I have a CAD model that I use inside a .dll VB code to do some operations.
Is it possibile to hide this model to the user enclosing it into the dll?
Yes, you can add the file as an embedded resource to your dll (add it to your vs project and set embedded ressource in properties).
Use Assembly.GetManifestResourceStream to retrieve the data (presumably you have to store it as a temporary file to do something useful with it).
Is the model highly complex?
If not too bad, can you just create it "on the fly" (when you need it) (i.e. create a block then add a hole then ...)
Depending on what you are doing, another option might be UDF (user defined feature)
Production: NX10.0.3.5 MP5 + patch/TC11.2
Apparently I've turned into a gearhead