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
作者:Petter Liu
出处:http://www.cnblogs.com/wintersun/
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该文章也同时发布在我的独立博客中-Petter Liu Blog。