Private variables in .net 4.0 tasks

Question

doing a little experimenting to find out how things work. I have the following code...

 for (int i = 0; i < 20; i++)
 {
      Task.Factory.StartNew(() => MethodTest(i));
 }

I'm wondering why MethodTest receives the int 20 almost always (unless I'm stepping through in debugger).

Obviously there's something missing in my understanding as I assumed that when 'i' is passed it would be part of a managed thread's local storage.

Answer 1

You are closing over the loop variable - try this:

 for (int i = 0; i < 20; i++)
 {
      int x = i;
      Task.Factory.StartNew(() => MethodTest(x));
 }

The important thing to understand is that you are creating a closure over the variable i, and not its current value.

By the time the thread pool is starting the first thread (they first land in the queue) the variable i will almost certainly be 20 as you have broken out of the loop. Now each thread that is started will look at the value of the variable i at that point in time.

The fix as suggested is to create a new variable inside the scope of the loop and assign the current value of i to that variable. Since a new variable is used on every iteration of the loop each created thread is now closing over its "own" variable, which is isolated and will not change.

The standard reference to explain what is going is "Closing over the loop variable considered harmful".