.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1。“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<int, bool>对象：

static Func<int, bool> 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<int, bool>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<int> values = new List<int>();
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。

异常陷阱

问题

请问您是否知道下面的代码有什么问题？

public static IEnumerable<string> ToString(IEnumerable<int> source)
{
    if (source == null)
    {
&