Hi All, got the solution from a MS newsgroup, don't know if this is a bug but ?, it seems if you set a reference by browsing to the projects' output folder this can cause the problem I'm experiencing, the proper way (??!!!) is to select the "projects" tab in the add reference dialog, then, when you switch configurations VS does all the re-referencing for you (and copies (if you are in debug config) the debug info to the run folder of the solution) I think this is bad ! - why give you more than one way to achieve something if they appear to do the same thing but actually don't ? v misleading