c# - Different summation results with Parallel.ForEach

Question

I have a foreach loop that I am parallelizing and I noticed something odd. The code looks like

double sum = 0.0;

Parallel.ForEach(myCollection, arg =>
{
     sum += ComplicatedFunction(arg);
});

// Use sum variable below

When I use a regular foreach loop I get different results. There may be something deeper down inside the ComplicatedFunction but it is possible that the sum variable is being unexpectantly affected by the parallelization?

Answer 1

it is possible that the sum variable is being unexpectantly affected by the parallelization?

Yes.
Access to a double is not atomic and the sum += ... operation is never thread-safe, not even for types that are atomic. So you have multiple race conditions and the result is unpredictable.

You could use something like:

double sum = myCollection.AsParallel().Sum(arg => ComplicatedFunction(arg));

or, in a shorter notation

double sum = myCollection.AsParallel().Sum(ComplicatedFunction);