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;}}}}