C# in Depth

Capturing variables in anonymous methods153Console.WriteLine (captured);captured = "before second invocation";x();The output of listing 5.11 is as follows:directly before x is invokedchanged by xbefore second invocationLet’s look at how this happens. First, we declare the variable captured and set its valuewith a perfectly normal string literal. So far, there’s nothing special about the variable.We then declare x and set its value using an anonymous method that capturescaptured. The delegate instance will always print out the current value of captured,and then set it to “changed by x”.Just to make it absolutely clear that just creating the delegate instance didn’t readthe variable and stash its value away somewhere, we now change the value of capturedto “directly before x is invoked”. We then invoke x for the first time. It reads the valueof captured and prints it out—our first line of output. It sets the value of captured to“changed by x” and returns. When the delegate instance returns, the “normal”method continues in the usual way. It prints out the current value of captured, givingus our second line of output.The normal method then changes the value of captured yet again (this time tobefore second invocation) and invokes x for the second time. The current value ofcaptured is printed out, giving our last line of output. The delegate instance changescaptured to changed by x and returns, at which point the normal method has run outof code and we’re done.That’s a lot of detail about how a pretty short piece of code works, but there’s reallyonly one crucial idea in it: the captured variable is the same one that the rest of the methoduses. For some people, that’s hard to grasp; for others it comes naturally. Don’t worry ifit’s tricky to start with—it’ll get easier over time. Even if you’ve understood everythingeasily so far, you may be wondering why you’d want to do any of this. It’s about time wehad an example that was actually useful.5.5.3 What’s the point of captured variables?To put it simply, captured variables get rid of the need for you to write extra classesjust to store the information a delegate needs to act on, beyond what it’s passed asparameters. Before ParameterizedThreadStart existed, if you wanted to start a new(non-threadpool) thread and give it some information—the URL of a page to fetch,for instance—you had to create an extra type to hold the URL and put the action ofthe ThreadStart delegate instance in that type. It was all a very ugly way of achievingsomething that should have been simple.As another example, suppose you had a list of people and wanted to write amethod that would return a second list containing all the people who were under agiven age. We know about a method on List that returns another list of everythingmatching a predicate: the FindAll method. Before anonymous methods and capturedLicensed to Rhona Hadida