C# in Depth

C# 2 and 3: new features on a solid base55fixes this with anonymous methods, and introduces a simpler syntax for the cases whereyou still want to use a normal method to provide the action for the delegate. You can alsocreate delegate instances using methods with compatible signatures—the method signatureno longer has to be exactly the same as the delegate’s declaration.Listing 2.4 demonstrates all these improvements.Listing 2.4 Improvements in delegate instantiation brought in by C# 2static void HandleDemoEvent(object sender, EventArgs e){Console.WriteLine ("Handled by HandleDemoEvent");}...EventHandler handler;handler = new EventHandler(HandleDemoEvent);BSpecifies delegatetype and methodhandler(null, EventArgs.Empty);handler = HandleDemoEvent;CImplicitly convertsto delegate instancehandler(null, EventArgs.Empty);handler = delegate(object sender, EventArgs e){ Console.WriteLine ("Handled anonymously"); };handler(null, EventArgs.Empty);handler = delegate{ Console.WriteLine ("Handled anonymously again"); };handler(null, EventArgs.Empty);MouseEventHandler mouseHandler = HandleDemoEvent;mouseHandler(null, new MouseEventArgs(MouseButtons.None,0, 0, 0, 0));Specifies actionwith anonymousmethodThe first part of the main code B is just C# 1 code, kept for comparison. The remainingdelegates all use new features of C# 2. The conversion involved C makes eventsubscription code read a lot more pleasantly—lines such as saveButton.Click +=SaveDocument; are very straightforward, with no extra fluff to distract the eye. Theanonymous method syntax D is a little cumbersome, but does allow the action tobe very clear at the point of creation, rather than being another method to look atbefore you understand what’s going on. The shortcut used E is another example ofanonymous method syntax, but this form can only be used when you don’t need theparameters. Anonymous methods have other powerful features as well, but we’ll seethose later.The final delegate instance created F is an instance of MouseEventHandler ratherthan just EventHandler—but the HandleDemoEvent method can still be used due tocontravariance, which specifies parameter compatibility. Covariance specifies returntype compatibility. We’ll be looking at both of these in more detail in chapter 5. Eventhandlers are probably the biggest beneficiaries of this, as suddenly the Microsoftguideline to make all delegate types used in events follow the same convention makesDEUsesanonymousmethodshortcutFUsesdelegatecontravarianceLicensed to Rhona Hadida