156: Anonymous Types within Query Expressions in Visual C#

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Listing 156: Anonymous Types within Query Expressions
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using using using using System; SystemCollectionsGeneric; SystemLinq; SystemIO;
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Introducing Query Expressions
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// static void FindMonthOldFiles( string rootDirectory, string searchPattern) { IEnumerable<FileInfo> files = from fileName in DirectoryGetFiles( rootDirectory, searchPattern) where FileGetLastWriteTime(fileName) < DateTimeNowAddMonths(-1) select new FileInfo(fileName); 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, fileLastWriteTime); } } //
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OUTPUT 155:
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\TestData\Billcs(8/10/2007 9:33:55 PM) \TestData\Contactcs(8/19/2007 11:40:30 PM) \TestData\Employeecs(8/17/2007 1:33:22 AM) \TestData\Personcs(10/22/2007 10:00:03 PM)
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Sorting To order the items using a query expression we rely on the orderby clause (see Listing 157)
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Listing 157: Sorting Using an Query Expression with an orderby Clause
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// static void ListByFileSize1( string rootDirectory, string searchPattern) {
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15: Query Expressions
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IEnumerable<string> fileNames = from fileName in DirectoryGetFiles( rootDirectory, searchPattern) orderby (new FileInfo(fileName))Length, fileName ascending select fileName; foreach (string fileName in fileNames) { // As simplification, current directory is // assumed to be a subdirectory of // rootDirectory string relativePath = fileNameSubstring( EnvironmentCurrentDirectoryLength); ConsoleWriteLine("{0}", relativePath); } } //
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Listing 157 uses the orderby clause to sort the files returned by DirectoryGetFiles() first by file size and then by filename in ascending order Multiple sort criteria are separated by a comma such that first the items are ordered by size, and if the size is the same they are ordered by filename ascending and descending are contextual keywords indicating the sort order If the keyword is omitted, the default is ascending
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Let In Listing 158, we have a query that is very similar to that in Listing 157, except that the type argument of IEnumerable<T> is FileInfo One of the problems with the groupby clause in Listing 158 is that in order to evaluate the size of the file, an instance of FileInfo needs to be available in both the orderby clause and the select clause
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Listing 158: Projecting a FileInfo Collection and Sorting by File Size
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// static void ListByFileSize2( string rootDirectory, string searchPattern) { IEnumerable<FileInfo> files =
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Introducing Query Expressions
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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 // rootDirectory string relativePath = fileFullNameSubstring( EnvironmentCurrentDirectoryLength); ConsoleWriteLine("{0}({1})", relativePath, fileLength); } } //
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Unfortunately, although the 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
using using using using System; SystemCollectionsGeneric; SystemLinq; SystemIO;
// 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
15: Query Expressions
string relativePath = fileFullNameSubstring( EnvironmentCurrentDirectoryLength); ConsoleWriteLine("{0}({1})", relativePath, fileLength); } } //
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/groupby clause No operator is needed to separate out the expressions
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