public List<Customer> GetALL()

        {

            List<Customer> customers = new List<Customer>();

            customers=SqlHelper.ExcuteReader("select * from T_Customer");//就是在这里抛出异常的,可是SqlHelper是个静态类啊

            return customers;

        }

public static List<Customer> ExcuteReader(string str, params SqlParameter[] para)

        {

            using (SqlConnection sql = new SqlConnection(connection))

            {

                sql.Open();

                using (SqlCommand com = sql.CreateCommand())

                {

                    com.CommandText = str;

                    com.Parameters.AddRange(para);

                    using(SqlDataReader reader=com.ExecuteReader())

                    {

                        if (reader==null)

                        {

                            return null;

                        }

                        List<Customer> customers=new List<Customer>();