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Gary Wheeler wrote:

You don't even want to know what it was like when the reader jammed...

public class NthPrimeNumber
{
    public string GetNumber()
    {
        long number = 0;

        // All prime numbers are denoted by 6n+1 or 6n-1 format. So, assumming for 
        // 10001st number, n is 100000 (in fact it will be lesser than this)
        bool[] isComposite = new bool[1000002];

        // Since this method will find more than 10001 prime numbers, track how many have been
        // found already and once we have 10001 as the count, stop
        int primeNumberCount = 0;
        int upperBoundRoot = (int)Math.Sqrt(1000001);

        for (int i = 2; i <= upperBoundRoot; i++)
        {
            if (!isComposite[i])
            {
                primeNumberCount++;
                if (primeNumberCount == 10001)
                {
                    number = i;
                    break;
                }
                for (int k = i * i; k <= 1000001; k += i)
                    isComposite[k] = true;
            }
        }

        // if the count is not 10001 yet, there are more numbers to find
        for (int i = upperBoundRoot; i < 1000001; i++)
        {
            if (!isComposite[i])
            {
                primeNumberCount++;

                if (primeNumberCount == 10001)
                {
                    number = i;
                    break;
                }
            }
        }

        return number.ToString();
    }
}