Get data from RotateTransform3D in C#

hi, in my code i set that RotateTransform3D as a children inside a Transform3DGroup with this:

myGroupArray[0].Children.Add(newRotateTransform3D(newAxisAngleRotation3D(newVector3D(0,0,1),Convert.ToDouble(5)),newPoint3D(0,0,0)));

and in another function i try to recover my "5" with this:

RotateTransform3D rotation =newRotateTransform3D();
rotation =(RotateTransform3D)myGroupArray[0].Children[0];

now, even doing

MessageBox.Show(rotation.Rotation.Angle.ToString());

results in error because Rotation3D does not contain an Angle property

How can i get my "5" back?