Input string was not in a correct format

Har fundet en løsning:

SqlParameter[] parms = new SqlParameter[5];
                parms[0] = new SqlParameter("@kundeId", SqlDbType.Int);
                parms[0].Value = kundeId;
                parms[1] = new SqlParameter("@kategoriId", SqlDbType.Int);
                parms[1].Value = int.Parse(RadComboBox1.SelectedValue.ToString());
                parms[2] = new SqlParameter("@lokationsId", SqlDbType.Int);
                parms[2].Value = partnerId;
                parms[3] = new SqlParameter("@startDato", SqlDbType.SmallDateTime);
                parms[3].Value = DateTime.Parse(this.RadDatePicker1.SelectedDate.ToShortDateString());
                parms[4] = new SqlParameter("@slutDato", SqlDbType.SmallDateTime);
                parms[4].Value = DateTime.Parse(this.RadDatePicker2.SelectedDate.ToShortDateString());
               
                SqlHelper.ExecuteNonQuery(System.Configuration.ConfigurationSettings.AppSettings["dataSource"], "sr_insertKalenderAftale", parms);