Sieve Of Eratosthenes

SieveOfEratosthenes.cs

using System;
using System.Collections.Generic;
using System.Linq;

namespace Primes
{
	public static IEnumerable<int> SieveOfEratosthenes(int upperLimit)
	{
		int sieveBound = (upperLimit - 1) / 2,
			upperSqrt = ((int)Math.Sqrt(upperLimit) - 1) / 2;

		bool[] PrimeBits = new bool[sieveBound + 1];

		yield return 2;

		for (int i = 1; i <= upperSqrt; i++)
		{
			if (!PrimeBits[i])
			{
				yield return 2 * i + 1;

				for (int j = i * 2 * (i + 1); j <= sieveBound; j += 2 * i + 1)
				{
					PrimeBits[j] = true;
				}
			}
		}

		for (int i = upperSqrt + 1; i <= sieveBound; i++)
		{
			if (!PrimeBits[i])
			{
				yield return 2 * i + 1;
			}
		}
	}
}