Run Addin without UI

I am using the COM API through Interop assembly code below to scan the document for some reporting. 

public Application ustnApp = new ApplicationClass();

ustnApp.OpenDesignFile(fileName);

  (Blank UI)

Interestingly, when I run it in CONNECT i always get his blank UI popping up, while the blank UI is NOT showing up if I use V8i. How can that be possible?

If I don't want any UI to show up (only want the code to quietly read the Microstation file and generate the report), what should I do?

Thanks,