对 C# 中的 Parallel.ForEach 循环感到困惑

Question

I'm new to C# and am basically trying to generate large Integers using the BigInteger class in C#, by feeding it a byte[] array of randomly filled values. I have another method, CheckForPrime(BigIngeger b) that checks for a prime number and simply returns true if the number is prime. So to run everything, I'm using a Parallel ForEach loop which loops infinitely until a condition is false. Here is my code:

private static IEnumerable<bool> IterateUntilFalse(bool condition)
{
    while (condition) yield return true;
}

and here is my attempt at a Parallel.ForEach loop that runs till the condition is false:

public void PrintOutPrimes(int numPrimes)
{
    byte[] makeBytes = new byte[512];
    BigInteger makePrime;
    RandomNumberGenerator r = RandomNumberGenerator.Create();
    ConcurrentBag<BigInteger> bbag = new ConcurrentBag<BigInteger>();

    int trackNum = 0;
    bool condition = (trackNum <= numPrimes);

    Parallel.ForEach(IterateUntilFalse(condition), (ignored, state) =>
    {
        if (trackNum > numPrimes) state.Stop();
        r.GetNonZeroBytes(makeBytes);
        makePrime = BigInteger.Abs(new BigInteger(makeBytes));
        if (CheckForPrime(makePrime))
        {
            bbag.Add(makePrime);
            if(bbag.TryPeek(out makePrime))
            {
                Console.WriteLine(makePrime);
            }
            Interlocked.Increment(ref trackNum);
        }
    });
}

I want to generate large prime numbers using the performance of Parallel.ForEach, and then print them out as it finds them. The problem seems like the code is "bypassing" the if statement in the loop and adding non-prime numbers to the concurrent bag too. Is there a way to avoid this and guarantee that only the prime numbers be added to the bag and then printed sequentially?

Answer 1

bool condition is not a condition, it's the result of evaluating the condition once. Basically it is always true and will never change. Therefore, your iterator will yield true forever.

To solve that, you need a real condition that can be evaluated several times, like a Lambda expression. But even with a lambda this won't work well due to race conditions in your local variables.

Actually I don't really see why you use Parallel.Foreach at all. If you want 500 primes, then use Parallel.For , not Parallel.Foreach .

Instead of generating a huge amount of random numbers and checking them for primality, choose a random number and increment it until you find a prime number. That way it's guaranteed that you get one prime number for every random number input.

Concept:

Parallel.For(0, numPrimes, (i, state)=>
{
    byte[] makeBytes = new byte[512];
    r.GetNonZeroBytes(makeBytes);
    BigInteger makePrime = BigInteger.Abs(new BigInteger(makeBytes));
    while (!IsPrime(makePrime))
       makePrime += 1;       // at some point, it will become prime
    bbag.Add(makePrime);
});

You probably want to start with an odd number and increment by 2.

You definitely want one RNG per thread instead of accessing a single global RNG, except you have a thread-safe RNG.

You also want the byte[] to be part of the state, so that it does not get recreated per loop and messes with garbage collection.