C# in Depth

34 CHAPTER 2 Core foundations: building on C# 1your last will and testament. It is a set of instructions—“pay the bills, make a donationto charity, leave the rest of my estate to the cat,” for instance. You write it before yourdeath, and leave it in an appropriately safe place. After your death, your attorney will(you hope!) act on those instructions.A delegate in C# acts like your will does in the real world—as a sequence of actionsto be executed at the appropriate time. Delegates are typically used when the code thatwants to execute the actions doesn’t know the details of what that action should be. Forinstance, the only reason that the Thread class knows what to run in a new thread whenyou start it is because you provide it with a ThreadStart delegate instance.We’ll start our tour of delegates with the four absolute basics, without which noneof the rest would make sense.2.1.1 A recipe for simple delegatesIn order for delegates to do anything, four things need to happen:■ The delegate type needs to be declared.■ There must be a method containing the code to execute.■ A delegate instance must be created.■ The delegate instance must be invoked.Let’s take each of the steps of this recipe in turn.DECLARING THE DELEGATE TYPEA delegate type is effectively just a list of parameter types and a return type. It specifieswhat kind of action can be represented by instances of the type. For instance, considera delegate type declared like this:delegate void StringProcessor (string input);The code says that if we want to create an instance of StringProcessor, we’re going toneed a method with one parameter (a string) and a void return type (the methoddoesn’t return anything). It’s important to understand that StringProcessor really isa type. It has methods, you can create instances of it, pass around references to instances,the whole works. There are obviously a few “special features,” but if you’re ever stuckwondering what will happen in a particular situation, first think about what would happenif you were just using a “normal” reference type.NOTESource of confusion: the ambiguous term “delegate”—Delegates are often misunderstoodbecause the word “delegate” is used to describe both a “delegatetype” and a “delegate instance.” The distinction between these two isexactly the same as the one that exists between any other type andinstances of that type—the string type itself is different from a particularsequence of characters, for example. I’ve used the terms “delegate type”and “delegate instance” throughout this chapter to try to keep it clearexactly what I’m talking about at any point.We’ll use the StringProcessor delegate type when we consider the next ingredient.Licensed to Rhona Hadida