Question

I am having a problem where my program executes and then just hangs somewhere during execution. I basically want to know what causes this.

for (long i = maxNumber; i > 2; i--)
{
    IsPrime = true;
    for (long g = 2; g < i; g++)
    {
        long temp = i % g;
        if (temp == 0)
        {
            IsPrime = false;                            
            break;
        }
    }
    if (IsPrime == true)
    {
        largestPrimeFactor = i;                        
        break;
    }
}

Answer 1

If I tried it correctly, your code is trying to find the largest prime between 0 and maxNumber. Use the Sieve of Eratosthenes for finding all prime numbers between 0 and the square root of maxNumber. Then, you can iterate from maxNumber to 0 for a number indivisible by all of the prime numbers you’ve just found.

EDIT :
Tried this

            var sqrtMax = (int)Math.Sqrt(maxNumber);
            var primeCandidates = Enumerable.Range(2, sqrtMax-1)
             .ToDictionary(number => number, isComposite => false);

            foreach (var number in primeCandidates.Keys.ToArray())
            {
                if (primeCandidates[number])
                {
                    continue;
                }
                Parallel.ForEach(Enumerable.Range(2, sqrtMax / number - 1).Select(times => number * times),multiples=>
                    primeCandidates[multiples] = true);
            }
            var primeList = primeCandidates.Where(number => !number.Value).Select(pair=>pair.Key).ToArray();
            var maxPrime = maxNumber;


            while (primeList.AsParallel().Any(prime=> maxPrime%prime==0))
            {
                maxPrime--;
            }

and it find maxPrime in less than 3 seconds for maxNumber = 600881475134 (the parallelization was because I thought it would take long time)