ushort using virtual void volatile while Contextual Keywords: add alias ascending by descending dynamic equals from get global group into join let on orderby partial remove select set value var where yield
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The groupby clause returns a collection of IGrouping<TKey, TElement> objects just as the GroupBy() standard query operator did (see 14)
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The select clause defines an anonymous type, renaming IGrouping<TKey, TElement>Key to be IsContextualKeyword and naming the subcollection property Items With this change, the nested foreach uses wordGroupItems rather than wordGroup directly, as shown in Listing 1510 Another potential property to add to the anonymous type would be the count of items within the subcollection However, this is available on wordGroup ItemsCount(), so the benefit of adding it to the anonymous type directly is questionable
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Query Continuation with into Following the groupby query is a second query that projects out an anonymous type from the grouping Rather than write an additional query, you can extend the query with a query continuation clause using the contextual keyword into that allows you to name each item returned by the groupby clause with a range variable (groups in Listing 1511) The into clause serves as a generator for additional query commands specifically, a select clause, as shown in Listing 1512
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Listing 1512: Selecting without the Query Continuation
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using System; using SystemCollectionsGeneric; using SystemLinq; // private static void GroupKeywords1() { var selection = from word in Keywords group word by wordContains('*') into groups select new { IsContextualKeyword = groupsKey, Items = groups }; // } //
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15: LINQ with Query Expressions
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The ability to run additional queries on the results of an existing query using into is not specific to groupby clauses, but rather is a feature of all query expressions Query continuation provides a form of shorthand in place of writing multiple individual query expressions The query in Listing 1512 is identical to the one in Listing 1511, but without using into In other words, into shortcuts the need to write a second query using the results of the first query; it serves as a pipeline operator, combining the results of the first query with those of the second query
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BEGINNER TOPIC Distinct Members Often, it is desirable to return only distinct items from within a collection all duplicates are combined into a single item Query expressions don t have explicit syntax for distinct members, but the functionality is available via the query operator Distinct(), as introduced in the preceding chapter Listing 1513 demonstrates calling it directly from the query expression, and Output 158 shows the results
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Listing 1513: Obtaining Distinct Members from a Query Expression
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using System; using SystemCollectionsGeneric; using SystemLinq; // public static void ListMemberNames() { IEnumerable<string> enumerableMethodNames = ( from method in typeof(Enumerable)GetMembers( SystemReflectionBindingFlagsStatic | SystemReflectionBindingFlagsPublic) select methodName)Distinct(); foreach(string method in enumerableMethodNames) { ConsoleWrite(" {0},", method); } } //
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OUTPUT 158:
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Enumerable methods are: First, FirstOrDefault, Last, LastOrDefault, Single, SingleOrDefault, ElementAt, ElementAtOrDefault, Repeat, Empty, Any, All, Count, LongCount, Contains, Aggregate, Sum, Min, Max, Average, Where, Select, SelectMany, Take, TakeWhile, Skip, SkipWhile, Join, GroupJoin, OrderBy, OrderByDescending, ThenBy, ThenByDescending, GroupBy, Concat, Distinct, Union, Intersect, Except, Reverse, SequenceEqual, AsEnumerable, ToArray, ToList, ToDictionary, ToLookup, DefaultIfEmpty, OfType, Cast, Range
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In this example, typeof(Enumerable)GetMembers() returns a list of all the members (methods, properties, and so on) on SystemLinqEnumerable However, many of these members are overloaded, sometimes more than once Rather than displaying the same member multiple times, Distinct() is called from the query expression This eliminates the duplicate names from the list (I cover the details of typeof() and GetMembers() in 17)
ADVANCED TOPIC Query Expression Compilation Under the covers, a query expression is a series of method calls to the underlying API The CIL itself does not have any concept of query expressions In fact, except for some corner cases with expression trees, there was no change to the underlying CLR in order to support query expressions Rather, query expressions were supported via changes to the C# compiler only This worked because the compiler translates the query expression to method calls For example, the query expression from Listing 151 translates to a call to SystemLinqEnumerable s Where() extension method and becomes KeywordsWhere<string>() The criteria identified by the where clause are just like they were in the Where() (or FindAll()) method described in the preceding chapter
ADVANCED TOPIC Implementing Implicit Execution The capability of saving the selection criteria into selection (see Listing 151) rather than executing the query at the time of assignment is implemented through delegates The compiler translates the query expression to methods