Query Expressions and Other Flavors of LINQ in C#

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Query Expressions and Other Flavors of LINQ
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Over the course of the last two chapters, you have read about the features of the C# language that make LINQ possible, and you have read about the structure of LINQ query expressions Throughout this discussion, we have been focused on LINQ to Objects
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5: Writing Quer y Expre ssions
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As you know, LINQ comes in many different avors Two of them, LINQ to Objects and LINQ to XML, exactly follow the principles laid out in these chapters In particular, they support a composable query syntax that Here is the pattern they follow both consumes and returns in a deferred query:
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However, other avors of LINQ are not based on Most , but on of these are LINQ to SQL, which is based not on LINQ to SQL generally follows this pattern in deferred queries:
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This query operates on a variable type and returns a vari It is, therefore, fully composable able of type is a fairly complicated type As a result, it does not serve as a good introduction to avors of LINQ that are not based on Instead, I will create a very simple type and show you a very simple provider for it After you understand the principles involved, I ll come back and say a few more words about
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LINQ to MyNumberServer Query expressions are surprisingly obtuse about the data they query They are merely syntactic sugar on top of LINQ query methods They exist only because they provide developers with a simple, easy-to-use syntax for writing queries The translation from a query expression to a query method is not complex It is really just a pattern-matching exercise The compiler looks at a query expression, applies a few simple rules, and translates it into query methods If you create types that follow the few simple patterns that query expressions expect to see, you can use your types in a query expres In fact, there is nothsion, even if they are not based on other than the fact that it follows the ing special about simple patterns expected of any type used in a query expression
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Quer y Expre ssions and Other Flavors of LINQ
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Query expressions follow a pattern based on sequences of data You learned in the preceding chapter that sequences of data are generated by iterators in particular, by iterators that implement a method called Consider the simple class shown in Listing 510, which is found in the QueryProvider sample available on the book s web site
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Listing 510 Constructor This Simple Class Serves up Multiple Copies of the Number You Pass to Its
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This very simple class is designed to serve up a number times If you pass in the numbers 3 and 5 to its constructor, it serves up the number 3 ve times We want to be able to write queries against this type that ask it to show us the numbers it generates if we pass in any two arbitrary parameters For instance, if we pass in the numbers 2 and 7, we want to see what values the class returns and does not even Although it does not mention include generics, this simple class nevertheless follows two of the key patterns that a LINQ query expression expects to nd: It works with sequences of data It exposes the sequence in a method called interface returns the that
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5: Writing Quer y Expre ssions
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It looks like we are off to a good start The next step is to try to use this type in a LINQ query:
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Despite our efforts, the compiler bristles at this code and complains that it
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The compiler wants us to implement the operator for So far in this book, whenever we have wanted to write a query, we were As you have seen, working with a type that implemented type that ships with LINQ includes implementations of all the This time, however, we are workthe LINQ operators for It is now up to us to implement ing with a new type called the query operators we plan to use In the preceding chapter you learned how to write extension methods, operator Let s use that and you even saw an implementation of the : knowledge to create operators for
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