C# in Depth

154 CHAPTER 5 Fast-tracked delegatesvariables were around, it wouldn’t have made much sense for List.FindAll toexist, because of all the hoops you’d have to go through in order to create the rightdelegate to start with. It would have been simpler to do all the iteration and copyingmanually. With C# 2, however, we can do it all very, very easily:List FindAllYoungerThan(List people, int limit){return people.FindAll (delegate (Person person){ return person.Age < limit; });}Here we’re capturing the limit parameter within the delegate instance—if we’d hadanonymous methods but not captured variables, we could have performed a test againsta hard-coded limit, but not one that was passed into the method as a parameter. I hopeyou’ll agree that this approach is very neat—it expresses exactly what we want to do withmuch less fuss about exactly how it should happen than you’d have seen in a C# 1 version.(It’s even neater in C# 3, admittedly… 7 ) It’s relatively rare that you come across a situationwhere you need to write to a captured variable, but again that can certainly haveits uses.Still with me? Good. So far, we’ve only used the delegate instance within the methodthat creates it. That doesn’t raise many questions about the lifetime of the captured variables—butwhat would happen if the delegate instance escaped into the big bad world?How would it cope after the method that created it had finished?5.5.4 The extended lifetime of captured variablesThe simplest way of tackling this topic is to state a rule, give an example, and thenthink about what would happen if the rule weren’t in place. Here we go:A captured variable lives for at least as long as any delegate instancereferring to it.Don’t worry if it doesn’t make a lot of sense yet—that’s what the example is for. Listing5.12 shows a method that returns a delegate instance. That delegate instance is createdusing an anonymous method that captures an outer variable. So, what will happenwhen the delegate is invoked after the method has returned?Listing 5.12Demonstration of a captured variable having its lifetime extendedstatic ThreadStart CreateDelegateInstance(){int counter = 5;ThreadStart ret = delegate{Console.WriteLine(counter);counter++;};7In case you’re wondering: return people.Where(person => person.Age < limit);Licensed to Rhona Hadida