C# in Depth

146 CHAPTER 5 Fast-tracked delegatesprintReverse("Hello world");printRoot(2);printMean(samples);Listing 5.5 shows a few of the different features of anonymous methods.First, the syntax of anonymous methods: use the delegate keyword, followedby the parameters (if there are any), followed by the code for theaction of the delegate instance, in a block. The string reversal code Bshows that the block can contain local variable declarations, and the “listaveraging” code C demonstrates looping within the block. Basically, anythingyou can do in a normal method body, you can do in an anonymousmethod. 4 Likewise, the result of an anonymous method is a delegateinstance that can be used like any other one D. Be warned that contravariancedoesn’t apply to anonymous methods: you have to specify the parameter types thatmatch the delegate type exactly.In terms of implementation, we are still creating a method for each delegateinstance: the compiler will generate a method within the class and use that as theaction it uses to create the delegate instance, just as if it were a normal method. TheCLR neither knows nor cares that an anonymous method was used. You can see theextra methods within the compiled code using ildasm or Reflector. (Reflector knowshow to interpret the IL to display anonymous methods in the method that uses them,but the extra methods are still visible.)It’s worth pointing out at this stage that listing 5.5 is “exploded” compared withhow you may well see anonymous methods in real code. You’ll often see them usedas parameters to another method (rather than assigned to a variable of the delegatetype) and with very few line breaks—compactness is part of the reason forusing them, after all. For example, we mentioned in chapter 3 that List has aForEach method that takes an Action as a parameter and performs that actionon each element. Listing 5.6 shows an extreme example of this, applying the same“square rooting” action we used in listing 5.5, but in a compact form.Anonymousmethodsjust containnormal codeDInvokes delegatesas normalListing 5.6Extreme example of code compactness. Warning: unreadable code ahead!List x = new List();x.Add(5);x.Add(10);x.Add(15);x.Add(20);x.Add(25);x.ForEach(delegate(int n){Console.WriteLine(Math.Sqrt(n));});That’s pretty horrendous—especially when at first sight the last six characters appear tobe ordered almost at random. There’s a happy medium, of course. I tend to break my4One slight oddity is that if you’re writing an anonymous method in a value type, you can’t reference this fromwithin it. There’s no such restriction within a reference type.Licensed to Rhona Hadida