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

The canonical way to solve this is to copy the values into temporary variables which are declared inside the loop.

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 it to:

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);
}

This is simple and easy to understand. It states the developer's intent without cluttering the code with extra variables (as in the default way to solve this problem).