.NET发展至今，其实各处都有“延迟(Lazy)”的痕迹，一个小小的“Laziness”给我们带来了不少灵活性。“延迟”的关键就在于“只在需要的时候处理数据”，老赵曾经在多篇文章中提到了类似的概念，如《高阶函数、委托与匿名方法》及《您善于使用匿名函数吗?》。不过“延迟”本身也会给您带来一些陷阱，某些陷阱您很有可能也曾经遇到过。这篇文章便是总结了延迟特性的集中常见陷阱，并给出应对方案。
重复运算
问题
“延迟”的本意是“减少计算”，但是如果您使用不当，很可能反而会造成“重复计算”。例如，我们首先构建一个方法，它接受一个参数n，返回一个Func对象：
static Func DivideBy(int n) { return x => { bool divisible = x % n == 0; Console.WriteLine( "{0} can be divisible by {1}? {2}", x, n, divisible ? "Yes" : "No"); return divisible; }; } |
返回的Func对象会根据传入的参数x，返回一个表示x能否被n整除的布尔值。在这过程中，还会向控制台输出一句话，例如：“10 can be divisible by 3? No”。每当看到这句话，则表明“经过了一次判断”。那么您是否知道，下面的代码会输出什么结果呢?
List values = new List(); for (int i = 0; i < 10; i++) values.Add(i); var divideByTwo = values.Where(DivideBy(2)); var divideByTwoAndThree = divideByTwo.Where(DivideBy(3)); var divideByTwoAndFive = divideByTwo.Where(DivideBy(5)); foreach (var i in divideByTwoAndThree) { } foreach (var i in divideByTwoAndFive) { } |
结果如下：
0 can be divisible by 2? Yes 0 can be divisible by 3? Yes 1 can be divisible by 2? No 2 can be divisible by 2? Yes 2 can be divisible by 3? No 3 can be divisible by 2? No 4 can be divisible by 2? Yes 4 can be divisible by 3? No 5 can be divisible by 2? No 6 can be divisible by 2? Yes 6 can be divisible by 3? Yes 7 can be divisible by 2? No 8 can be divisible by 2? Yes 8 can be divisible by 3? No 9 can be divisible by 2? No 0 can be divisible by 2? Yes 0 can be divisible by 5? Yes 1 can be divisible by 2? No 2 can be divisible by 2? Yes 2 can be divisible by 5? No 3 can be divisible by 2? No 4 can be divisible by 2? Yes 4 can be divisible by 5? No 5 can be divisible by 2? No 6 can be divisible by 2? Yes 6 can be divisible by 5? No 7 can be divisible by 2? No 8 can be divisible by 2? Yes 8 can be divisible by 5? No 9 can be divisible by 2? No |
您是否发现，无论是在遍历divideByTwoAndThree和divideByTwoAndFive序列时，都会从原有的values序列里重新判断每个元素是否能够被2整除?这就是.NET 3.5中“Where”的延迟特性，如果您在这里没有意识到这点，就可能会产生重复计算，浪费了计算能力。
解决方案
解决这个问题的方法就是在合适的时候进行“强制计算”。例如：
var divideByTwo = values.Where(DivideBy(2)).ToList(); var divideByTwoAndThree = divideByTwo.Where(DivideBy(3)); var divideByTwoAndFive = divideByTwo.Where(DivideBy(5)); |
结果就变成了：
0 can be divisible by 2? Yes 1 can be divisible by 2? No 2 can be divisible by 2? Yes 3 can be divisible by 2? No 4 can be divisible by 2? Yes 5 can be divisible by 2? No 6 can be divisible by 2? Yes 7 can be divisible by 2? No 8 can be divisible by 2? Yes 9 can be divisible by 2? No 0 can be divisible by 3? Yes 2 can be divisible by 3? No 4 can be divisible by 3? No 6 can be divisible by 3? Yes 8 can be divisible by 3? No 0 can be divisible by 5? Yes 2 can be divisible by 5? No 4 can be divisible by 5? No 6 can be divisible by 5? No 8 can be divisible by 5? No |
此时，在获得divideByTwo序列时，就会立即进行计算，这样在遍历后两者时就不会重复计算1，3，5等元素了。
