Threading - issues

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Hi!

I've done some code using threading just to check if writing code improperly
can cause serious issues when using somehow safe data types.

So in my understanding a stack ( collection type ) won't give any item more
than once. But it does !!!

Again I wrote this code improperly to see what happens but I don't
understand why it happens so if you understand the issue only than reply ;)

private void btnTestThreading_Click(object sender, EventArgs e)
{
Stack<int> numbersStack = new Stack<int>();
for (int i = 0; i < 100; i++)
numbersStack.Push(i);

for (int i = 0; i < 10; i++)
{
Thread th = new Thread(delegate()
{
// i'm putting this as a reference so it should be
shared nicely
TestThreading(ref numbersStack, i);
}
);

th.Start();
}


}

private void TestThreading(ref Stack<int> numbers, int threadNumber)
{
for (int i = 0; i < 10; i++)
{
Thread.Sleep(numbers.Peek() * 10);
Console.WriteLine("Thread with number " + threadNumber +
"\t\t processed item " + numbers.Pop());
}
}


In my understanding the worst case scenario here would be 2 threads
accessing stack in the same time ( i'm not using any locks ) and than the
same item would be 'processed' twice. But what happens is 1 thread is getting
the same item twice...
HOW it's possible ? I would understand if it was other thread but it's the
same one. If you delete Thread.Sleep it stops ( at least at mine ;)

Thanks
Jedrzej

.

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