[abenedik]

The ReadModel3D method returns a Model3D object. This is a base class for GeometryModel3D and Model3DGroup classes. In case the ReadModel3D reads a file that defines only one simple 3D model, then the method returns a GeometryModel3D object. When a more complex 3D model is defined in a file, then a Model3DGroup - this object is used to hierarchically organized the GeometryModel3D and other Model3DGroup in the Children collection.

Therefore, it is much better to check the type of the returned object before using it (except in case when you are reading a fixed file and are sure of the content of the file).