如何突破 C# 中的内部嵌套 Parallel.Foreach 循环？

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.");
            }
        });
    });
}

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.