Все, задача решена! Всем спасибо!Pro100Oleh, совет очень помог
Вот рабочий код:

        public List<int> AddObjectType(out int id, ObjectType objectType) {
            var ret = new List<int>();
            using (var sqlConnection = new SqlConnection(_conString)) {
                var sqlCommand = new SqlCommand("spAddObjectType", sqlConnection);
                sqlCommand.CommandType = CommandType.StoredProcedure;
                sqlCommand.Parameters.Add(new SqlParameter("flId", SqlDbType.Int));
                sqlCommand.Parameters["flId"].Direction = ParameterDirection.Output;
                sqlCommand.Parameters.Add(new SqlParameter("flName", objectType.Name));
                sqlCommand.Parameters.Add(new SqlParameter("flDescription", objectType.Description));
                sqlCommand.Parameters.Add(new SqlParameter("flCount", 3));

                try {
                    sqlConnection.Open();
                    var adapter = new SqlDataAdapter(sqlCommand);
                    var dataSet = new DataSet("ds");
                    adapter.Fill(dataSet);
                    id = (int) sqlCommand.Parameters["flId"].Value;
                    foreach (DataRow row in dataSet.Tables[0].Rows) {
                        ret.Add((int) row["ObjectId"]);
                    }
                } catch (Exception ex) {
                    throw ex;
                }
            }

            return ret;
        }