Это должно быть что-то вроде:
object nullobj = Missing.Value;
doc = wordApp.Documents.Open(ref file,
ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj);
doc.Activate();
doc.Visible = true;
int dialogResult = wordApp.Dialogs[Microsoft.Office.Interop.Word.WdWordDialog.wdDialogFilePrint].Show(ref nullobj);
if (dialogResult == 1)
{
doc.PrintOut(ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj, ref nullobj, ref nullobj,
ref nullobj, ref nullobj);
}