class ScalingConverter : IValueConverter
    {
        public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
        {
            if (value == null || parameter == null)
                return DependencyProperty.UnsetValue;
            double v = (double)value;
            double p = (double)parameter;
            return v * p;
        }

        public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
        {
            throw new NotSupportedException();
        }
    }

    public class DoubleExtension : TypedValueExtension<double>
    {
        public DoubleExtension(double value) : base(value) { }
    }

    public class TypedValueExtension<T> : MarkupExtension
    {
        public TypedValueExtension(T value) { Value = value; }
        public T Value { get; set; }
        public override object ProvideValue(IServiceProvider sp) { return Value; }
    }