class MyBrushClass
{
    public static readonly DependencyProperty MyBrushProperty = DependencyProperty.RegisterAttached("MyBrush", typeof(SolidColorBrush), typeof(MyBrushClass));


    public Geometry MyBrush
    {
        get
        {
            return (SolidColorBrush)GetValue(MyBrushProperty);
        }
        set
        {
            //Вот сюда заходит норм
            SetValue(MyBrushProperty, Value);
        }
    }

}