Every Rule Has an Exception in C#.NET

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In his review of this text, Nick Paldino found exceptions to the rule I just stated His comments on the subject are clear, so I ll include them almost exactly as he presented them to me: You can pass an anony, and Re ecmous type out of a method by passing it as type tion will be able to parse it Anders Hejlsberg gave a presentation at the MVP summit in 2007 that bound the output of an anonymous type to ; he could do that because the uses re eca tion to get the information on the type Also, the extension methods that LINQ uses actually are outside of the method When anonymous extension method on types are used, they are passed to the class Technically, that is a means of passing an anonythe mous type out of a method
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4: C# 30 Te chnical Over view
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Anonymous types are also used in LINQ queries Consider this LINQ to Objects code fragment borrowed from one of the samples in 2:
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The clause contains an anonymous type You could create your own type and use it instead, but it is a common and useful practice to write code like that shown here Because this LINQ query uses an anonymous type, developers must use type inference to declare its return type In the section Composable in the preceding chapter, you learned that query expressions both return and In this case is some anonymous operate on instances of This means that again we must use type with two elds of type Without type inference, code type inference to declare the identi er of this type would not be possible Consider this code fragment, which operates on the variable returned from the query we are studying:
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In the current context, is an anonymous type, so again it would be impossible to declare it explicitly This is yet another case where we could not go forward if type inference did not exist stateEven if we could explicitly declare the type used in this ment, it would probably cause us more trouble than it would be worth The problem is that the type of could easily change if we made small changes to the query expression from which it is derived In complex LINQ queries, especially those that use composability, this could cause a cascading and type series of changes All of this is avoided by using the keyword inference In this section you have learned about anonymous types and type inference You have seen that these subjects are inextricably linked, because only rarely can you make use of anonymous types without also needing to use type inference
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Lambdas are important both as a stand-alone C# 30 feature and as a signi cant part of the LINQ architecture The C# 20 technologies called delegates and generic methods serve as stepping-stones and building blocks for lambdas Thus, I will cover them rst and then focus on lambdas I will also brie y mention a C# 20 technology called anonymous methods that has been eclipsed by lambdas
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Delegates Delegates provide a means of declaring a variable that references an individual method Developers can invoke the delegate, and the delegate, in turn, calls the method it references Because the delegate is just a variable, you can pass it to other methods and perform similar tricks with it that would otherwise be impossible In many other languages, delegates are called and implemented as function pointers In C#, however, a delegate is implemented as a class, rather than as a pointer The advantage of this system is that you can fully typecheck a class, whereas a pointer is less type-safe Consider the code shown in Listing 46 It shows a simple delegate type , used to reference the method called
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