Question

I need to run a method with a given parameter in a thread. I’ve noticed that when I run it,
the parameter is wrong. For the example given, I have an array int[] output with the numbers 1-7. For each number, I create a thread with the method WriteInt(i). I expect the output to be 1-7 in any order, but I consistently see some numbers missed and others duplicated. What is going on and what would the correct way be to start these threads?

(The list is only there to join the threads afterwards)

class Program
{
    static void Main(string[] args)
    {
        int[] output = { 1, 2, 3, 4, 5, 6, 7 };

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

        foreach (int i in output)
        {
            Thread thread = new Thread(() => WriteInt(i));
            thread.Start();
            runningThreads.Add(thread);
        }
        foreach(Thread t in runningThreads)
        {
            t.Join();
        }
    }

    private static void WriteInt(int i)
    {
        Console.WriteLine(i);
    }
}

Example output:

3
3
4
5
6
7

Answer 1

The closure created by the lambda (() => WriteInt(i)) is getting closing over the variable i, not the value set to i within each iteration. As the thread runs, it uses the value set within i at that point in time, which is likely already been changed due to the foreach loop processing.

You need a temporary:

foreach (int i in output)
{
        int temp = i;
        Thread thread = new Thread(() => WriteInt(temp));
        thread.Start();
        runningThreads.Add(thread);
}

For details on what’s happening, see Eric Lippert’s post titled Closing over the loop variable considered harmful.

Also, in C# 5 (VS2012), this is no longer an issue for foreach loops. It will still happen with a for loop, however.