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);
    }
}