C#:エラトステネスの篩を使って素数を求める ref: https://qiita.com/gushwell/items/5d3988a2f5de0587c88f

PrimesSavingMemory.cs

 static IEnumerable<int> Primes(int maxnum) {
     // var sieve = new BitArray(maxnum + 1, true);
     bool[] sieve = Enumerable.Repeat(true, maxnum + 1).ToArray();
     int squareroot = (int)Math.Sqrt(maxnum);
     for (int i = 2; i <= squareroot; i++) {
         if (sieve[i] == false)
             continue;
         for (int n = i * 2; n <= maxnum; n += i)
             sieve[n] = false;
     }
     for (int i = 2; i <= maxnum; i++)
         if (sieve[i] == true)
             yield return i;
 }

PrimesStartDash.cs

 static IEnumerable<int> Primes(int maxnum) {
     var sieve = new BitArray(maxnum + 1, true);
     int squareroot = (int)Math.Sqrt(maxnum);
     for (int i = 2; i <= squareroot; i++) {
         if (sieve[i] == false)
             continue;
         for (int n = i * 2; n <= maxnum; n += i)
             sieve[n] = false;
         if (sieve[i])
             yield return i;  // ここで列挙してしまう
     }
     for (int i = squareroot + 1; i <= maxnum; i++)
         if (sieve[i] == true)
             yield return i;
 }

PrimesStartDash2.cs

 static IEnumerable<int> Primes(int maxnum) {
    yield return 2;
    yield return 3;
    var sieve = new BitArray(maxnum + 1);
    int squareroot = (int)Math.Sqrt(maxnum);
    for (int i = 2; i <= squareroot; i++) {
        if (sieve[i] == false) {
            for (int n = i * 2; n <= maxnum; n += i)
                sieve[n] = true;
        }
        for (int n = i * i + 1; n <= maxnum && n < (i + 1) * (i + 1); n++) {
            if (!sieve[n])
                yield return n;
        }
    }
 }

SieveOfEratosthenes.cs

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

namespace Gushwell.Etude {

    class Program {
        static void Main(string[] args) {
            foreach(var n in Primes(200))
                Console.Write("{0,3} ",n);
            Console.WriteLine();
            Console.ReadLine();
        }

        static IEnumerable<int> Primes(int maxnum) {
            int[] sieve = Enumerable.Range(0, maxnum + 1).ToArray();
            sieve[1] = 0;  // 0 : 素数ではない
            int squareroot = (int)Math.Sqrt(maxnum);
            for (int i = 2; i <= squareroot; i++) {
                if (sieve[i] <= 0)
                    continue;
                for (int n = i * 2; n <= maxnum; n += i)
                    sieve[n] = 0;
            }
            return sieve.Where(n => n > 0);
        }
    }
}