WPF 依赖项属性：为什么需要指定所有者类型

csharp
public class MyControl : DependencyObject
{
    public static readonly DependencyProperty MyProperty =
        DependencyProperty.Register("My", typeof(string), typeof(MyControl));
    public string My
    {
        get { return (string)GetValue(MyProperty); }
        set { SetValue(MyProperty, value); }
    }
}

csharp
public class MyButton : Button
{
    public static readonly DependencyProperty BackgroundColorProperty =
        DependencyProperty.Register("BackgroundColor", typeof(Brush), typeof(MyButton));
    public Brush BackgroundColor
    {
        get { return (Brush)GetValue(BackgroundColorProperty); }
        set { SetValue(BackgroundColorProperty, value); }
    }
}

xaml