an efficient method for detecting if a positive integer is a prime number

prime.cs

/* This code written by Bryan Miller 9th May 2021, but the actual prime detection
methods contain code found on various websites and forums.

To create DLL prime.dll, csc /t:library prime.cs
To build EXE myprogram.exe referencing that DLL, csc /r:prime.dll myprogram.cs */

using System;

namespace IsPrimeNs {
  
  public class IsPrimeCls {
    
    public static bool IsPrime(int n){
        if (n == 1) return false;
        if (n == 2 || n == 3 || n == 5) return true;
        if (n % 2 == 0 || n % 3 == 0 || n % 5 == 0) return false;

        var boundary = (int)Math.Floor(Math.Sqrt(n));

        // You can do less work by observing that at this point, all primes 
        // other than 2 and 3 leave a remainder of either 1 or 5 when divided by 6. 
        // The other possible remainders have been taken care of.
        int i = 6; // start from 6, since others below have been handled.
        while (i <= boundary)
        {
            if (n % (i + 1) == 0 || n % (i + 5) == 0)
                return false;

            i += 6;
        }

        return true;

    }    
    
    
  }
}