C# static Dbhelper

 public class SQLHelper {
        private static string connString = "";
        public static int Update(string sql) {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            try {
                conn.Open();
                int i = cmd.ExecuteNonQuery();
                conn.Close();
                return i;
            }
            catch (Exception ex) {
                throw new Exception(ex.Message);
            }
            finally {
                conn.Close();
            }
        }
        public static object GetSingleResult(string sql) {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            try {
                conn.Open();
                object result = cmd.ExecuteScalar();
                conn.Close();
                return result;
            }
            catch (Exception ex) {
                throw ex;
            }
            finally {
                conn.Close();
            }

        }
        public static SqlDataReader GetReader(string sql) {
            SqlConnection conn = new SqlConnection(connString);
            SqlCommand cmd = new SqlCommand(sql, conn);
            SqlDataReader objReader = null;
            try {
                conn.Open();
                objReader = cmd.ExecuteReader(CommandBehavior.CloseConnection);
                return objReader;
            }
            catch (Exception ex) {
                conn.Close();
                throw ex;
            }
        }
    }

在使用static中不可以使用Using来关闭数据库连接，因为static是静态的，只可以初始化一次，如果使用using释放，之后它为空，会出现未将对象初始化