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Ultimately, the call to yields a class that we call , which contains a single method called A mimic state machine is implemented in the class such that calls to loop in the original what would happen if we called the simple method The difference is that the results from the loop are returns the rst retrieved one at a time, so that the rst call to returns the next value, and value from the loop, the next call to so on, until the items generated by the loop are exhausted loop can differ, depending on the The actual location of the type of query you write In our case, it is actually called from inside the , as shown in the next section implementation of Deferred execution ensures that LINQ never wastes time performing calculations you don t actually need This is made possible by LINQ s reliance on sequences of numbers that are retrieved from a class that impleinterface This is true even if we declare a list like ments the this:
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Even with a seemingly static list like this, behind the scenes the C# compiler pattern and pulls the numbers from a uses the loop, grabbing each current item one at a time Consider the following variation of the code from Listing 414:
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A newcomer to LINQ might suppose that this method prints the numbers is equal to 3 when the query expression is exe1 and 2, because cuted However, the code actually prints the values 1, 2, and 3 The variis equal to 4 when the code reaches the loop, and that able is when the query expression is executed Why is execution of the query deferred A primary reason is because it enables composition to work as expected You can link two, three, four, or more queries using the compositional style of development, and none of the queries will execute until you begin to iterate over the results This means that each query can be combined into a single computation that is executed only once When you are using query expression syntax, execution is always deferred Only operators that must be called using query method syntax or might execute immediately For instance, when you call , execution is immediate:
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The , , and methods are all called using query expresoperator is called using query sion syntax and are all deferred The method syntax and is not deferred
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We are now deep inside the implementation of LINQ to Objects and near the end of our journey Just one piece is missing: How are LINQ operators, , actually implemented such as This chapter has told you several times that code that appears to say one thing is actually translated by the compiler into something else The compiler translates automatic properties into standard properties with funny names Anonymous types are translated into real classes that have funny loops actually call the internames Behind the scenes method Most surprising of all, the compiler transface with its interface lates yield iterators into classes that implement the Given this background, it should come as no surprise that the compiler translates query expressions into something else Consider the following query, which you have seen several times:
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Behind the scenes, at compile time, this query is translated into the following code:
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Or if you prefer, the compiler is smart enough to work with this shortened version of the statement:
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These three statements are semantically quite similar, even if they differ syntactically The second and third statements are valid C# code and can
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be used in lieu of the rst implementation They are said to use query method syntax, whereas the rst statement is a query expression
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