Question

我遇到了

的问题

foreach(var category in categories)
{
    foreach(var word in words)
    {
        var waitCallback = new WaitCallback(state =>
        {
            DoSomething(word, category);
        });

        ThreadPool.QueueUserWorkItem(waitCallback);
    }
}

执行DoSomething时，它会接收每个捕获变量的最新值，而不是我想要的值。我可以想象一个解决方案，但它想象你们可以提出更好的解决方案

Answer 1

解决此问题的规范方法是将值复制到临时变量中，这些变量在循环中声明为

。

foreach(var category in categories) { var catCopy = category; foreach(var word in words) { var wordCopy = word; var waitCallback = new WaitCallback(state => { DoSomething(wordCopy, catCopy); }); ThreadPool.QueueUserWorkItem(waitCallback); } }

Answer 2

Refactor来：

foreach(var category in categories) {
  foreach(var word in words) {
    DoSomethingAsync(word, category);
  }
}

...

private void DoSomethingAsync(string word, string category) {
  var waitCallback = new WaitCallback(state => DoSomething(word, category));
  ThreadPool.QueueUserWorkItem(waitCallback);
}

这很简单易懂。它说明了开发人员的 intent ，而没有使用额外的变量使代码混乱（如解决此问题的默认方式）。

Answer 3

作为参考，我想以下可以解决我的问题：

foreach(var category in categories)
{
    foreach(var word in words)
    {
        var waitCallback = new WaitCallback(state =>
        {
            var kv = (KeyValuePair<string, string>)state;
            DoSomething(kv.Key, kv.Value);
        });

        var state2 = new KeyValuePair<string, string>(word, category);
        ThreadPool.QueueUserWorkItem(waitCallback, state2);
    }
}

Answer 4

我会写出这样的全部内容，它会避开问题并且完全没有问题：

var callbacks = words.SelectMany(w => categories.Select(c =>
    new WaitCallback(state => {
        DoSomething(w, c);
    })
));

foreach (var callback in callbacks)
    ThreadPool.QueueUserWorkItem(callback);

如何避免捕获变量？

4 个答案: