IbanLagen

//Business.cs

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

namespace IbanLagen
{
    class Business
    {
        public bool controleIban(string invoer)
        {
            bool geldig;

            if (invoer.Length == 16)
            {
                string land = invoer.Substring(0, 2);
                long rekening = Convert.ToInt64(invoer.Substring(4, 10));
                int controle = Convert.ToInt32(invoer.Substring(14, 2));

                if (land != "BE")
                {
                    geldig = false;
                }
                else
                {
                    if ((rekening % 97) != controle)
                    {
                        geldig = false;
                    }
                    else
                    {
                        geldig = true;
                    }
                }
            }
            else
            {
                geldig = false;
            }
            return geldig;

            string melding = "";
            if (!geldig)
            {
                melding = "g";
                Console.ForegroundColor = ConsoleColor.Red;
            }
            else
            {
                Console.ForegroundColor = ConsoleColor.Green;
            }
        }
    }
}


//Program.cs
using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace IbanLagen
{
    class Program
    {
        static void Main(string[] args)
        {
            Business rekening = new Business();
            Console.WriteLine("Geef een volledig nummer zonder spaties");
            string invoer = Console.ReadLine();

            string melding = "";
            if (!rekening.controleIban(invoer))
            {
                melding = "g";
                Console.ForegroundColor = ConsoleColor.Red;
            }
            else
            {
                Console.ForegroundColor = ConsoleColor.Green;
            }
            Console.WriteLine("Dit is {0}een geldig IBAN nummer", melding);
            Console.ReadLine();
        }
    }
}