Convert a text representing a number to a currency word version.

gistfile1.cs

public static class NumberWordConverter
{
    #region Lookups

    private static readonly Dictionary<char, string> units = new Dictionary<char, string>()
                                                                    {
                                                                        {'0', ""},
                                                                        {'1', " One"},
                                                                        {'2', " Two"},
                                                                        {'3', " Three"},
                                                                        {'4', " Four"},
                                                                        {'5', " Five"},
                                                                        {'6', " Six"},
                                                                        {'7', " Seven"},
                                                                        {'8', " Eight"},
                                                                        {'9', " Nine"},
                                                                    };

    private static readonly Dictionary<char, string> tens = new Dictionary<char, string>()
                                                                {
                                                                    {'0', ""},
                                                                    {'1', " Ten"},
                                                                    {'2', " Twenty"},
                                                                    {'3', " Thirty"},
                                                                    {'4', " Forty"},
                                                                    {'5', " Fifty"},
                                                                    {'6', " Sixty"},
                                                                    {'7', " Seventy"},
                                                                    {'8', " Eighty"},
                                                                    {'9', " Ninety"},
                                                                };

    private static readonly Dictionary<char, string> teens = new Dictionary<char, string>()
                                                                    {
                                                                        {'0', " Ten"},
                                                                        {'1', " Eleven"},
                                                                        {'2', " Twelve"},
                                                                        {'3', " Thirteen"},
                                                                        {'4', " Fourteen"},
                                                                        {'5', " Fiveteen"},
                                                                        {'6', " Sixteen"},
                                                                        {'7', " Seventeen"},
                                                                        {'8', " Eighteen"},
                                                                        {'9', " Nineteen"},
                                                                    };

    private static readonly string[] numberScales = new string[]
                                                        {
                                                            "",
                                                            " Thousand",
                                                            " Million",
                                                            " Billion",
                                                            " Trillion",
                                                            " Quadrillion",
                                                            " Quintillion",
                                                            " Sextillion",
                                                            " Septillion"
                                                        };

    #endregion

    public static string ToCurrencyWords(this string text, string major = "Dollars", string minor = "Cents", int minorSize = 2)
    {
        string[] parts = (text + ".00").Split('.');     // Forces all numbers to #.00 format.  
                                                        // If supplied number contained a fraction, then the split array now contain
                                                        // 3 elements so will ignore our extra addition.

        // Force fractional parts to 1 extra decimal places then round away from zero
        parts[1] = Math
            .Round(int.Parse(parts[1].PadRight(minorSize + 1, '0').Substring(0, minorSize + 1)) / 10d, MidpointRounding.AwayFromZero)
            .ToString(CultureInfo.InvariantCulture);

        string majorText = parts[0].ToNumber(major);
        string minorText = parts[1].ToNumber(minor);

        return majorText
                + (majorText.Length > 0 && minorText.Length > 0 ? " and " : "")
                + minorText;
    }

    public static string ToNumber(this string text, string postFix)
    {
        int length = text.Length + (3 - text.Length%3)%3;

        Stack<char> chars = new Stack<char>(text.PadLeft(length, '0').ToCharArray());
        Queue<string> modifiers = new Queue<string>(numberScales);

        string result = string.Empty;
        while (chars.Count > 0)
        {

            result = ConvertTriad(chars.Pop(), chars.Pop(), chars.Pop(), modifiers.Dequeue(), chars.Count > 0) + result;
        }

        return (string.IsNullOrWhiteSpace(result) ? string.Empty : result.Trim() + " " + postFix);
    }

    private static string ConvertTriad(char unit, char ten, char hundred, string modifier, bool hasMore)
    {
        string result = "";
        string tempModifier = string.Empty; // In the case all 3 chars are '0' we skip this level
        if (hundred != '0')
        {
            result = units[hundred] + " hundred";
            tempModifier = modifier;        // Have a hundred number, restore the modifier in case no tens or units
        }

        if (ten == '0' && unit == '0')
            return result + tempModifier;


        if (!string.IsNullOrEmpty(result) || hasMore)
            result += " and";

        return result + ((ten == '1') ? teens[unit] : tens[ten] + units[unit]) + modifier;
    }
}