c# qr codes Introducing LINQ to Amazon in C#.NET

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12.4.1 Introducing LINQ to Amazon
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The example we ll introduce in this section will use LINQ s extensibility to allow for language-integrated queries against a book catalog. LINQ queries will be converted
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Extending LINQ
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to REST URLs, which are supported by Amazon s web services. These services return XML data, which we ll be able to convert from XML to .NET objects using LINQ to XML. A use case for this example could be the following:
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Search for books on Amazon using a LINQ query Display the results in XHTML using LINQ to XML Import the selected books into a database using LINQ to SQL
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The goal here is not to create a complete solution, so we won t demonstrate all of this at this point. We ll focus on the first part of the scenario. We already performed this kind of operation in the prior chapters, but this time we ll create a LINQ provider that can be used to write queries without worrying about the details of the dialog with Amazon. We won t support the complete set of operators that could be used in a LINQ query. This would be too complex to present in the context of this book. Anyway, since we are calling an underlying web service, we need to restrict the query possibilities to what the service supports. For the moment, let s look at the client code we would like to be able to write:
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var query = from book in new LinqToAmazon.AmazonBookSearch() where book.Title.Contains("ajax") && (book.Publisher == "Manning") && (book.Price <= 25) && (book.Condition == BookCondition.New) select book;
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This piece of code is nearly self-explanatory. This is LINQ to Amazon code. It expresses a query against Amazon, but does not execute it. The query variable contains a query. The query will be executed when we start enumerating the results. The following piece of code makes the transition from the LINQ to Amazon world to the familiar LINQ to Objects world:
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var sequence = query.AsEnumerable();
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The query gets executed when AsEnumerable is called and an enumeration of the resulting books is created. The next steps could be to use LINQ to Objects to perform grouping operations on the results:
var groups = from book in query.AsEnumerable() group book by book.Year into years
Querying a web service: LINQ to Amazon
orderby years.Key descending select new { Year = years.Key, Books = from book in years select new { book.Title, book.Authors } };
This query can be used for displaying the results like this:
Published in 2006 Title=Ruby for Rails : Ruby Techniques for Rails Developers Title=Wxpython in Action Authors=... Published in 2005 Title=Ajax in Action Authors=... Title=Spring in Action (In Action series) Authors=...
Authors=...
Published in 2004 Title=Hibernate in Action (In Action series) Authors=... Title=Lucene in Action (In Action series) Authors=...
Here is the code that produces this kind of results:
foreach (var group in groups) { Console.WriteLine("Published in " + group.Year); foreach (var book in group.Books) { Console.Write(" "); ObjectDumper.Write(book); } Console.WriteLine(); }
What a great way to query a catalog of books! Don t you think that this code is comprehensible and clearly expresses the intention It s certainly better than having to construct a web request and having to know all the details of the Amazon API. Let s see what s needed to implement LINQ to Amazon.
12.4.2 Requirements
This time, the data we ll query will not be in memory. When the data is in memory, we can query it continuously and retrieve the results one by one. In our now-classic LINQ to Objects example, each time we perform an iteration in foreach, a new result is pulled from the original list down through our query processing:
Extending LINQ
using System.Linq; using LinqInAction.LinqBooks.Common; static class LinqInAction { static void Main() { var books = from book in SampleData.Books where book.Price < 30 select book.Title; foreach (String book in books) { } } }
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