Csharp/C Sharp by API/System/ICustomFormatter

implements ICustomFormatter

  
using System;
using System.Text;
using System.Globalization;
public class WordyFormatProvider : IFormatProvider, ICustomFormatter {
    static readonly string[] _numberWords = "zero one two three four five six seven eight nine minus point".Split();
    IFormatProvider _parent;   
    public WordyFormatProvider() : this(CultureInfo.CurrentCulture) { }
    public WordyFormatProvider(IFormatProvider parent) {
        _parent = parent;
    }
    public object GetFormat(Type formatType) {
        if (formatType == typeof(ICustomFormatter)) return this;
        return null;
    }
    public string Format(string format, object arg, IFormatProvider prov) {
        if (arg == null || format != "W")
            return string.Format(_parent, "{0:" + format + "}", arg);
        StringBuilder result = new StringBuilder();
        string digitList = string.Format(CultureInfo.InvariantCulture,
                                          "{0}", arg);
        foreach (char digit in digitList) {
            int i = "0123456789-.".IndexOf(digit);
            if (i == -1) continue;
            if (result.Length > 0) result.Append(" ");
            result.Append(_numberWords[i]);
        }
        return result.ToString();
    }
}

public class MainClass {
    public static void Main() {
        double n = -123.45;
        IFormatProvider fp = new WordyFormatProvider();
        Console.WriteLine(string.Format(fp, "{0:C} in words is {0:W}", n));
    }
}