1.创建项目结构，如下： 

引用关系如下（此图来源网络）：

2.IService创建接口，如下（用户类接口）：

using Model;
using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace IService
{
    /// <summary>
    /// 用户数据访问接口
    /// </summary>
    public interface IUserService
    {

        /// <summary>
        /// 获取所有的用户
        /// </summary>
        /// <returns>用户DataTable</returns>
        DataTable GetAllUser();

        /// <summary>
        /// 添加用户
        /// </summary>
        /// <param name="UserVo">用户</param>
        /// <returns>受影响的行数</returns>
        int AddUser(UserVo user);
    }
}

3.在SqlService和OracleService实现IService接口,如下（SqlService用户实现类）：

using IService;
using Model;
using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace SQLServerService
{
    public class SqlUserService : IUserService
    {

        /// <summary>
        /// 根据用户名ID，删除用户
        /// </summary>
        /// <param name="id">用户ID</param>
        /// <returns>受影响的行数</returns>
        public int DeleteUserByName(string id)
        {
            StringBuilder sqlSB = new StringBuilder();
            sqlSB.Append("delete from HT_OIL_USER where id=@id");
            SqlParameter[] pa ={
                        SqlHelper.GetParameter("@id", DbType.String, id)
                                  };
            return SqlHelper.ExecuteSql(sqlSB.ToString(), pa);
        }

        /// <summary>
        /// 获取所有的用户
        /// </summary>
        /// <returns></returns>
        public DataTable GetAllUser()
        {
            StringBuilder sqlSB = new StringBuilder();
            sqlSB.Append("select id,USERNAME,PASSWORD from HT_OIL_USER");
            sqlSB.Append(" Order by AddTime asc");
            return SqlHelper.ExecuteDt(sqlSB.ToString());

        }
    }
}

4.Manager中，创建工厂类，根据配置的数据库标识，实例化不同的Service对象

using IService;
using System;
using System.Collections.Generic;
using System.Configuration;
using System.Linq;
using System.Reflection;
using System.Text;
using System.Threading.Tasks;

namespace Manager
{
    public class DalFactory
    {

        private static readonly string AssemblyName = "OracleService";
        // private static readonly string db = "Oracle";
        private static readonly string db = ConfigurationManager.AppSettings["DB"].ToLower();
        /// <summary>
        /// 实例化IUserService对象
        /// </summary>
        /// <returns></returns>
        public static IUserService CreateUserService()
        {
            //string className = AssemblyName + "." + db + "UserService";
            //return (IUserService)Assembly.Load(AssemblyName).CreateInstance(className);

            if (db == "oracle")
            {
                return new OracleService.OracleUserService();
            }
            else
            {
                return new SQLServerService.SqlUserService();
            }
        }
    }
}