The Sieve of Eratosthenes in C#

In some of the Project Euler problems we have needed a source of primes. One algorithm for finding primes is called the Sieve of Eratosthenes. This algorithm is both pretty simple to understand and to implement. It is also fairly fast and usable, at least for the lower primes.

My implementation is based upon the algorithm described on the Wikipedia page and some helpful optimizations I found in an article at Black Wasp. The differences from the original algorithm and the solution at Black Wasp is that it finds the primes incrementally and that it only looks for a new prime when asked.

My code should be pretty self explanatory, but the basic idea is that we have a list of known primes and for each odd number we check it against the these. If a number is not evenly divisible by any of the known primes, then we have found a new one and can add it to our list.

public class Eratosthenes : IEnumerable<ulong>
{
    private readonly List<ulong> knownPrimes;

    public Eratosthenes()
    {
        knownPrimes = new List<ulong> {2, 3};
    }

    public IEnumerator<ulong> GetEnumerator()
    {
        // Return the ones we know first
        foreach (var prime in knownPrimes)
            yield return prime;

        // Then find new ones
        var possible = knownPrimes.Last();
        while (true)
            if (IsPrime(possible += 2))
            {
                yield return possible;
                knownPrimes.Add(possible);
            }
    }

    IEnumerator IEnumerable.GetEnumerator()
    {
        return GetEnumerator();
    }

    private bool IsPrime(ulong value)
    {
        var sqrt = (ulong) Math.Sqrt(value);
        return ! knownPrimes
            .TakeWhile(x => x <= sqrt)
            .Any(x => value%x == 0);
    }
}

How do you use it? Simple! For example, to print the first 500 primes you can do this:

var primes = new Eratosthenes().Take(500);

foreach(var prime in primes)
{
    Console.WriteLine(prime);
}

And that's all there is to it! What do you think? Let me know if you have any questions about it or any suggestions to how it can be improved. I want to know 😉