Number as Words

//  Write a program that converts a number in the range [0…999] to words, corresponding to the English pronunciation.
using System;

class NumberAsWords
{
    static void Main()
    {
        int number = 0;
        while (true)
        {
            Console.WriteLine("Enter value between 0 and 999!");
            string userInput = Console.ReadLine();
            number = 0;
            if (int.TryParse(userInput, out number) && number >= 0 && number <= 999)
            {
                break;
            }
        }

        string and = "and";
        string space = " ";
        string oneHundred = "Hundred";
        string zero = "Zero";

        int firstDigit = 0;
        int tenToNineteen = 0;
        int secondDigit = 0;
        int thirdDigit = 0;

        if (number == 0 || number == 100)
        {
            if (number == 0)
            {
                Console.WriteLine(zero);
                return;
            }
            else
            {
                Console.WriteLine(oneHundred);
                return;
            }
        }
        else if (number >= 1 && number <= 9)
        {
            Units(number);
            Console.WriteLine();
            return;
        }
        else if (number >=10 && number <= 19)
        {
            TenToNineteen(number);
            Console.WriteLine();
            return;
        }
        else if (number >= 20 && number <= 99)
        {
            if (number % 10 == 0)
            {
                number /= 10;
                Tens(number);
                Console.WriteLine();
                return;
            }
            else
            {
                firstDigit = number % 10;
                secondDigit = number / 10;
                Tens(secondDigit);
                Console.Write(space);
                Units(firstDigit);
                Console.WriteLine();
                return;
            }

        }
        else if (number > 100)
        {
            if (number % 100 == 0)
            {
                number /= 100;
                Units(number);
                Console.WriteLine(space + oneHundred);
                return;
            }
            else if (number % 10 == 0)
            {
                thirdDigit = number / 100;
                secondDigit = (number / 10) % 10;
                Units(thirdDigit);
                Console.Write(" {0} {1} ", oneHundred, and);
                Tens(secondDigit);
                Console.WriteLine();
                return;
            }
            else if (number % 100 >= 11 && number % 100 <= 19)
            {
                tenToNineteen = number % 100;
                thirdDigit = number / 100;
                Units(thirdDigit);
                Console.Write(" {0} {1} ", oneHundred, and);
                TenToNineteen(tenToNineteen);
                Console.WriteLine();
                return;
            }
            else if ((number / 10) % 10 == 0)
            {
                firstDigit = number % 10;
                thirdDigit = number / 100;
                Units(thirdDigit);
                Console.Write(" {0} {1} ", oneHundred, and);
                Units(firstDigit);
                Console.WriteLine();
                return;
            }
            else
            {
                firstDigit = number % 10;
                secondDigit = (number / 10) % 10;
                thirdDigit = number / 100;
                Units(thirdDigit);
                Console.Write(" {0} {1} ", oneHundred, and);
                Tens(secondDigit);
                Console.Write(space);
                Units(firstDigit);
                Console.WriteLine();
                return;
            }
        }
    }

    static void Units(int units)
    {
        switch (units)
        {
            //case "0": Console.Write("Zero"); break;
            case 1: Console.Write("One"); break;
            case 2: Console.Write("Two"); break;
            case 3: Console.Write("Three"); break;
            case 4: Console.Write("Four"); break;
            case 5: Console.Write("Five"); break;
            case 6: Console.Write("Six"); break;
            case 7: Console.Write("Seven"); break;
            case 8: Console.Write("Eight"); break;
            case 9: Console.Write("Nine"); break;
        }
    }
    static void TenToNineteen(int tenToNineteen)
    {
        switch (tenToNineteen)
        {
            case 10: Console.Write("Ten"); break;
            case 11: Console.Write("Еleven"); break;
            case 12: Console.Write("Тwelve"); break;
            case 13: Console.Write("Thirteen"); break;
            case 14: Console.Write("Fourteen"); break;
            case 15: Console.Write("Fifteen"); break;
            case 16: Console.Write("Sixteen"); break;
            case 17: Console.Write("Seventeen"); break;
            case 18: Console.Write("Eighteen"); break;
            case 19: Console.Write("Nineteen"); break;
        }
    }
    static void Tens(int tens)
    {
        switch (tens)
        {
            case 1: Console.Write("Ten"); break;
            case 2: Console.Write("Twenty"); break;
            case 3: Console.Write("Thirty"); break;
            case 4: Console.Write("Forty"); break;
            case 5: Console.Write("Fifty"); break;
            case 6: Console.Write("Sixty"); break;
            case 7: Console.Write("Seventy"); break;
            case 8: Console.Write("Eighty"); break;
            case 9: Console.Write("Ninety"); break;
        }
    }
}

// Examples:
//numbers   number as words

//0         Zero
//9         Nine
//10        Ten
//12        Twelve
//19        Nineteen
//25        Twenty five
//98        Ninety eight
//273       Two hundred and seventy three
//400       Four hundred
//501       Five hundred and one
//617       Six hundred and seventeen
//711       Seven hundred and eleven
//999       Nine hundred and ninety nine