Nice loop.cs

using System;
using System.Collections.Generic;

namespace PrimeFactorisatorInator {
	class Program {
		static void Main(string[] args) {
			while (true) {
				App ();
			}
		}

		static void App () {
			Console.Write ("Indtast et heltal mellem 2 og {0} der skal deles op i primtal: ", ulong.MaxValue);
			string userInput = Console.ReadLine ();
			Console.WriteLine ("Udregner heltal, vent venligst");
			Console.WriteLine ();

			ulong userNumber = Convert.ToUInt64 (userInput);

			// TODO: Check for numbers less than 2

			// The list containing the prime numbers 
			var primes = new List<ulong> ();
			// Run the factorizer program
			PrimeFactorizer (userNumber, ref primes);
			
			if (primes.Count == 1) {											// If the number is a prime number, the function with return a list of length 1
				Console.WriteLine ("Tallet {0} er et primtal!", userNumber);
			} else {
				Console.WriteLine ("Primtalsfaktorene er: ");
				foreach (int p in primes)
					Console.Write (p + " ");
			}
			Console.WriteLine ("\n----------------------------\n");
		}
		
		/// <summary>
		/// Will return a list of all the prime numbers that the number "n" can be divided with
		/// </summary>
		/// <param name="n"></param>
		/// <returns>List of primes that when multiplied will </returns>
		static void PrimeFactorizer (ulong n, ref List<ulong> primes) {
			// Go through all the even divisors (which will always be 2)
			CheckFactors (ref n, 2, ref primes);

			// Go through all the odd divisors
			for (ulong i = 3; i <= n; i += 2) 
				CheckFactors (ref n, i, ref primes);

			// If the above checks fails, it means the number is a prime number
			if (n > 2) 
				primes.Add (n);
		}

		/// <summary>
		/// This function will check for divisors on a number and add it to a list of primes
		/// </summary>
		/// <param name="currentNumber">Number to check divisors on</param>
		/// <param name="divisor">The divisor to check with</param>
		/// <param name="primes">The list to add the prime numbers to</param>
		static void CheckFactors (ref ulong currentNumber, ulong divisor, ref List<ulong> primes) {
			while (currentNumber % divisor == 0) {  // Go through all divisions with the divisor
				primes.Add (divisor);				// Add the divisor to the list of primes
				currentNumber /= divisor;			// Divide with the current prime number
			}
		}
	}
}