using System;

using System.Text;

public class Program

{

    public static void Main()

    {

        Console.WriteLine(120.66m.ToCurrencyWords().ToUpperInvariant());

    }

}

​

public static class DecimalToWordExtension

{

    /// <summary>

    /// Convert a decimal number to currency words

    /// </summary>

    /// <param name="value">The value</param>

    /// <returns></returns>

    public static string ToCurrencyWords(this decimal value)

    {

        string decimals = "";

        string input = Math.Round(value, 2).ToString();

​

        if (input.Contains("."))

        {

            var pointIndex = input.IndexOf(".");

            decimals = input.Substring(pointIndex + 1);

            // remove the decimal part from input

            input = input.Remove(pointIndex);

        }

​

        var words = new StringBuilder();

        words.Append(GetWords(input));

​

        if (decimals.Length > 0)

            words.Append($" and {decimals} / 100 dollars");

        else

            words.Append(" and 00 / 100 dollars");

​

        return words.ToString();

    }

​

    private static string GetWords(string input)

    {

        // These are seperators for each 3 digit in numbers.

        string[] seperators = { "", " thousand ", " million ", " billion " };

​

        // Counter is indexer for seperators. Each 3 digit will increase this.

        int i = 0;

​

        var strWords = "";

​

        while (input.Length > 0)

        {

            // Get the 3 last numbers from input and store it. If there is no 3 numbers just use it.

            string _3digits = input.Length < 3 ? input : input.Substring(input.Length - 3);

            // Remove the 3 last digits from input.

            input = input.Length < 3 ? "" : input.Remove(input.Length - 3);

​

            int number = int.Parse(_3digits);

            _3digits = Get3DigitsWords(number);

​

            // apply the seperator.

            _3digits += seperators[i];

            // since we are getting numbers from right to left then we must append resault to strWords like this.

            strWords = _3digits + strWords;

​

            // 3 digits converted. count and go for next 3 digits

            i++;

        }

        return strWords;

    }

​

    private static string Get3DigitsWords(int number)

    {

        string[] ones =

        {

            "one", "two", "three", "four", "five", "six", "seven", "eight", "nine", "ten", "eleven",

            "twelve", "thirteen", "fourteen", "fifteen", "sixteen", "seventeen", "eighteen", "ninteen"

        };

​

        string[] tens = { "ten", "twenty", "thirty", "fourty", "fifty", "sixty", "seventy", "eighty", "ninety" };

​

        string word = "";

​

        if (number > 99 && number < 1000)

        {

            int i = number / 100;

            word = word + ones[i - 1] + " hundred ";

            number = number % 100;

        }

​

        if (number > 19 && number < 100)

        {

            int i = number / 10;

            word = word + tens[i - 1] + " ";

            number = number % 10;

        }

​

        if (number > 0 && number < 20)

        {

            word = word + ones[number - 1];

        }

​

        return word;

    }

}