SqlHelper简单实现（通过Expression和反射）6.Providor模式（工厂+策略）可配置数据库选择

基本思想，将MsSqlDbUtility和MySqlDbUtility设计成单例模式，通过App.config或Web.config进行默认的数据库设置，然后通过DbUtilityFactory获取DbUtility。

1.接口：

 using System;
 using System.Collections.Generic;
 using System.Data;
 using System.Linq.Expressions;
 using RA.DataAccess.MsSqlDbUtility;

 namespace RA.DataAccess
 {
     public interface IDbUtility
     {
         /// <summary>
         /// 获取Sql描述对象
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <returns></returns>
         SqlSession<T> GetSqlExpression<T>() where T : class;
         /// <summary>
         /// 获取多条数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="exp"></param>
         /// <returns></returns>
         List<T> GetList<T>(SqlSession<T> exp) where T : class;
         /// <summary>
         /// 获取多条数据，并填入DTO中
         /// </summary>
         /// <typeparam name="Target"></typeparam>
         /// <typeparam name="T"></typeparam>
         /// <param name="exp"></param>
         /// <returns></returns>
         List<Target> GetList<Target, T>(SqlSession<T> exp) where T : class where Target : class;
         /// <summary>
         /// 分页
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="By">OrderBy表达式</param>
         /// <param name="pageIndex"></param>
         /// <param name="pageSize"></param>
         /// <returns></returns>
         List<T> Paged<T>(Expression<Func<T, object>> By, int pageIndex, int pageSize = ) where T : class;
         /// <summary>
         /// 获取单条数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="func">linq表达式，代表条件</param>
         /// <returns></returns>
         T GetSingle<T>(Expression<Func<T, bool>> func) where T : class;
         /// <summary>
         /// 删除一条数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="func">linq表达式，代表条件</param>
         /// <returns></returns>
         int Delete<T>(Expression<Func<T, bool>> func) where T : class;
         /// <summary>
         /// 添加一条数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="obj"></param>
         /// <returns></returns>
         int Add<T>(T obj) where T : class;
         /// <summary>
         /// 直接执行Sql
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="sql"></param>
         /// <returns></returns>
         int RunSingleSql<T>(string sql) where T : class;
         /// <summary>
         /// 通过Sql获取DataTable，业务层不使用
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="sql"></param>
         /// <returns></returns>
         DataTable GetDataBySql<T>(string sql) where T : class;
         /// <summary>
         /// 更新一条数据
         /// </summary>
         /// <typeparam name="T">数据表实体</typeparam>
         /// <param name="obj">实体对象</param>
         /// <param name="func">linq表达式，代表条件</param>
         /// <returns></returns>
         int Update<T>(T obj, Expression<Func<T, bool>> func) where T : class;
         /// <summary>
         /// 添加多条数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="objs">实体对象列表</param>
         /// <returns></returns>
         int AddList<T>(List<T> objs) where T : class;
         /// <summary>
         /// 计数
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <param name="func">linq表达式，代表条件</param>
         /// <returns></returns>
         int Count<T>(Expression<Func<T, bool>> func = null) where T : class;
         /// <summary>
         /// 获取一个字段数据
         /// </summary>
         /// <typeparam name="T"></typeparam>
         /// <typeparam name="Target"></typeparam>
         /// <param name="field">linq表达式，代表字段</param>
         /// <param name="func">linq表达式，代表条件</param>
         /// <returns></returns>
         Target Scala<T, Target>(Expression<Func<T, Target>> field, Expression<Func<T, bool>> func);
     }
 }

2.工厂类：

 using System;
 using System.Configuration;

 namespace RA.DataAccess
 {
     public class DbUtilityFactory
     {
         public static IDbUtility GetDbUtility()
         {
             var getDbType = ConfigurationManager.AppSettings["DbType"];
             if (getDbType == "MySql")
             {
                 return MySqlDbUtility.DbUtility.GetInstance();
             }
             else if (getDbType == "MsSql")
             {
                 return MsSqlDbUtility.DbUtility.GetInstance();
             }
             else
             {
                 throw new NotSupportedException("不支持的数据库");
             }
         }
     }

 }