Messagebox.show is not inove

in below code messagebox.show not invoking.

in rea = new RRecord(path); method call it will work. again  mdd.DeleteOne(rea, remove); method call it will not work.

 using (RecordExpectations recorder = RecorderManager.StartRecording())
            {
                MessageBox.Show(null);
                recorder.Return(DialogResult.OK);
            }

            rea = new RRecord(path);
            Isolate.NonPublic.WhenCalled(aed, "RExist").WithExactArguments("rec").WillReturn(false);
            mdd.DeleteOne(rea, remove);