C# in Depth

150 CHAPTER 5 Fast-tracked delegatesto call different method overloads) then the compiler needs more help. To show youwhat I mean, let’s look at how we start threads. There are four thread constructors in.NET 2.0:public Thread (ParameterizedThreadStart start)public Thread (ThreadStart start)public Thread (ParameterizedThreadStart start, int maxStackSize)public Thread (ThreadStart start, int maxStackSize)The two delegate types involved arepublic delegate void ThreadStart()public delegate void ParameterizedThreadStart(object obj)Now, consider the following three attempts to create a new thread:new Thread(delegate() { Console.WriteLine("t1"); } );new Thread(delegate(object o) { Console.WriteLine("t2"); } );new Thread(delegate { Console.WriteLine("t3"); } );The first and second lines contain parameter lists—the compiler knows that it can’t convertthe anonymous method in the first line into a ParameterizedThreadStart, or convertthe anonymous method in the second line into a ThreadStart. Those lines compile,because there’s only one applicable constructor overload in each case. The third line,however, is ambiguous—the anonymous method can be converted into either delegatetype, so both of the constructor overloads taking just one parameter are applicable. Inthis situation, the compiler throws its hands up and issues an error. You can solve thiseither by specifying the parameter list explicitly or casting the anonymous method to theright delegate type.Hopefully what you’ve seen of anonymous methods so far will have provokedsome thought about your own code, and made you consider where you could usethese techniques to good effect. Indeed, even if anonymous methods could only dowhat we’ve already seen, they’d still be very useful. However, there’s more to anonymousmethods than just avoiding the inclusion of an extra method in your code.Anonymous methods are C# 2’s implementation of a feature known elsewhere as closuresby way of captured variables. Our next section explains both of these terms andshows how anonymous methods can be extremely powerful—and confusing if you’renot careful.5.5 Capturing variables in anonymous methodsI don’t like having to give warnings, but I think it makes sense to include one here: ifthis topic is new to you, then don’t start this section until you’re feeling reasonablyawake and have a bit of time to spend on it. I don’t want to alarm you unnecessarily,and you should feel confident that there’s nothing so insanely complicated that youwon’t be able to understand it with a little effort. It’s just that captured variables canbe somewhat confusing to start with, partly because they overturn some of your existingknowledge and intuition.Licensed to Rhona Hadida