A prime generator. I think it works all right

primegenerator.cs

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

namespace Primer
{
    class PrimeGenerator
    {
        private readonly BigInteger _checkStep = 5000;

        private static IEnumerable<BigInteger> GetBigIntegers(BigInteger start)
        {
            while (true)
                yield return start++;
        }

        private static IEnumerable<BigInteger> GetMultiples(BigInteger number, BigInteger min, BigInteger max)
        {
            for (var n = (min/number)*number; n < max; n += number)
                yield return n;
        }
 
        public IEnumerable<BigInteger> GetPrimes()
        {
            /*
             * nonPrimes contains a list of number that are not primes.
             * This list is filled by multiplying all known prime numbers by any number.
             */
            List<BigInteger> nonPrimes = new List<BigInteger>();

            /*
             * checkLimit is the number up to which nonPrimes have yet been calculated.
             */
            BigInteger checkLimit = _checkStep;

            /*
             * primes contains a list of found primes.
             */
            List<BigInteger> primes = new List<BigInteger>();

            /*
             * Loop trough an infinite list of integers, starting at 2.
             * For this reason, this function should be used with linq (eg. x.Take(n))
             */
            foreach(var number in GetBigIntegers(2)){
                /*
                 * If this number is outside of nonPrimes boundaries, generate a new set.
                 */
                if (number == checkLimit)
                {
                    /*
                     * Replace the set with a new set of all multiples of the found primes.
                     */
                    var min = checkLimit;
                    checkLimit += _checkStep;

                    nonPrimes.Clear();
                    foreach (var p in primes.Where(p => p < checkLimit / 2))
                        nonPrimes.AddRange(GetMultiples(p, min, checkLimit));
                }

                /*
                 * If the number is in the nonPrimes list, it is not a prime.
                 */
                if (nonPrimes.Contains(number)) continue;

                /*
                 * Add the number to the known primes list and return the number.
                 */
                primes.Add(number);
                yield return number;

                /*
                 * Add all multiples of this prime to the nonPrimes list.
                 */
                if (checkLimit > number*2)
                    nonPrimes.AddRange(GetMultiples(number, number, checkLimit));

            }
        }
    }
}