C# in Depth

Capturing variables in anonymous methods151Stick with it, though! The payback can be massive in terms of code simplicity andreadability. This topic will also be crucial when we come to look at lambda expressionsand LINQ in C# 3, so it’s worth the investment. Let’s start off with a few definitions.5.5.1 Defining closures and different types of variablesThe concept of closures is a very old one, first implemented in Scheme, but it’s been gainingmore prominence in recent years as more mainstream languages have taken it onboard. The basic idea is that a function 5 is able to interact with an environment beyondthe parameters provided to it. That’s all there is to it in abstract terms, but to understandhow it applies to C# 2, we need a couple more terms:■■An outer variable is a local variable or parameter 6 whose scope includes an anonymousmethod. The this reference also counts as an outer variable of anyanonymous method where it can be used.A captured outer variable (usually shortened to just “captured variable”) is an outervariable that is used within an anonymous method. So to go back to closures,the function part is the anonymous method, and the environment it can interactwith is the set of variables captured by it.That’s all very dry and may be hard to imagine, but the main thrust is that an anonymousmethod can use local variables defined in the same method that declares it. Thismay not sound like a big deal, but in many situations it’s enormously handy—you canuse contextual information that you have “on hand” rather than having to set up extratypes just to store data you already know. We’ll see some useful concrete examplessoon, I promise—but first it’s worth looking at some code to clarify these definitions.Listing 5.10 provides an example with a number of local variables. It’s just a singlemethod, so it can’t be run on its own. I’m not going to explain how it would work orwhat it would do yet, but just explain how the different variables are classified.Listing 5.10Examples of different kinds of variables with respect to anonymous methodsvoid EnclosingMethod(){int outerVariable = 5;string capturedVariable = "captured";BOuter variable(uncaptured)COuter variablecaptured byanonymousmethodif (DateTime.Now.Hour==23){int normalLocalVariable = DateTime.Now.Minute;Console.WriteLine(normalLocalVariable);}ThreadStart x = delegate(){string anonLocal="local to anonymous method";EDLocal variable ofnormal methodLocal variableof anonymousmethod5This is general computer science terminology, not C# terminology.6Excluding ref and out parameters.Licensed to Rhona Hadida