UF pc, C# - rextester

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//Rextester.Program.Main is the entry point for your code. Don't change it.
//Compiler version 4.0.30319.17929 for Microsoft (R) .NET Framework 4.5

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

namespace Rextester
{
    public class Program
    {
        public static void Main(string[] args)
        {
            Random rnd = new Random();
            var number = rnd.Next(0, 9999);
            
            var start = DateTime.Now.AddYears(-25);
            int range = (DateTime.Now.AddYears(-15) - start).Days;
                        
            var date = start.AddDays(rnd.Next(range)).ToString("yyMMdd");
            
            while(!ModulusCheck(date+number.ToString("0000"))){
                number = rnd.Next(0, 9999);
            }
            Console.WriteLine(date+"-"+number.ToString("0000"));
        }
        
        private static bool ModulusCheck(string nr)
        {
            decimal n;
            decimal checkSum = 0;
            decimal checkNum;
            var pnumArr = nr.ToCharArray();

if (nr.Length == 12)
            {
                for (var i = 2; i < 11; i++)
                {
                    checkNum = 0;
                    n = (decimal)pnumArr[i] - 48;
                    if (i % 2 == 0)
                    {
                        checkNum += n * 2;
                    }
                    else
                    {
                        checkNum += n;
                    }

if (checkNum >= 10)
                    {
                        checkSum += 1 + (checkNum - 10);
                    }
                    else
                    {
                        checkSum += checkNum;
                    }
                }
            }
            else
            {
                for (var i = 0; i < pnumArr.Count() - 1; i++)
                {
                    checkNum = 0;
                    n = (decimal)pnumArr[i] - 48;
                    if (i % 2 == 0)
                    {
                        checkNum += n * 2;
                    }
                    else
                    {
                        checkNum += n;
                    }

if (checkNum >= 10)
                    {
                        checkSum += 1 + (checkNum - 10);
                    }
                    else
                    {
                        checkSum += checkNum;
                    }
                }
            }

var checkSum2 = checkSum.ToString();
            var entalet = Convert.ToDecimal(checkSum2.Substring(checkSum2.Length - 1, 1));

if (entalet == 0)
            {
                entalet = 10;
            }

if (10 - entalet == ((decimal)pnumArr[pnumArr.Count() - 1] - 48))
            {
                return true;
            }

return false;
        }
    }
}

Show compiler warnings [ + ] Show input
Compilation time: 0,16 sec, absolute running time: 0,09 sec, cpu time: 0,11 sec, average memory usage: 13 Mb, average nr of threads: 3