14: Collection Interfaces with Standard Query Operators in C#.NET

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14: Collection Interfaces with Standard Query Operators
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Listing 1416: Executing LINQ Queries in Parallel
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// IEnumerable<string> fileList = DirectoryGetFiles( rootDirectory, searchPattern); var items = fileList AsParallel() Select( file => { FileInfo fileInfo = new FileInfo(file); return new { FileName = fileInfoName, Size = fileInfoLength }; }); //
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As Listing 1416 shows, the change in code to enable parallel support is minimal All that it uses is a NET Framework 4 introduced standard query operator, AsParallel(), on the static class SystemLinqParallelEnumerable Using this simple extension method, however, the runtime begins executing over the items within the fileList collection and returning the resultant objects in parallel Each parallel operation in this case isn t particularly expensive (although it is relative to what other execution is taking place), but consider CPU-intensive operations such as encryption or compression Paralyzing the execution across multiple CPUs can decrease execution time by a magnitude corresponding to the number of CPUs An important caveat to be aware of (and the reason why AsParallel() appears in an Advanced Block rather than the standard text) is that parallel execution can introduce race conditions such that an operation on one thread can be intermingled with an operation on a different thread, causing data corruption To avoid this, synchronization mechanisms are required on data with shared access from multiple threads in order to force the operations to be atomic where necessary Synchronization itself, however, can introduce deadlocks that freeze the execution, further complicating the effective parallel programming More details on this and additional multithreading topics are covered in 18 and 19
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Standard Query Operators
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Counting Elements with Count() Another common query performed on a collection of items is to retrieve the count To support this LINQ includes the Count() extension method Listing 1417 demonstrates that Count() is overloaded to simply count all elements (no parameters) or to take a predicate that only counts items identified by the predicate expression
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Listing 1417: Counting Items with Count()
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using System; using SystemCollectionsGeneric; using SystemLinq; class Program { static void Main() { IEnumerable<Patent> patents = PatentDataPatents; ConsoleWriteLine("Patent Count: {0}", patents Count()); ConsoleWriteLine("Patent Count in 1800s: {0}", patentsCount(patent => patentYearOfPublicationStartsWith("18"))); } // }
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In spite of the simplicity of writing the Count() statement, IEnumerable<T> has not changed, so the executed code still involves iterating over all the items in the collection Whenever a Count property is directly available on the collection, it is preferable to use that rather than LINQ s Count() method (a subtle difference) Fortunately, ICollection<T> includes the Count property, so code that calls the Count() method on a collection that supports ICollection<T> will cast the collection and call Count directly However, if ICollection<T> is not supported, EnumerableCount() will proceed to enumerate all the items in the collection rather than call the built-in Count mechanism If the purpose of checking the count is only to see whether it is greater than zero (if(patentsCount() > 0){}), a preferable approach would be to use the Any() operator (if(patentsAny()){}) Any() attempts to iterate over only one of the items in the collection to return a true result, rather than the entire sequence
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14: Collection Interfaces with Standard Query Operators
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Deferred Execution One of the most important concepts to remember when using LINQ is deferred execution Consider the code in Listing 1418 and the corresponding output in Output 145
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Listing 1418: Filtering with SystemLinqEnumerableWhere()
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using System; using SystemCollectionsGeneric; using SystemLinq; // IEnumerable<Patent> patents = PatentDataPatents; bool result; patents = patentsWhere( patent => { if (result = patentYearOfPublicationStartsWith("18")) { // Side effects like this in a predicate // are used here to demonstrate a // principle and should generally be // avoided ConsoleWriteLine("\t" + patent); } return result; }); ConsoleWriteLine("1 Patents prior to the 1900s are:"); foreach (Patent patent in patents) { } ConsoleWriteLine(); ConsoleWriteLine( "2 A second listing of patents prior to the 1900s:"); ConsoleWriteLine( " There are {0} patents prior to 1900", patentsCount()); ConsoleWriteLine(); ConsoleWriteLine( "3 A third listing of patents prior to the 1900s:"); patents = patentsToArray();
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