DBHelper，之前常用的代码，保存起来。

using System;
using System.Collections.Generic;
using System.Configuration;
using System.Data;
using System.Data.SqlClient;

namespace PersistentSys.DAL
{
    public static class DBHelper
    {
		//创建数据库连接字符串
        private static string _conStr = ConfigurationManager.ConnectionStrings["db"].ConnectionString;
		//创建数据库连接对象
        private static SqlConnection _con;
		//数据库连接
        public static SqlConnection Con
        {
            get
            {
				//创建连接
                if (_con==null)
                    _con = new SqlConnection(_conStr);
				//连接状态打开
                if (_con.State!=ConnectionState.Open)
                    _con.Open();
				return _con;
            }
        }
		
		//获取表数据
        public static DataTable GetTable(string sql)
        {
            SqlDataAdapter adapter = new SqlDataAdapter(sql,_conStr);
            DataSet set = new DataSet();
            adapter.Fill(set);
            return set.Tables[0];
        }
		
		//执行增删改查的方法，使用SqlParameter传递数据
        public static bool ExecuteNonQuery(string sql,List<SqlParameter> paras=null)
        {
            SqlCommand cmd = Con.CreateCommand();
            cmd.CommandText = sql;
            if (paras!=null)
                cmd.Parameters.AddRange(paras.ToArray());// 添加参数列表
            try
            {
                return cmd.ExecuteNonQuery() > 0;//返回受影响行数
            }
            catch (Exception e)
            {
                Console.WriteLine(e);
                return false;
            }
            finally
            {
                cmd.Connection.Close();
            }
        }
    }
}

　　以上内容是简化版。

-------------------------------------------------------------

　　更新一个完整版：

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Data;
using System.Data.SqlClient;
using System.Configuration;
namespace MySchool.DAL
{
    public static class DBHelper
    {
        private static string _constr = ConfigurationManager.ConnectionStrings["myschool"].ConnectionString;

        private static SqlConnection _con;

        public static SqlConnection Con
        {
            get
            {
                if (_con == null)
                    _con = new SqlConnection(_constr);
                if (_con.State != ConnectionState.Open)
                    _con.Open();
                return _con;
            }
        }


        private static SqlCommand PrepareCMD(string sql,List<SqlParameter> paras, CommandType type)
        {
            SqlCommand cmd = Con.CreateCommand();
            cmd.CommandText = sql;
            
            if (paras!=null)
                // 绑定 命令对象与 参数列表
                cmd.Parameters.AddRange(paras.ToArray());
            cmd.CommandType = type;//指定 CMD 对象操作类型（SQL文本，存储过程)
            return cmd;
        }


        public static object ExecuteScalar(string sql, List<SqlParameter> paras = null, CommandType type= CommandType.Text)
        {
            SqlCommand c = PrepareCMD(sql, paras,type);
            try
            {
                return c.ExecuteScalar();
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return null;
            }
            finally
            {
                c.Connection.Close();
            }
        }

        public static SqlDataReader ExecuteReader(string sql, List<SqlParameter> paras=null, CommandType type= CommandType.Text)
        {
            SqlCommand c = PrepareCMD(sql, paras, type);
            try
            {
                return c.ExecuteReader(CommandBehavior.CloseConnection);
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return null;
            }
            
        }

        public static bool ExecuteNonQuery(string sql,List<SqlParameter> paras=null,CommandType type = CommandType.Text)
        {
            SqlCommand c = PrepareCMD(sql,paras, type);
            
            try
            {
                return c.ExecuteNonQuery()>0;
            }
            catch (Exception ex)
            {
                Console.WriteLine(ex.Message);
                return false;
            }
            finally
            {
                
                c.Connection.Close();
            }
        }


        public static DataSet GetSet(string sql)
        {
            SqlDataAdapter adapter = new SqlDataAdapter(sql, _constr);
            DataSet set = new DataSet();
            adapter.Fill(set);
            return set;
        }

        public static DataTable GetTable(string sql)
        {
            return GetSet(sql).Tables[0];
        }
    }
}