C# in Depth

142 CHAPTER 5 Fast-tracked delegatesListing 5.2Demonstration of method group conversions and delegate contravariancestatic void LogPlainEvent(object sender, EventArgs e){Console.WriteLine ("An event occurred");}...Button button = new Button();button.Text = "Click me";button.Click += LogPlainEvent;button.KeyPress += LogPlainEvent;button.MouseClick += LogPlainEvent;Form form = new Form();form.AutoSize=true;form.Controls.Add(button);Application.Run(form);We’ve managed to completely remove the two handler methods that dealt specificallywith key and mouse events, using one event handling method B for everything. Ofcourse, this isn’t terribly useful if you want to do different things for different types ofevents, but sometimes all you need to know is that an event occurred and, potentially,the source of the event. The subscription to the Click event C only uses the implicitconversion we discussed in the previous section because it has a simple EventArgsparameter, but the other event subscriptions D involve the conversion and contravariancedue to their different parameter types.I mentioned earlier that the .NET 1.0/1.1 event handler convention didn’t makemuch sense when it was first introduced. This example shows exactly why the guidelinesare more useful with C# 2. The convention dictates that event handlers shouldhave a signature with two parameters, the first of which is of type object and is theorigin of the event, and the second of which carries any extra information about theevent in a type deriving from EventArgs. Before contravariance became available,this wasn’t useful—there was no benefit to making the informational parameterderive from EventArgs, and sometimes there wasn’t much use for the origin of theevent. It was often more sensible just to pass the relevant information directly in theform of normal parameters, just like any other method. Now, however, you can use amethod with the EventHandler signature as the action for any delegate type thathonors the convention.Demonstrating covariance of return types is a little harder as relatively few built-indelegates are declared with a nonvoid return type. There are some available, but it’s easierto declare our own delegate type that uses Stream as its return type. For simplicitywe’ll make it parameterless: 2delegate Stream StreamFactory();Uses methodgroup conversionB Handlesall eventsWe can now use this with a method that is declared to return a specific type of stream,as shown in listing 5.3. We declare a method that always returns a MemoryStream with2Return type covariance and parameter type contravariance can be used at the same time, although you’reunlikely to come across situations where it would be useful.CUses conversionD and contravarianceLicensed to Rhona Hadida