Capturing iteration variables

首先要理解Lambda表达式的延迟执行（Deferred execution）

An important feature of most query operators is that they execute not when
constructed, but when enumerated.

其次来理解捕获迭代变量

When you capture iteration variables in for and foreach statements, C# treats those
iteration variables as though they were declared outside the loop。

Action[] actions = new Action[3];
for (int i = 0; i < 3; i++)
     actions [i] = () => Console.Write (i);   // 用Lambda表达式来实例化Action委托。//这样的代码基本上就是一个Bug。
foreach (Action a in actions) a(); // 333。执行。

最后需要理解这样的外部变量的生命周期扩展到了相应的委托。

int seed = 0;
Func<int> natural = () => seed++;
Console.WriteLine (natural()); // 0
Console.WriteLine (natural()); // 1
Console.WriteLine (seed); // 2