Currently, when you select multiple shapes, you can rotate them altogether using the rotate icon displayed on top of the ManipulationAdorner visual (the rectangle that encloses the shapes). However, there is no property or a method that allows to programmatically provide a rotation angle for the adorner. 

Add a RotationAngle property or a Rotate() method or something similar that will automatically update the rotation of the ManipulationAdorner along with the shapes inside.