Something

 if (OnSiteMovie.SelectedIndex == -1)
                OnSiteMovie.Focus();
            else if (OnSiteDay.SelectedIndex == -1)
                OnSiteDay.Focus();
            else if (OnSiteNr.SelectedIndex == -1)
                OnSiteNr.Focus();
            else if (OnSiteTime.SelectedIndex == -1)
                OnSiteTime.Focus();
            else if (OnSiteCategory.SelectedIndex == -1)
                OnSiteCategory.Focus();
            else
            {
                int cCode = CreateCode();
                this.CodeTextBox.Text = cCode.ToString();
            }