This is a function for creating a valid-ish credit card number

CardGenerator.cs

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

namespace CardGenerator
{
	/*
	* Original instructions https://www.nerdwallet.com/blog/credit-cards/credit-card-numbers-valid-website-luhn-algorithm/
	*/

    static class Extensions
    {
		public static long NextCard(this Random r)
        {
            //var start = r.LongRandom(1111111111111111, 9999999999999999);
            int process = 0;
            List<int> start = new List<int>();
            while (process % 10 != 0)
            {
                start = new List<int>();
                var work = new int[10];
                for (int i = 0; i < 16; i++) start.Add(r.Next(1, 9));

                start.CopyTo(work, 6);
                work.DoMath(x => x * 2);

                for (int i = 0; i < work.Count(); i++)
                {
                    if (work[i] >= 10)
                    {
                        work[i] = work[i] % 10 + (int)(work[i] / 10);
                    }
                }

                process = (work.Sum() * 9) % 10 + added; 
            }

            long ret = 0;
            for (int i = 1; i <= 16; i++)
            {
                ret = ret + (i * start[i]);
            }

            return ret;
        }
		
		public static IEnumerable<T> DoMath<T>(this IEnumerable<T> e, Func<T, T> operation)
        {
            var o = new List<T>();
            var a = e.ToArray();
            for (int i = 0; i < e.Count(); i++) o.Add(operation.Invoke(a[i]));
            e = o.AsEnumerable();
            return o.AsEnumerable();
        }
	}
}