LINQ 小心Access to modified closure 问题

http://www.cnblogs.com/wintersun/archive/2010/06/15/1758628.html

LINQ 小心Access to modified closure 问题

      最近在VisualStudio中Edit Source Code, Resharp plugin 在一处CODE上提示：Access to modified closure 。后面得知这是和闭包有关系，先看下面的CODE：

   1:              // First build a list of actions

   2:              List<Action> actions = new List<Action>();

   3:              for (int counter = 0; counter < 10; counter++)

   4:              {

   5:                  actions.Add(() => Console.WriteLine(counter));

   6:              }

   7:              // Then execute them

   8:              foreach (Action action in actions)

   9:              {

  10:                  action();

  11:              }

       一眼看上去，你以为会output 0-9，但实际上output 十个10. 这是什么原因呢？我们知道匿名函数有Capture变量的特性，上面我们声名了一个counter变量，然后相同counter变量被所有的Action实例捕捉到，所以将输出十行10的字符。如何解决这个问题，只要引入一个额外的变量在这个循环中就可以，如下的CODE：

   1:              // First build a list of actions

   2:              List<Action> actions = new List<Action>();

   3:              for (int counter = 0; counter < 10; counter++)

   4:              {

   5:                  int counter1 = counter;

   6:                  actions.Add(() => Console.WriteLine(counter1));

   7:              }

   8:              // Then execute them

   9:              foreach (Action action in actions)

  10:              {

  11:                  action();

  12:              }

       再看几个例子，都是有问题的：

   1:              // First build a list of actions

   2:  foreach (Attribute a in requiredAttributes) 

   3:  { 

   4:      result = result.Where(p => p.Attributes.Contains(a)); 

   5:  } 

   1:  List<string> keys = FillKeys() 

   2:  foreach (string key in keys){ 

   3:      q = q.Where(c => c.Company.Name.Contains(key)); 

   4:  } 

      完了，希望这篇POST对您开发有帮助！

      还可以参考：

       Comparing capture strategies: complexity vs power