How to break out of an inner nested Parallel.Foreach loop in C#?

Question

Why doesn't this break statement work? My goal is to break out of the inner loop only, but keep processing the outer loop. It is hitting the Console.WriteLine() no matter what I do.

I have tried:

1. innerLoopState.Break()
2. innerLoopState.Stop()

Neither stop the loop before the Console.WriteLine()...

private static ParallelOptions ParallelCityOptions => new ParallelOptions
{
    MaxDegreeOfParallelism = 2
};

private static ParallelOptions ParallelPeopleOptions => new ParallelOptions
{
    MaxDegreeOfParallelism = 3
}

[Test]
public void NestedParallelForeach()
{
    var listOfCities = new List<string> {"Atlanta", "Denver", "Scranton",};
    var listOfPeople = new List<string>() {"Bob", "Sally", "Craig", "Bill"};

    Parallel.ForEach(listOfCities, ParallelCityOptions, city =>
    {
        Parallel.ForEach(listOfPeople, ParallelPeopleOptions, (person, innerLoopState) =>
        {

            if (person == "Bill")
            {
                // do something
                innerLoopState.Break();
                Console.WriteLine("Should never hit this line.");
            }
        });
    });
}

question from:https://stackoverflow.com/questions/65924213/how-to-break-out-of-an-inner-nested-parallel-foreach-loop-in-c

Answer 1

The direct answer to your question is: curry the outer loop state and break it as you wish:

Parallel.ForEach(listOfCities, ParallelCityOptions, (city, outerLoopState) =>
{
    Parallel.ForEach(listOfPeople, ParallelPeopleOptions, (person, innerLoopState) =>
    {

        if (person == "Bill")
        {
            // do something
            outerLoopState.Break();
            innerLoopState.Break();
            Console.WriteLine("Should never hit this line.");
        }
    });
});

However all of the elements in your array will be processed by your code at once, since there's so few compared to the number of cores a modern computer has. Simply requesting a parallel loop break won't magically break all threads, it will just stop new ones from being processed.