Prime Numbers

namespace Problem_0003
{
    using System;
    using System.Diagnostics;

    /// <summary>
    /// Largest prime factor
    /// Problem 3
    /// The prime factors of 13195 are 5, 7, 13 and 29.
    /// What is the largest prime factor of the number 600851475143 ?
    /// </summary>
    class Program
    {
        static void Main(string[] args)
        {
            Stopwatch watch = Stopwatch.StartNew();

            ulong number = 600851475143;
            ulong boundary = (ulong)Math.Floor(Math.Sqrt(number));

            ulong largestPrimeFactorResult = 0;

            for (ulong i = boundary; i > 1; i--)
            {
                if (number % i == 0 && IsPrime(i))
                {
                    largestPrimeFactorResult = i;
                    break;
                }
            }

            watch.Stop();
            decimal elapsedSeconds = watch.ElapsedMilliseconds / 1000;

            Console.WriteLine($"Result        : {largestPrimeFactorResult}");
            Console.WriteLine($"Execution time: {elapsedSeconds}");
        }

        public static bool IsPrime(ulong number)
        {
            if (number <= 1)
            {
                return false;
            }

            if (number == 2 || number == 3 || number == 5)
            {
                return true;
            }

            if ((number & 1) == 0)
            {
                return false;
            }

            if (number % 5 == 0)
            {
                return false;
            }

            ulong boundary = (ulong)Math.Floor(Math.Sqrt(number));

            ulong reminder = number % 6;

            if (reminder == 0 || reminder == 2 || reminder == 3 || reminder == 4)
            {
                return false;
            }
            else
            {
                for (ulong i = 6; i < boundary; i += 6)
                {
                    if (number % (i + 1) == 0)
                    {
                        return false;
                    }

                    if (number % (i + 5) == 0)
                    {
                        return false;
                    }
                }
            }

            return true;
        }
    }
}