using System;

using System.Text.RegularExpressions;

public class Program

{

    public static void Main()

    {

        if(IsInvalidFaxNumber("070-408-96785")){

            Console.WriteLine("bad");

        }

        else{

            Console.WriteLine("good");

        }

        if(IsInvalidFaxNumber("")){

            Console.WriteLine("bad");

        }

                else{

            Console.WriteLine("good");

        }

        try{

        if(IsInvalidFaxNumber(null)){

            Console.WriteLine("bad");

        }

                    else{

            Console.WriteLine("good");

        }

        }

        catch{

            Console.WriteLine("threw exception");

        }

    }

     private static bool IsInvalidFaxNumber(string number)

        {

            return (!string.IsNullOrEmpty(number) ||CheckPhoneNumberFormat(number));

        }

    private static bool returnFalse(){

    throw new Exception("");

    }

        public static bool CheckPhoneNumberFormat(string fieldValue)

        {

            Regex regPattern = new Regex(@"(^[\+][0-9][0-9\-]{0,19}[0-9]$)|(^[0-9][0-9\-]{0,20}[0-9]$)");

            return regPattern.IsMatch(fieldValue);

        }

}

​

​