DepedencyProperty_CreateResultStars.cs

public class CreateResultStars
{
    private readonly MultiplyByTwo _multiplier;

    public string Stars { get; private set; }

    public CreateResultStars(MultiplyByTwo multiplier)
    {
        _multiplier = multiplier;

        var dpd = DependencyPropertyDescriptor.FromProperty(MultiplyByTwo.ResultProperty, typeof(MultiplyByTwo));
        dpd.AddValueChanged(_multiplier, (o, e) => RecalculateStars());

        RecalculateStars();
    }

    private void RecalculateStars()
    {
        Stars = new string('*', _multiplier.Result);
    }
}