C# in Depth

38 CHAPTER 2 Core foundations: building on C# 1how they’re used differently when we create the delegate instances E. We’ve createdtwo instances of the Person class so that we can see the difference that the targetof a delegate makes. When jonsVoice is invoked F, it calls the Say method onthe Person object with the name Jon; likewise, when tomsVoice is invoked, it usesthe object with the name Tom. I’ve included both the ways we’ve seen of invokingdelegate instances—calling Invoke explicitly and using the C# shorthand—just forinterest’s sake. Normally you’d just use the shorthand. The output for listing 2.1 isfairly obvious:Jon says: Hello, son.Tom says: Hello, Daddy!(An airplane flies past.)Frankly, there’s an awful lot of code in listing 2.1 to display three lines of output. Evenif we wanted to use the Person class and the Background class, there’s no real need touse delegates here. So what’s the point? Why can’t we just call methods directly? Theanswer lies in our original example of an attorney executing a will—just because youwant something to happen, that doesn’t mean you’re always there at the right timeand place to make it happen yourself. Sometimes you need to give instructions—todelegate responsibility, as it were.I should stress that back in the world of software, this isn’t a matter ofobjects leaving dying wishes. Often the object that first creates a delegateinstance is still alive and well when the delegate instance isinvoked. Instead, it’s about specifying some code to be executed at aparticular time, when you may not be able to (or may not want to)change the code that is running at that point. If I want something tohappen when a button is clicked, I don’t want to have to change thecode of the button—I just want to tell the button to call one of my methodsthat will take the appropriate action. It’s a matter of adding a level ofindirection—as so much of object-oriented programming is. As we’ve seen, this addscomplexity (look at how many lines of code it took to produce so little output!) butalso flexibility.Now that we understand a bit more about simple delegates, we’ll take a brief look atcombining delegates together to execute a whole bunch of actions instead of just one.Reasons fordelegatesexisting2.1.2 Combining and removing delegatesSo far, all the delegate instances we’ve looked at have had a single action. The truth isa little bit more complicated: a delegate instance actually has a list of actions associatedwith it. This is called the invocation list of the delegate instance. The static Combine andRemove methods of the System.Delegate type are responsible for creating new delegateinstances by respectively splicing together the invocation lists of two delegateinstances or removing the invocation list of one delegate instance from another.Before we look at the details, it’s important to note that delegate instances areimmutable. Once you’ve created a delegate instance, nothing about it can be changed.This makes it safe to pass around delegate instances and combine them with othersLicensed to Rhona Hadida