MultiBinding in Silverlight 5

using System;
using System.Net;
using System.Windows;
using System.Windows.Controls;
using System.Windows.Documents;
using System.Windows.Ink;
using System.Windows.Input;
using System.Windows.Media;
using System.Windows.Media.Animation;
using System.Windows.Shapes;
using System.Text.RegularExpressions;

namespace SilverlightMarkupExtensions
{
    public class StringFormatMultiValueConverter : IMultiValueConverter 
    {
        public StringFormatMultiValueConverter()
        {

        }

        private static readonly Regex ArgRefRegex = new Regex(@"%(\d+)");

        public object Convert(object[] values, Type targetType, object parameter, System.Globalization.CultureInfo culture)
        {
            string format = parameter as String;
            string result;
            if (format != null)
            {
                format = ArgRefRegex.Replace(format, "{$1}");
                result = String.Format(culture, format, values);
            }
            else
            {
                result = String.Concat(values);
            }
            return result;
        }


        public object[] ConvertBack(object value, Type[] targetTypes, object parameter, System.Globalization.CultureInfo culture)
        {
            if (targetTypes.Length == 1) return new object[] { value };
            throw new NotSupportedException("ConvertBack is not supported by " + GetType().Name);
        }
    }
}