Debit card

// Debit card program

using System;

class Submission
{
    // -------------------------
    // Subprograms
    // -------------------------
    static bool validate_number(string number_on_card)
    {
        bool valid = true;
        int number_of_characters = number_on_card.Length;
        if (number_of_characters == 16 || number_of_characters == 19)
        {
            foreach (char character in number_on_card)
            {
                if (!Char.IsDigit(character) && character != ' ')
                {
                    valid = false;
                }
            }
        }
        else
        {
            valid = false;
        }
        return valid;
    }

    static void input_card_details()
    {
        bool valid_card = false;
        string number_input = "";
        while (!valid_card)
        {
            Console.WriteLine("Enter the 16 digit number:");
            number_input = Console.ReadLine();
            valid_card = validate_number(number_input);
        }
    }

    // -------------------------
    // Main program
    // -------------------------
    public static void Main(string[] args)
    {
        input_card_details();
        Console.WriteLine("Card details valid.");
    }

}