Parallel.For(): Update variable outside of loop

Question

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.

Answer 1

sum += y; is actually sum = sum + y;. You are getting incorrect results because of the following race condition:

1. Thread1 reads sum
2. Thread2 reads sum
3. Thread1 calculates sum+y1, and stores the result in sum
4. Thread2 calculates sum+y2, and stores the result in sum

sum is now equal to sum+y2, instead of sum+y1+y2.