I've got SolidWorks.Interop.sldworks and 'using SolidWorks.Interop.Sldworks;' in my project... I can open .slddrw files with opendoc6 too.... (and startup SW--using ver 20.5.0 a.k.a. sw2012)

I need to do a file save/conversion... from looking at the macro-recorder's output... I need to use Part.SaveAs3(outputfile,0,0)...

what I have in C# looks like this:

Type swType = Type.GetTypeFromProgID("SldWorks.Application");

ISldWorks app = (ISldWorks)Activator.CreateInstance(swType);

.....snip....

            if (app != null)

            {

                app.Visible = true;

                app.OpenDoc6(sourceFile, 3, 0, "", ref openErrors, ref openWarnings);

                saveStatus = app.ActiveDoc.Part.SaveAs3(destinationFile, 0, 0);

                closeSuccessful = app.CloseAllDocuments(true);

          }

The variable sourceFile is a full path to the file... and I've tried destinationFile as just a filename and as a full path&filename....  anyway.. here's the problem:

I can get SW started up and the file opens... butI get this exception thrown when executing the app.ActiveDoc.Part.SaveAs3 method:

System.Runtime.InteropServices.COMException was unhandled

  Message=Invalid index. (Exception from HRESULT: 0x8002000B (DISP_E_BADINDEX))

  Source=mscorlib

  ErrorCode=-2147352565

Am I using the right call for this task?