prime.cs

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

namespace Prime
{
    static class Prime
    {
        static List<ulong> primes = new List<ulong> { (long)2, (long)3 };

        public static IEnumerable<ulong> Gets()
        {
            Func<ulong, bool> f = (x) =>
            {
                foreach (var prime in primes)
                    if (x % prime > 0) return true;
                return false;
            };
            foreach (var prime in primes) yield return prime;
            for (ulong i = primes.Last() + 1; ; i++)
            {
                if(f(i))
                {
                    primes.Add(i);
                    yield return i;
                };
            }
        }
    }
}