C# in Depth

Capturing variables in anonymous methods159GUIDELINES FOR USING CAPTURED VARIABLESThe following is a list of suggestions for using captured variables:■ If code that doesn’t use captured variables is just as simple as code that does,don’t use them.■ Before capturing a variable declared by a for or foreach statement, considerwhether your delegate is going to live beyond the loop iteration, and whetheryou want it to see the subsequent values of that variable. If not, create anothervariable inside the loop that just copies the value you do want.■ If you create multiple delegate instances (whether in a loop or explicitly) thatcapture variables, put thought into whether you want them to capture the samevariable.■ If you capture a variable that doesn’t actually change (either in the anonymousmethod or the enclosing method body), then you don’t need to worry as much.■ If the delegate instances you create never “escape” from the method—in otherwords, they’re never stored anywhere else, or returned, or used for startingthreads—life is a lot simpler.■ Consider the extended lifetime of any captured variables in terms of garbagecollection. This is normally not an issue, but if you capture an object that isexpensive in terms of memory, it may be significant.The first point is the golden rule. Simplicity is a good thing—so any time the use of acaptured variable makes your code simpler (after you’ve factored in the additionalinherent complexity of forcing your code’s maintainers to understand what the capturedvariable does), use it. You need to include that extra complexity in your considerations,that’s all—don’t just go for minimal line count.We’ve covered a lot of ground in this section, and I’m aware that it can be hard totake in. I’ve listed the most important things to remember next, so that if you need tocome back to this section another time you can jog your memory without having toread through the whole thing again:■ The variable is captured—not its value at the point of delegate instance creation.■ Captured variables have lifetimes extended to at least that of the capturing delegate.■ Multiple delegates can capture the same variable…■ …but within loops, the same variable declaration can effectively refer to differentvariable “instances.”■ for/foreach loop declarations create variables that live for the duration of theloop—they’re not instantiated on each iteration.■ Captured variables aren’t really local variables—extra types are created wherenecessary.■ Be careful! Simple is almost always better than clever.We’ll see more variables being captured when we look at C# 3 and its lambda expressions,but for now you may be relieved to hear that we’ve finished our rundown of thenew C# 2 delegate features.Licensed to Rhona Hadida