158: Projecting a FileInfo Collection and Sorting by File Size in C#

Draw QR Code ISO/IEC18004 in C# 158: Projecting a FileInfo Collection and Sorting by File Size
Listing 158: Projecting a FileInfo Collection and Sorting by File Size
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using using using using System; SystemCollectionsGeneric; SystemLinq; SystemIO;
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// static void ListByFileSize2( string rootDirectory, string searchPattern) { IEnumerable<FileInfo> files = from fileName in DirectoryGetFiles( rootDirectory, searchPattern) orderby new FileInfo(fileName)Length, fileName select new FileInfo(fileName); foreach (FileInfo file in files) { // As simplification, current directory is // assumed to be a subdirectory of
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Introducing Query Expressions
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// rootDirectory string relativePath = fileFullNameSubstring( EnvironmentCurrentDirectoryLength); ConsoleWriteLine("{0}({1})", relativePath, fileLength); } } //
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Unfortunately, although the end result is correct, Listing 158 ends up instantiating a FileInfo object twice for each item in the source collection FileInfo is instantiated not only in the select clause, but also when the orderby clause is evaluated To avoid unnecessary overhead like this overhead that could potentially be expensive the query expression syntax includes a let expression, as demonstrated in Listing 159
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Listing 159: Ordering the Results in a Query Expression
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using using using using System; SystemCollectionsGeneric; SystemLinq; SystemIO;
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// static void ListByFileSize3( string rootDirectory, string searchPattern) { IEnumerable<FileInfo> files = from fileName in DirectoryGetFiles( rootDirectory, searchPattern) let file = new FileInfo(fileName) orderby fileLength, fileName select file; foreach (FileInfo file in files) { // As simplification, current directory is // assumed to be a subdirectory of // rootDirectory string relativePath = fileFullNameSubstring( EnvironmentCurrentDirectoryLength); ConsoleWriteLine("{0}({1})", relativePath, fileLength); } } //
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15: LINQ with Query Expressions
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The let clause provides a location to place an expression that is used throughout the query expression To place a second let expression, simply add it as an additional clause to the query after the first from clause but before the final select/group by clause No operator is needed to separate out the expressions
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Grouping Another common collection scenario is the grouping of items In SQL, this generally involves aggregating the items into a summary header or total an aggregate value However, C# is more expressive than this In addition to providing aggregate information about each grouping, query expressions allow for the individual items in the group to form a series of subcollections to each item in the overall parent list For example, it is possible to group the contextual keywords separately from the regular keywords and automatically associate the individual words within the keyword type grouping to each other Listing 1510 and Output 156 demonstrate the query expression
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Listing 1510: Grouping Together Query Results
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using System; using SystemCollectionsGeneric; using SystemLinq; // private static void GroupKeywords1() { IEnumerable<IGrouping<bool, string>> selection = from word in Keywords group word by wordContains('*'); foreach (IGrouping<bool, string> wordGroup in selection) { ConsoleWriteLine(EnvironmentNewLine + "{0}:", wordGroupKey "Contextual Keywords" : "Keywords"); foreach (string keyword in wordGroup) { ConsoleWrite(" " + (wordGroupKey keywordReplace("*", null) : keyword)); }
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} } //
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OUTPUT 156:
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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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There are several things to note in this listing First, each item in the list is of type IGrouping<bool, string> The type parameters of IGrouping<TKey, TElement> are determined by the data type following group and by that is, TElement is a string because word is a string Type parameter TKey is determined by the data type following by In this case, wordContains() returns a Boolean, so TKey is a bool A second characteristic of a query expression s groupby clause is the fact that it enables a nested foreach loop via which the code can iterate over the subcollection mentioned earlier in this section In Listing 1510, we first iterate over the groupings and print out the type of keyword as a header Nested within the first iteration is a second foreach loop that prints each keyword as an item below the header Third, we can append a select clause to the end of a groupby clause, allowing support for projection (see Listing 1511 and Output 157) More generally, the addition of the select clause is enabled via query continuation any query body that handles the elements of the first query can be appended to the first query body
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