[quote user="James Manfield"]I ran the code again with this modification but it made no difference to the result. [/quote]
How can you be sure: there are numerous ways in which the active model (from OpenDesignFIle) behaves differently, and has different properties (e.g. no view is open) to an inactive model (from OpenDesignFileForProgram). See my previous comments about Debug.Assert to verify your assumptions.
[quote user="James Manfield"]All that happened was that each file was opened in a visible window which made the code take longer to run[/quote]
Do you want a fast app. that give the wrong result or a slower app. that gives the right result?
See my previous comments about Debug.Assert to verify your assumptions. The best way to develop your code is using an active model reference (from OpenDesignFIle): MicroStation behaves as you expect. When that's all working you can try the OpenDesignFileForProgram version, if there's a good reason for that.