Question

using System.Threading.Tasks;

const int _Total = 1000000;
[ThreadStatic]
static long count = 0;

static void Main(string[] args)
{
    Parallel.For(0, _Total, (i) =>
    {
        count++;
    });

    Console.WriteLine(count);
}

I get different result every time, can anybody help me and tell me why?

Answer 1

Most likely your “count” variable isn’t atomic in any form, so you are getting concurrent modifications that aren’t synchronized. Thus, the following sequence of events is possible:

• Thread 1 reads “count”
• Thread 2 reads “count”
• Thread 1 stores value+1
• Thread 2 stores value+1

Thus, the “for” loop has done 2 iterations, but the value has only increased by 1. As thread ordering is “random”, so will be the result.

Things can get a lot worse, of course:

• Thread 1 reads count
• Thread 2 increases count 100 times
• Thread 1 stores value+1

In that case, all those 100 increases done by thread 2 are undone. Although that can really only happen if the “++” is actually split into at least 2 machine instructions, so it can be interrupted in the middle of the operation. In the one-instruction case, you’re only dealing with interleaved hardware threads.