Question

I have a method that accepts a List<int> called DoWork. I have a huge List<int> Ids. I split the huge list into 4 sub lists:

List<List<int>> t = (List<List<int>>)SplitColumn<int>(Ids);

(SplitColumn is slightly modified from the answer to splitting a list into sub lists).

I paused the program and inspected t with the debugger and it is four lists divided exactly as I would expect.

Then, what I’m trying to do is spawn four threads (one for each sublist). The part I’m having trouble with is passing the four lists. I am getting out of bounds problems, I’m not sure what’s going on here:

        List<Thread> threads = new List<Thread>();

        for(int i = 0; i < t.Count; i++) 
        {
            threads.Add(new Thread(() => DoWork(t[i])));
        }

        foreach (Thread thread in threads)
        {
            thread.Start();
        }


        foreach (Thread thread in threads)
        {
            thread.Join();
        }

Answer 1

This is a classic, called capturing the loop variable.

In this code, the same variable i is shared by all threads. By the time the threads are running the main thread will have made i == t.Count, hence the range exception.

    for(int i = 0; i < t.Count; i++) 
    {
        threads.Add(new Thread(() => DoWork(t[i])));
    }

To fix it:

    for(int i = 0; i < t.Count; i++) 
    {
        int copy = i;
        threads.Add(new Thread(() => DoWork(t[copy])));
    }