The sub components of assembly are already loaded into a memory but just hidden. unless the component is lightweight. So OpenDoc6 may return this warning however it should still return the pointer to IModelDoc2. You may need then to call the SldWorks::ActivateDoc3 to activate the doc and bring foreground. You can also try IModelDoc2::Visible property to just display the model.
Regards, Artem Taturevych | Snr. Developer | IC3D ANZ
IC3DSteel – New Steel Solution for SolidWorks
translationXpert – SolidWorks files language translator
Thanks, that worked.