如何获取DependencyProperty的默认值

public static readonly DependencyProperty MyPropertyProperty = DependencyProperty.Register(
    "MyProperty",
    typeof(string),
    typeof(MyControl),
    new PropertyMetadata(
        "default value", // 默认值
        new PropertyChangedCallback(OnMyPropertyChanged)));

private static void OnMyPropertyChanged(DependencyObject d, DependencyPropertyChangedEventArgs e)
{
    MyControl control = d as MyControl;

    if (e.NewValue != DependencyProperty.UnsetValue)
    {
        // 属性已被设置
        string newValue = e.NewValue as string;
        control.DoSomething(newValue);
    }
    else
    {
        // 属性未被设置，使用默认值
        control.DoSomething(control.MyProperty);
    }
}