Captured variables in ParameterizedThreadStart

Question

I have the following code that creates 10 threads which in turn write out messages to the console:

for (int i = 0; i < 10; i++)
{
    {
        Thread thread = new Thread((threadNumber) =>
            {
                for (int j = 0; j < 10; j++)
                {
                    Thread.Sleep(200);
                    Console.WriteLine(string.Format("Thread: {0}, Line: {1}", threadNumber, j));
                }                           
            });
        thread.Start(i);
    }
}

My understanding is that ParameterizedThreadStart takes an object for which a copy of the reference is sent to the thread. If that is the case since I have not made a local copy of i within each loop all new threads would point to the same memory location meaning certain thread numbers could be 'missed out'. Having run this though (and even against a larger number of threads/sleep times) each value of i has its own thread. Can anyone explain why?

score 4 · Accepted Answer · answered Jul 13 '13 at 17:50

You haven't applied anything deferred or "captured" in the sense of creating an anonymous function that would wrap i.

The lambda function here does not reference i anywhere and its state is completely internalized/contained so no issues here:

(threadNumber) =>
{
    for (int j = 0; j < 10; j++)
    {
        Thread.Sleep(200);
        Console.WriteLine(string.Format("Thread: {0}, Line: {1}", threadNumber, j));
    }                           
});

The Start call here:

thread.Start(i);

Passes i by value (i.e. copies its value) because it is a "value type" and it's not captured in any kind of anonymous function. In this sense, it is passed as any normal struct would to any normal method (because this is exactly what is happening).

If instead you had written your lambda as this using i instead of your threadNumber:

{
    for (int j = 0; j < 10; j++)
    {
        Thread.Sleep(200);
        Console.WriteLine(string.Format("Thread: {0}, Line: {1}", i, j));
    }                           
});

Then you would be in trouble. In this case i is referring to the original variable location and will be evaluated whenever the thread executes. This means it could be the current value of i when it was created (unlikely just due to processing times), or the value set later on in the for loop, or the last possible value 10, and quite possibly have the number skip or shared between iterations.

Thanks for the detailed explanation Chris, it was the copy by value part that was tripping me up! — Maverick, Jul 13 '13 at 19:40
@Maverick note even in the "error" case he gives, you can still get the correct result by adding a line `var i1 = i` before the closure, and using `i1` everywhere you use `i`. — lobsterism, Jul 14 '13 at 02:09

Captured variables in ParameterizedThreadStart

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