asp.net c# print barcode The query expression pattern in C#.NET

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Listing 12.7 The query expression pattern
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delegate R Func<T1,R>(T1 arg1); delegate R Func<T1,T2,R>(T1 arg1, T2 arg2); class C { public C<T> Cast<T>(); } class C<T> { public C<T> Where(Func<T,bool> predicate); public C<U> Select<U>(Func<T,U> selector); public C<U> SelectMany<U,V>(Func<T,C<U>> selector, Func<T,U,V> resultSelector); public C<V> Join<U,K,V>(C<U> inner, Func<T,K> outerKeySelector, Func<U,K> innerKeySelector, Func<T,U,V> resultSelector); public C<V> GroupJoin<U,K,V>(C<U> inner, Func<T,K> outerKeySelector, Func<U,K> innerKeySelector, Func<T,C<U>,V> resultSelector); public O<T> OrderBy<K>(Func<T,K> keySelector); public O<T> OrderByDescending<K>(Func<T,K> keySelector); public C<G<K,T>> GroupBy<K>(Func<T,K> keySelector); public C<G<K,E>> GroupBy<K,E>(Func<T,K> keySelector, Func<T,E> elementSelector);
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} class O<T> : C<T> { public O<T> ThenBy<K>(Func<T,K> keySelector); public O<T> ThenByDescending<K>(Func<T,K> keySelector); } class G<K,T> : C<T> { public K Key { get; } }
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The query expression pattern for VB has not been provided by Microsoft at the time of this writing.
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You should refer to the C# 3.0 specification to learn about the details of this pattern. Because query expressions are translated into method invocations by means of a syntactic mapping, types have considerable flexibility in how they implement the query expression pattern. In the context of this book, there are a few things you need to know to understand the examples we re about to work out:
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A generic type is used in the query expression pattern to illustrate the proper relationships between parameter and result types, but it is possible to implement the pattern for nongeneric types as well. The standard query operators we described in chapters 3 and 4 provide an implementation of the query operator pattern for any type that implements the IEnumerable<T> interface. Although we re used to working on collections with the standard query operators in the context of LINQ to Objects and LINQ to XML, you can see that IEnumerable<T> is not part of the pattern. This means we can use LINQ with any object and not just enumerations/sequences. The standard query operators are implemented as extension methods, but the patterns methods can be implemented as extension methods or as instance methods, because the two have the same invocation syntax. The methods can request delegates or expression trees as their parameters because lambda expressions are convertible to both. Although recommended for completeness, providing an implementation of all the previously listed methods is not required.
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Custom implementations of the basic query operators
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Everything we covered in the first part of this section is the foundation we needed to start creating custom implementations of the basic query operators. We are now ready to see some examples. To give you a good overview of how it s possible to implement the LINQ query expression pattern, here s what we re going to demonstrate next:
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We ll show you examples of generic implementation as well as nongeneric. We ll build operators that work on IEnumerable<T> as well as operators that work on other kinds of objects. We ll build operators that receive delegates as well as operators that receive expression trees. Some of our operators will be defined as extension methods, some as instance methods. To keep things simple, we ll provide implementations of Where and Select only.
Let s jump right into our examples.
12.3.3 Example 1: tracing standard query operators execution
In our first example, we ll create custom implementations of the Where and Select operators. Our methods will just delegate the processing to the standard Enumerable.Where and Enumerable.Select implementations. Listing 12.8 shows two operators implemented in a class named CustomImplementation inside the LinqInAction.Extensibility namespace.
Listing 12.8 Custom implementations of Where and Select with the standard generic signatures (CustomImplementation.csproj)
using System; using System.Collections.Generic; using System.Linq; namespace LinqInAction.Extensibility { public static class CustomImplementation { public static IEnumerable<TSource> Where<TSource>( this IEnumerable<TSource> source, Func<TSource, Boolean> predicate) { Console.WriteLine("in CustomImplementation.Where<TSource>");
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return Enumerable.Where(source, predicate); } public static IEnumerable<TResult> Select<TSource, TResult>( this IEnumerable<TSource> source, Func<TSource, TResult> selector) { Console.WriteLine( "in CustomImplementation.Select<TSource, TResult>"); return Enumerable.Select(source, selector); } } }
In order to use these new implementations of the two operators, all we need to do is import the LinqInAction.Extensibility namespace instead of the System.Linq namespace:
//using System.Linq; using LinqInAction.Extensibility; using LinqInAction.LinqBooks.Common; class TestCustomImplementation { static void Main() { var books = from book in SampleData.Books where book.Price < 30 select book.Title; ObjectDumper.Write(books); } }
Of course, executing this program will display our trace information:
in CustomImplementation.Where<TSource> in CustomImplementation.Select<TSource, TResult> Funny Stories LINQ rules Bonjour mon Amour
That s it for our first example. You ve just seen how to provide your own implementation of the Where and Select operators. Here, we ve simply added some trace information, which can be useful if you want to better understand how queries
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