Figure 142: IEnumerator<T> and IEnumerator Interfaces in C#.NET

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Figure 142: IEnumerator<T> and IEnumerator Interfaces
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Standard Query Operators
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As Figure 142 shows, three calls in this Listing 1416 trigger the lambda expression, and each time it is fairly implicit If the lambda expression was expensive (such as a call to a database) it would be important to minimize the lambda expression s execution First, the execution is triggered within the foreach loop As I described earlier in the chapter, the foreach loop breaks down into a MoveNext() call and each call results in the lambda expression s execution for each item in the original collection While iterating, the runtime invokes the lambda expression for each item to determine whether the item satisfies the predicate Second, a call to Enumerable s Count() (the function) triggers the lambda expression for each item once more Again, this is subtle since Count (the property) is very common on collections that have not been queried with a standard query operator Third, the call to ToArray() (or ToList(), ToDictionary(), or ToLookup()) triggers the lambda expression for each item However, converting the collection with one of these To methods is extremely helpful Doing so returns a collection on which the standard query operator has already executed In Listing 1416, the conversion to an array means that when Length is called in the final ConsoleWriteLine(), the underlying object pointed to by patents is in fact an array (which obviously implements IEnumerable<T>), and therefore, SystemArray s implementation of Length is called and not SystemLinqEnumerable s implementation Therefore, following a conversion to one of the collection types returned by a To method, it is generally safe to work with the collection (until another standard query operator is called) However, be aware that this will bring the entire result set into memory (it may have been backed by a database or file before this) Furthermore, the To method will snapshot the underlying data so that no fresh results will be returned upon requerying the To method result I strongly encourage readers to review the sequence diagram in Figure 142 along with the corresponding code and understand the fact that the deferred execution of standard query operators can result in subtle triggering of the standard query operators; therefore, developers should use caution to avoid unexpected calls
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14: Collection Interfaces with Standard Query Operators
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To avoid such repeated execution, it is necessary to cache the data the executed query retrieves To do this, you assign the data to a local collection using one of the To collection methods During the assignment call of a To method, the query obviously executes However, iterating over the assigned collection after that will not involve the query expression any further In general, if you want the behavior of an in-memory collection snapshot, it is a best practice to assign a query expression to a cached collection to avoid unnecessary repeated executions of the standard query operator
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Sorting with OrderBy() and ThenBy() Another common operation on a collection is to sort it This involves a call to SystemLinqEnumerable s OrderBy(), as shown in Listing 1417 and Output 146
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Listing 1417: Ordering with SystemLinqEnumerableOrderBy()/ThenBy()
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using System; using SystemCollectionsGeneric; using SystemLinq; // IEnumerable<Patent> items; Patent[] patents = PatentDataPatents; items = patentsOrderBy( patent => patentYearOfPublication)ThenBy( patent => patentTitle); Print(items); ConsoleWriteLine(); items = patentsOrderByDescending( patent => patentYearOfPublication)ThenByDescending( patent => patentTitle); Print(items); //
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The OrderBy() call takes a lambda expression that identifies the key on which to sort In Listing 1417, the initial sort uses the year the patent was published
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Standard Query Operators
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OUTPUT 146:
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Bifocals (1784) Steam Locomotive(1815) Electrical Telegraph(1837) Phonograph(1877) Kinetoscope(1888) Flying machine (1903) Backless Brassiere(1914) Droplet deposition apparatus(1989) Droplet deposition apparatus(1989) Backless Brassiere(1914) Flying machine (1903) Kinetoscope(1888) Phonograph(1877) Electrical Telegraph(1837) Steam Locomotive(1815) Bifocals (1784)
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However, notice that the OrderBy() call takes only a single parameter, which uses the name keySelector, to sort on To sort on a second column, it is necessary to use a different method: ThenBy() Similarly, code would use ThenBy() for any additional sorting OrderBy() returns an IOrderedEnumerable<T> interface, not an IEnumerable<T> Furthermore, IOrderedEnumerable<T> derives from IEnumerable<T>, so all the standard query operators (including OrderBy()) are available on the OrderBy() return However, repeated calls to OrderBy() would undo the work of the previous call such that end result would sort by only the keySelector in the final OrderBy() call As a result, be careful not to call OrderBy() on a previous OrderBy() call Instead, you should specify additional sorting criteria using ThenBy() Although ThenBy() is an extension method, it is not an extension of IEnumerable<T>, but rather IOrderedEnumerable<T> The method, also defined on SystemLinqExtensionsEnumerable, is declared as follows:
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public static IOrderedEnumerable<TSource> ThenBy<TSource, TKey>( this IOrderedEnumerable<TSource> source, Func<TSource, TKey> keySelector)
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In summary, use OrderBy() first, followed by zero or more calls to ThenBy() to provide additional sorting columns The methods OrderByDescending() and ThenByDescending() provide the same functionality
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