Listing primes using list comprehensions with the aid of PLINQ in C#

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

namespace PrimeLister
{
    /// <summary>
    /// Visual Studio Community 2017
    /// 
    /// Listing primes using list comprehensions with the aid of PLINQ
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            int cNumber = InputInteger();

            Console.WriteLine("The number {0} has following factors:", cNumber);
            List<int> facs = factors(cNumber);
            PrintOut(facs);

            if (prime(cNumber))
                Console.WriteLine("It is a prime number");
            else
                Console.WriteLine("It is not a prime number");

            Console.WriteLine("The primes up to this number are:");
            var STW = Stopwatch.StartNew();                
                List <int> prims = primes(cNumber); 
            STW.Stop();
            PrintOut(prims);

            Console.WriteLine("Total number of primes up to {0} is: {1}", cNumber, prims.Count());
            Console.WriteLine("Total time calculating: {0}", STW.Elapsed.ToString(@"mm\:ss\.fff"));

            Console.ReadKey();
        }

        /// <summary>
        /// Some way where it is only possible to input numbers
        /// </summary>
        /// <returns>on success, the inputted number</returns>
        static int InputInteger()
        {
            string intStr;
            int numb;
            do
            {
                Console.Write("Input an integer: ");
                intStr = string.Empty;
                intStr = Console.ReadLine();
            } while (!Int32.TryParse(intStr, out numb));
            return numb;
        }

        /// <summary>
        /// Print out a formatted list of integers on the console
        /// </summary>
        /// <param name="aList">here a list of integers</param>
        static void PrintOut(List<int> aList)
        {
            const int cNumbersOnaLine = 15;

            int Counter = 0;
            foreach (var result in aList)
            {
                Console.Write("{0,6} ", result);
                Counter++;
                if (Counter > cNumbersOnaLine)
                {
                    Counter = 0;
                    Console.WriteLine();
                }
            }
            Console.WriteLine();
        }

        /// <summary>
        /// Get a list of factors from a number
        /// </summary>
        /// <param name="number">The number to handle</param>
        /// <returns>The list of factors</returns>
        static List<int> factors(int number)
        {
            return new List<int>(from x in Enumerable.Range(1, number)
                                 where number % x == 0
                                 select x);
        }

        /// <summary>
        /// Test if a number is prime
        /// </summary>
        /// <param name="number"></param>
        /// <returns>true or false</returns>
        static bool prime(int number)
        {
            //return factors(number).Count() == 2; or
            return factors(number).SequenceEqual(new List<int>() { 1, number });
        }

        /// <summary>
        /// Get a list of primes up to a certain number.
        /// Leaving AsParallel out it's 4 times slower on my system
        /// but depending on how may numbers you choose!
        /// </summary>
        /// <param name="upto">the maximum</param>
        /// <returns>A list of primes in the range 2 until upto</returns>
        static List<int> primes(int upto)
        {
            return new List<int>(from x in Enumerable.Range(2, upto).AsParallel().AsOrdered()
                                 where prime(x)
                                 select x);
        }
    }
}