Number as words

using System;
//Write a program that converts a number in the range [0…999] to words, corresponding to the English pronunciation.
class NumberAsWords
{
    static void Main()
    {
        Console.Write("Please enter a  number (0...999) : ");
        int num = int.Parse(Console.ReadLine());
        string[] num0To19={"null","one","two","three","four","five","six","seven","eight","nine","ten","eleven","twelve","thirteen","fourteen","fifteen","sixteen","seventeen","eighteen","nineteen"};
        string[] num0To19BigLetters = { "Null", "One", "Two", "Three", "Four", "Five", "Six", "Seven", "Eight", "Nine","Ten","Eleven", "Twelve", "Thirteen", "Fourteen", "Fifteen", "Sixteen", "Seventeen", "Eighteen", "Nineteen" };
        string[] num20To90 = {"twenty","thirty","forty","fifty","sixty","seventy","eighty","ninety"};
        string[] num20To90BigLetters = {"Twenty", "Thirty", "Forty", "Fifty", "Sixty", "Seventy", "Eighty", "Ninety" };
        string hundred="hundred";
        string h = "Hundred";
        int a = (num%10);
        int b = num%100;
        int c = num/100;
        string result = null;
        if (num == 100)
        {
            result = h;
        }

        if (num>=0&&num<=19)
        {
            result = num0To19BigLetters[num];
        }
        if (num>=20&&num<=99)
        {
            if (num%10==0)
            {
                result = num20To90BigLetters[(num/10)-2];
            }
            else
            {

                result=num20To90BigLetters[(num/10)-2] + " " + num0To19[(num%10)];
            }
        }
        if (num > 100 && num <= 999 && num % 100 == 0)
            {

                    result = num0To19BigLetters[num / 100] + " " + hundred;
            }
        if (num > 100 && num <= 999 && num % 100 != 0&&(num%100)%10==0)
        {
            result = num0To19BigLetters [num/100]+" " +hundred +" "+num20To90[((num % 100)%10)+2];
        }
        if (num > 100 && num <= 999 && num % 100 != 0&&num % 100 <=19 && (num % 100) % 10 != 0 && (num % 100) % 10 > 0&&(num % 100) % 10 <=19)
        {
            result=num0To19BigLetters [num/100]+" " +hundred + " "+num0To19[num%100];
        }
       if (num > 100 && num <= 999 && num % 100 != 0&&num % 100 >20&& (num % 100) % 10 != 0 )
        {
            result=num0To19BigLetters [num/100]+" " +hundred +" "+num20To90[((num%100)/10)-2] + " -  " + num0To19[(num%100)%10];
        }


        Console.WriteLine("{0}     {1}", num, result);
        Console.WriteLine();
    }
}