C# in Depth

144 CHAPTER 5 Fast-tracked delegatesListing 5.4 Demonstration of breaking change between C# 1 and C# 2delegate void SampleDelegate(string x);public void CandidateAction(string x){Console.WriteLine("Snippet.CandidateAction");}public class Derived : Snippet{public void CandidateAction(object o){Console.WriteLine("Derived.CandidateAction");}}...Derived x = new Derived();SampleDelegate factory = new SampleDelegate(x.CandidateAction);factory("test");Remember that Snippy 3 will be generating all of this code within a class called Snippetwhich the nested type derives from. Under C# 1, listing 5.4 would print Snippet.CandidateAction because the method taking an object parameter wasn’t compatiblewith SampleDelegate. Under C# 2, however, it is compatible and is the method chosendue to being declared in a more derived type—so the result is that Derived.CandidateAction is printed. Fortunately, the C# 2 compiler knows that this is a breakingchange and issues an appropriate warning.Enough doom and gloom about potential breakage, however. We’ve still got to seethe most important new feature regarding delegates: anonymous methods. They’re abit more complicated than the topics we’ve covered so far, but they’re also very powerful—anda large step toward C# 3.5.4 Inline delegate actions with anonymous methodsHave you ever been writing C# 1 and had to implement a delegate with a particularsignature, even though you’ve already got a method that does what you want butdoesn’t happen to have quite the right parameters? Have you ever had to implement adelegate that only needs to do one teeny, tiny thing, and yet you need a whole extramethod? Have you ever been frustrated at having to navigate away from an importantbit of code in order to see what the delegate you’re using does, only to find that themethod used is only two lines long? This kind of thing happened to me quite regularlywith C# 1. The covariance and contravariance features we’ve just talked about cansometimes help with the first problem, but often they don’t. Anonymous methods, whichare also new in C# 2, can pretty much always help with these issues.Informally, anonymous methods allow you to specify the action for a delegateinstance inline as part of the delegate instance creation expression. This means there’s3In case you skipped the first chapter, Snippy is a tool I’ve used to create short but complete code samples. Seesection 1.4.2 for more details.Licensed to Rhona Hadida