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As you can see, changing or adding a namespace import can make a serious difference in the behavior of your code. A given query can behave differently if you use System.Linq, LinqInAction.Extensibility, or another namespace! The design decision of relying on namespace imports to reference extension methods (and query operators) is questionable. Anyway, be careful about this and double-check the namespaces you import when in doubt.
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After demonstrating that the implementation you provide for the basic query operators doesn t have to work on generic types, we ll show you in a third example that your implementation doesn t necessarily have to work on sequences either.
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12.3.6 Example 3: non-sequence operator
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This last example of how to provide custom implementations of the operators used in query expressions demonstrates how you can integrate single objects in queries. The standard query operators provide an implementation of the query operator pattern for IEnumerable<T>. This allows you to work with collections like the array of Book objects provided by our SampleData.Books property. Let s suppose we want to work with a single object and not a sequence of objects. What can we do
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Extending LINQ
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In the following query, we work on a specific Publisher instance and use it in a way similar to how we d use a sequence of Publisher objects:
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from publisher in SampleData.Publishers[0] join book in SampleData.Books on publisher equals book.Publisher into books select new { Publisher = publisher.Name, Books = books};
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This query seems to make sense, but the problem is that it doesn t work as is with the standard query operators. This is because the standard query operators are designed to work only with IEnumerable<T>. The particular problem in our case is that the compiler complains that it cannot find GroupJoin for the Publisher type:
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error CS1936: Could not find an implementation of the query pattern for source type 'LinqInAction.LinqBooks.Common.Publisher'. 'GroupJoin' not found.
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The GroupJoin operator used because we are performing a join operation is defined the following way:
public static IEnumerable<TResult> GroupJoin<TOuter, TInner, TKey, TResult>( this IEnumerable<TOuter> outer, IEnumerable<TInner> inner, Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector)
You can clearly see that the outer argument is defined as a sequence (IEnumerable<TOuter>). All we need to do to make the compiler happy is provide a new implementation of GroupJoin that accepts a single element as the outer object instead of a sequence. Listing 12.10 shows how to write this additional version of GroupJoin.
Listing 12.10 Implementation of GroupJoin for a single element (NonSequenceOperator.cs)
using System; using System.Collections.Generic; using System.Linq; using LinqInAction.LinqBooks.Common; namespace LinqInAction.Extensibility { static class NonSequenceOperator { public static IEnumerable<TResult>
Querying a web service: LINQ to Amazon
GroupJoin<TOuter, TInner, TKey, TResult>( this TOuter outer, Outer argument as IEnumerable<TInner> inner, a single element Func<TOuter, TKey> outerKeySelector, Func<TInner, TKey> innerKeySelector, Func<TOuter, IEnumerable<TInner>, TResult> resultSelector) { ILookup<TKey, TInner> lookup = ignored for simplicity inner.ToLookup(innerKeySelector); yield return resultSelector(outer, lookup[outerKeySelector(outer)]); } } }
Validation of arguments
All we ve done here is change the type of the first argument and adapt the code to deal with a single object. Until now, we ve used only simple examples, but you should now be able to code your own query operators. We are now going to introduce a richer example. It will have methods request expression trees as their parameters instead of delegates.
12.4 Querying a web service: LINQ to Amazon
Querying a web service: LINQ to Amazon
In the previous section, we learned how to create custom query operators or implement the standard ones differently. This is a solution that works well for objects in memory, just like what LINQ to Objects offers. In this section, we ll consider a different scenario: We ll query a web service. More precisely, we ll query Amazon to get information about books. In this section, now that you know a lot about LINQ and how it works, we re going to create our own LINQ provider: LINQ to Amazon! In the next section, we are going to further refine our implementation. This example will allow us to address the case of query translation to another query language and remote evaluation. The query we ll write here will be translated into web queries and run on a remote web server. This requires a different extensibility mechanism than what we ve seen previously.
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