C# 数据库的基本操作（sqlserver）

<connectionStrings>
		<add name="ConnStr" connectionString="data source=xp-ba785745002d;database=databaseonline;Uid=sa;pwd=sasa"/>
	</connectionStrings>

public class SQLHelper
    {
        private SqlConnection sqlCon = null;
        private SqlCommand cmd = null;
        private SqlDataReader sdr = null;
        public SQLHelper()
        {
            sqlCon = new SqlConnection(ConfigurationManager.ConnectionStrings["ConnStr"].ConnectionString);
        }
      /// <summary>
      /// 打开数据库连接
      /// </summary>
      /// <returns></returns>
        private SqlConnection GetCon()
        {
            if (sqlCon.State==ConnectionState.Closed)
            {
              sqlCon.Open();  
            }
            return sqlCon;
        }
        /// <summary>
        /// 执行不带参数的增删改sql语句或存储过程
        /// </summary>
        /// <param name="cmdText">增删改sql语句或存储过程</param>
        /// <param name="ct">命令类型</param>
        /// <returns></returns>
        public int ExecuteNonQuery(string cmdText, CommandType ct)
        {
            int rex;
            try
            {
            SqlCommand sqlcom = new SqlCommand(cmdText,GetCon());
            sqlcom.CommandType = ct;
            rex =sqlcom.ExecuteNonQuery();
            }
            catch (Exception ex)
            {

                throw ex;
            }
            finally
            {
                if (sqlCon.State==ConnectionState.Open)
                {
                   sqlCon.Close();  
                }
            }
            return rex;
        }
        /// <summary>
        ///  执行带参数的增删改SQL语句或存储过程
        /// </summary>
        /// <param name="cmdText">增删改SQL语句或存储过程</param>
        /// <param name="ct">命令类型</param>
        /// <returns></returns>
        public int ExecuteNonQuery(string cmdText, SqlParameter[] paras, CommandType ct)
        {
            int res;
            using (cmd = new SqlCommand(cmdText, GetCon()))
            {
                cmd.CommandType = ct;
                cmd.Parameters.AddRange(paras);
                res = cmd.ExecuteNonQuery();
            }
            return res;
        }
        /// <summary>
        ///  执行带参数的查询SQL语句或存储过程
        /// </summary>
        /// <param name="cmdText">查询SQL语句或存储过程</param>
        /// <param name="paras">参数集合</param>
        /// <param name="ct">命令类型</param>
        /// <returns></returns>
        public DataTable ExecuteQuery(string cmdText, SqlParameter[] paras, CommandType ct)
        {
            DataTable dt = new DataTable();
            cmd = new SqlCommand(cmdText,GetCon());
            cmd.CommandType = ct;
            cmd.Parameters.AddRange(paras);
            using (sdr = cmd.ExecuteReader(CommandBehavior.CloseConnection))
            {
                dt.Load(sdr);
            }
            return dt;
        }
        /// <summary>
        /// 执行不带参数的查询SQL语句或存储过程
        /// </summary>
        /// <param name="cmdText">查询SQL语句或存储过程</param>
        /// <param name="ct">命令类型</param>
        /// <returns></returns>
        public DataTable ExecuteQuery(string cmdText, CommandType ct)
        {
            DataTable dt = new DataTable();
            cmd = new SqlCommand(cmdText, GetCon());
            cmd.CommandType = ct;
            using (sdr = cmd.ExecuteReader(CommandBehavior.CloseConnection))
            {
                dt.Load(sdr);
            }
            return dt;
        }
    }

public int logincheck(string admin, string pwd)
        {

            SqlParameter[] paras = new SqlParameter[] { 
            new SqlParameter("@AdminName",admin),
            new SqlParameter("@Password",pwd)
            };
            int i = Convert.ToInt32(sqlhelper.ExecuteQuery("Admin_check_login", paras, CommandType.StoredProcedure).Rows[0][0].ToString());
            return i;
        }

DataRow dr;
            string sql = "SELECT PKID, User_Name, Password, User_Grade,(SELECT UserGrade FROM UserGrade WHERE pkid = user_grade) AS UG FROM Users WHERE (User_Name =@UserName) AND (Password = @Password)";

            SqlConnection sqlConnection = new SqlConnection(ConfigurationManager.ConnectionStrings["DBConnection"].ConnectionString);
            SqlDataAdapter sqlAdapter1 = new SqlDataAdapter(sql, sqlConnection);
            sqlAdapter1.SelectCommand.Parameters.Add(new SqlParameter("@UserName", SqlDbType.NVarChar));
            sqlAdapter1.SelectCommand.Parameters.Add(new SqlParameter("@Password", SqlDbType.NVarChar));
            sqlAdapter1.SelectCommand.Parameters["@UserName"].Value = tbName.Text.Trim();
            sqlAdapter1.SelectCommand.Parameters["@Password"].Value = tbPwd.Text.Trim();

            DataSet product = new DataSet();
            sqlAdapter1.Fill(product,"Users");
            dr = product.Tables[0].Rows[0];