You can use GetValue/SetValue() for attached DependencyProperty, avoid calling the property directly even for normal DependencyProperty
this.SetValue(Selector.IsSelectedProperty, true);
this.GetValue(Selector.IsSelectedProperty) as bool;
Selector.IsSelectedProperty is a attached DependencyProperty, as an attached property, Selector defined both IsSelectedProperty and GetIsSelected()/SetIsSelected() method.
If normal DependencyProperty, e.g. Height Property, then it defined the HeightProperty and "Height" property (with getter and setter).
Regards
Joseph Leung