Sieve of Eratosthenes.

Eratosthenes.cs

public static class Eratosthenes
{
    public static IEnumerable<BigInteger> Sieve(Int64 maxValue)
    {
        if (maxValue < 10)
        {
            if (maxValue == 9 || maxValue == 8 || maxValue == 7)
            {
                return new List<BigInteger>() { 2, 3, 5, 7 };
            }
            else if (maxValue == 6 || maxValue == 5)
            {
                return new List<BigInteger>() { 2, 3, 5 };
            }
            else if (maxValue == 4 || maxValue == 3)
            {
                return new List<BigInteger>() { 2, 3 };
            }
            else
            {
                return new List<BigInteger>() { 2 };
            }
        }

        Int64 counter = 0;
        Int64 counterStart = 3;
        Int64 inc;
        Int64 sqrt = 3;

        Int64 ceil = maxValue >= Int64.MaxValue - 2 ? Int64.MaxValue - 2 : (Int64)maxValue;
        bool[] primeMembershipArray = (bool[])Array.CreateInstance(typeof(bool), new long[] { (ceil + 2) });

        primeMembershipArray[2] = true;

        // Set all odds as true
        for (counter = counterStart; counter <= maxValue; counter += 2)
        {
            if ((counter & 1) == 1) // Check if odd. &1 is the same as: %2
            {
                primeMembershipArray[counter] = true;
            }
        }

        while (sqrt * sqrt <= maxValue)
        {
            counter = sqrt * sqrt;
            inc = sqrt + sqrt;

            while (counter <= maxValue)
            {
                primeMembershipArray[counter] = false;
                counter += inc;
            }

            sqrt += 2;

            while (!primeMembershipArray[sqrt])
            {
                sqrt++;
            }
        }

        return Maths.GetRange<BigInteger>(2, maxValue).Where(l => (bool)primeMembershipArray.GetValue((Int64)l)); ;
    }
}