RE: ModelReference & Attachment base class for polymorphism

Unfortunately I do not see an Interface available similar to what you describe. Since an attachment is a specialized case of model reference the only way to treat both types in a similar sense via the COM object model would be to detect the model reference type as being an attachment; then use Attachment.MdlModelRefP passing the model reference to code that performs necessary operations in a similar manner.

HTH,
Bob