c# - How to avoid captured variables?

Question

I'm having a problem with

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

        ThreadPool.QueueUserWorkItem(waitCallback);
    }
}

When the DoSomething gets executed, it receives the latest value for each captured variable instead of the value I desired. I can imagine a solution for this, but it imagine you guys can come up with better solutions

Answer 1

解决此问题的规范方法是将值复制到在循环内声明的临时变量中。

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

将其重构为：

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);
}

这是简单易懂的。它说明了开发人员的意图，而不会用额外的变量使代码混乱（就像解决此问题的默认方式一样）。

Answer 3

作为参考，我想以下内容可以解决我的问题：

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

我会像这样写整件事，这可以避开问题，并且绝对不会对正在发生的事情产生任何疑问：

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

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