让Dapper在一个项目中支持多种库

　　如果想在一个项目中，用DapperPlus支持多种数据库该怎么做？

　　在《让Dapper支持Mock》中我们定义了DapperPlus，可以基于这个类，实现两个子类：MySqlDapperPlus，MsSqlDapperPls，在这两个子类的构造中适配对应的数据库类型，从注放容器中，获取IDbConnection实例，根据实例的类型来选取配置中的对应连接字符串，这里用到的是根据数据类型来配置，也是一种约定。

MySqlDapperPlus.cs

using Microsoft.Extensions.Configuration;
using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;

namespace WebDemo01.Services
{
    public class MySqlDapperPlus : DapperPlus
    {
        public MySqlDapperPlus(IEnumerable<IDbConnection> connections, IConfiguration configuration)
        {
            var connectionStrings = configuration.GetSection("ConnectionStrings").Get<Dictionary<string, string>>();          
            _connection = connections.FirstOrDefault(c => c.GetType().Name == "MySqlConnection");
            _connection.ConnectionString = connectionStrings.Where(s => s.Key.ToLower().Contains("mysql")).FirstOrDefault().Value;
        }
    }
}

MsSqlDapperPlus.cs

using Microsoft.Extensions.Configuration;
using System;
using System.Collections.Generic;
using System.Data;
using System.Linq;

namespace WebDemo01.Services
{
    public class MsSqlDapperPlus : DapperPlus
    {
        public MsSqlDapperPlus(IEnumerable<IDbConnection> connections, IConfiguration configuration)
        {
            var connectionStrings = configuration.GetSection("ConnectionStrings").Get<Dictionary<string, string>>();          
            _connection = connections.FirstOrDefault(c => c.GetType().Name == "SqlConnection");
            _connection.ConnectionString = connectionStrings.Where(s => s.Key.ToLower().Contains("mssql")).FirstOrDefault().Value;
        }
    }
}

　　这时，会有问题，DapperPlus没有无参构造，_connection访问级别也太低，所以要改造一下DapperPlus。

/// <summary>
    /// DappePlusr类
    /// </summary>
    public class DapperPlus : IDapperPlus
    {
        protected IDbConnection _connection;
        /// <summary>
        /// 无参构造函数
        /// </summary>
        public DapperPlus()
        {
        }
        //下面和原来的一样
    }

public void ConfigureServices(IServiceCollection services)
{
        services.AddControllers();
            
        services.AddScoped<IDbConnection, MySqlConnection>();
        services.AddScoped<IDbConnection, SqlConnection>();
        services.AddScoped<IDapperPlus, MySqlDapperPlus>();
        services.AddScoped<IDapperPlus, MsSqlDapperPlus>();
}

appsettings.json

"ConnectionStrings": {
    "MySqlConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=mysql_testdb",
    "MsSqlConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=mssql_testdb"
  }

　　如果是多个库，还要读写分离该怎么实现？其实和不分离是一样的，要改造《让Dapper读写分离》中的DapperPlusWrite和DapperPlusRead两个类，分别增加无参构造函数，和把_connection改成protected，方便子类中参访问到。

　　然后定义三个类：MySqlDapperPlusRead和MsSqlDapperPlusRead继承DapperPlusRead；MySqlDapperPlusWrite和MsSqlDapperPlusWrite继承DapperPlusWrite。在四个类的构造函数中，按照自己数据库的类型，Read或Write类型来取配置文件中的连接字符串即可。

　　Startup.cs

public void ConfigureServices(IServiceCollection services)
{
      services.AddControllers();
      services.AddScoped<IDbConnection, MySqlConnection>();
      services.AddScoped<IDbConnection, SqlConnection>();
      services.AddScoped<IDapperPlusRead, MySqlDapperPlusRead>();
      services.AddScoped<IDapperPlusRead, MsSqlDapperPlusRead>();
      services.AddScoped<IDapperPlusWrite, MySqlDapperPlusWrite>();
      services.AddScoped<IDapperPlusWrite, MsSqlDapperPlusWrite>();
}

　　appsettings.json

"ConnectionStrings": {
    "MySqlReadConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=read_mysql_testdb",
    "MySqlWriteConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=write_mysql_testdb",
    "MsSqlReadConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=read_mssql_testdb",
    "MsSqlWriteConnectionString": "server=127.0.0.1;uid=root;pwd=root;database=write_mssql_testdb"
  }

　　最后，在业务的Service中，读有两个，按类型区分，写有两个，按类型区分，代码如下：

public class GoodsService : IGoodsService
    {
        private readonly IDapperPlusWrite _mySqlDapperWrite;
        private readonly IDapperPlusWrite _msSqlDapperWrite;
        private readonly IDapperPlusRead _mySqlDapperRead;
        private readonly IDapperPlusRead _msSqlDapperRead;
        public ShopService(IEnumerable<IDapperPlusWrite> dapperWrites, IEnumerable<IDapperPlusRead> dapperReads)
        {
            foreach (var dapperWrite in dapperWrites)
            {
                switch (dapperWrite)
                {
                    case MySqlDapperPlusWrite mySqlDapperPlusWrite:
                        _mySqlDapperWrite = mySqlDapperPlusWrite;
                        break;
                    case MsSqlDapperPlusWrite msSqlDapperPlusWrite:
                        _msSqlDapperWrite = msSqlDapperPlusWrite;
                        break;
                }
            }
            foreach (var dapperRead in dapperReads)
            {
                switch (dapperRead)
                {
                    case MySqlDapperPlusRead mySqlDapperPlusRead:
                        _mySqlDapperRead = mySqlDapperPlusRead;
                        break;
                    case MsSqlDapperPlusRead msSqlDapperPlusRead:
                        _msSqlDapperRead = msSqlDapperPlusRead;
                        break;
                }

            }
        }
    }

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