I'm just looking in to the new .NET 4.0 features. With that, I'm attempting a simple calculation using Parallel.For and a normal for(x;x;x) loop. However, I'm getting different results about 50% of the time (no code change).

    long sum = 0;

    Parallel.For(1, 10000, y =>
        {
            sum += y;
        });

    Console.WriteLine(sum.ToString());

    sum = 0;

    for (int y = 1; y < 10000; y++)
    {
        sum += y;
    }
    Console.WriteLine(sum.ToString());

Surely I'm missing something simple or do not completely understand the concept. My guess is that the threads are trying to update "sum" at the same time. If so, is there an obvious way around it?