SieveOfEratosthenes.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading;

public class SieveOfEratosthenes
{
    public static void Main()
    {
        var n = int.Parse(Console.ReadLine());
        bool[] primes = new bool[n + 1];

        for (int i = 0; i <= n; i++)
        {
            primes[i] = true;
        }

        primes[0] = primes[1] = false;

        for (int p = 2; p <= n; p++)
        {
            if (primes[p])
            {
                FillPrimes(primes, p);
            }
        }

        DrawTable(primes);
    }

    private static void FillPrimes(bool[] primes, int step)
    {
        for (int i = 2 * step; i < primes.Length; i += step)
        {
            primes[i] = false;
                
            Console.Clear();
            //DrawTable(primes);
            //Thread.Sleep(200);
        }
    }

    private static void DrawTable(bool[] primes)
    {
        //StringBuilder output = new StringBuilder();

        int tableBoxes = Math.Min(primes.Length / 10, 20);
        int numberCount = primes.Length.ToString().Length;
        int rowLength = (tableBoxes * numberCount) + (tableBoxes + 1);

        Console.WriteLine(new string('_', rowLength));
        Console.Write("|");
        for (int i = 1; i < primes.Length; i++)
        {
            if (primes[i])
            {
                Console.BackgroundColor = ConsoleColor.White;
                Console.ForegroundColor = ConsoleColor.Black;
                Console.Write("{0," + numberCount + "}", i);
            }
            else
            {
                Console.BackgroundColor = ConsoleColor.Red;
                Console.ForegroundColor = ConsoleColor.White;
                Console.Write("{0," + numberCount + "}", " ");
            }

            Console.BackgroundColor = ConsoleColor.Black;
            Console.ForegroundColor = ConsoleColor.White;
            Console.Write("|");

            if (i % tableBoxes == 0)
            {
                Console.WriteLine();
                Console.Write(new string('-', (tableBoxes * numberCount) + (tableBoxes + 1)));
                Console.WriteLine();
                Console.Write("|");
            }
        }

        ////Console.WriteLine(output.ToString());

        ////var result = primes.Select((value, index) => new { index, value })
        ////          .ToDictionary(pair => pair.index, pair => pair.value)
        ////          .Where(e => e.Value)
        ////          .Select(pair => pair.Key);

        var result = new List<int>();
        for (int i = 0; i < primes.Length; i++)
        {
            if (primes[i])
            {
                result.Add(i);
            }
        }

        Console.WriteLine("Primes: {0}", string.Join(", ", result));
    }
}