It should be something along the lines of:
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);
}