extrefGenerate, C#

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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)
        {
             var currVal = DateTime.Now.Ticks;
             Console.WriteLine("Tick:"+currVal);
             Console.WriteLine("externalRef:"+GenerateT4TExternalReference(currVal));

}
        
         public static string GenerateT4TExternalReference(long extticks = 0)
        {
             
            var ticks = extticks != 0 ? extticks.ToString().Substring(5, 13) : DateTime.Now.Ticks.ToString().Substring(5, 13);
            Console.WriteLine("Short Tick:"+ticks);
            var mod10 = GetMod10Digit(ticks).ToString();
            Console.WriteLine("LuhnChecksum:"+mod10);
            var strToBase = ticks + mod10;
            Console.WriteLine("Tick+check:"+strToBase);
            var longNumber = Convert.ToInt64(strToBase);
             
            return ConvertTo(longNumber);
        }
    
        private static int GetMod10Digit(string data)
        {

int sumOfDigits = data.Where((e) => e >= '0' && e <= '9')
            .Reverse()
            .Select((e, i) => ((int)e - 48) * (i % 2 == 0 ? 1 : 2))
            .Sum((e) => e / 10 + e % 10);
            //// If the final sum is divisible by 10, then the credit card number is valid. If it is not divisible by 10, the number is invalid.
            return sumOfDigits % 10;
        }
        
        public static readonly string baseDef = "ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz0123456789-_";
        public static string ConvertTo(long val)
        {
 
            string result = string.Empty;
            int targetBase = baseDef.Length;
 
            do
            {
                result = baseDef[Convert.ToInt32(val % targetBase)] + result;
                val = val / targetBase;
            }
            while (val > 0);
 
            return result;
        }

}}

Show compiler warnings [ + ] Show input
Compilation time: 0,19 sec, absolute running time: 0,13 sec, cpu time: 0,12 sec, average memory usage: 17 Mb, average nr of threads: 4, absolute service time: 0,34 sec