class MySqlFlag {}
class SqlServerFlag {}

public void ConfigureServices(IServiceCollection services)
{
    var fsql1 = new FreeSqlBuilder().UseConnectionString(DataType.MySql, "str1")
        .Build<MySqlFlag>();
    var fsql2 = new FreeSqlBuilder().UseConnectionString(DataType.MySql, "str1")
        .Build<SqlServerFlag>();
    services.AddSingleton<IFreeSql<MySqlFlag>>(fsql1);
    services.AddSingleton<IFreeSql<SqlServerFlag>>(fsql2);
}

[Route("api/[controller]")]
[ApiController]
public class ValuesController : ControllerBase
{
    public ValuesController(IFreeSql<MySqlFlag> mysql, IFreeSql<SqlServerFlag> sqlserver)
    {
    }
}

public enum DbEnum { db1, db2, db3 }
var fsql = new FreeSqlCloud<DbEnum>(); //提示：泛型可以传入 string
fsql.DistributeTrace = log => Console.WriteLine(log.Split('\n')[0].Trim());
fsql.Register(DbEnum.db1, () => new FreeSqlBuilder()
    .UseConnectionString(DataType.Sqlite, @"Data Source=db1.db")
    .Build());
fsql.Register(DbEnum.db2, () => new FreeSqlBuilder()
    .UseConnectionString(DataType.Sqlite, @"Data Source=db2.db")
    .Build());
fsql.Register(DbEnum.db3, () => new FreeSqlBuilder()
    .UseConnectionString(DataType.Sqlite, @"Data Source=db3.db")
    .Build());

fsql.Select<T>();
fsql.Insert<T>();
fsql.Update<T>();
fsql.Delete<T>();
//...

fsql.Change(DbEnum.db3).Select<T>();
//同一线程，或异步await 后续 fsql.Select/Insert/Update/Delete 操作是 db3
fsql.Use(DbEnum.db3).Select<T>();
//单次有效

//对 fsql.CRUD 方法名 + 实体类型 进行拦截，自动定向到对应的数据库，达到自动 Change 切换数据库目的
fsql.EntitySteering = (_, e) =>
{
    switch (e.MethodName)
    {
        case "Select":
            if (e.EntityType == typeof(T))
            {
                //查询 T 自动定向 db3
                e.DBKey = DbEnum.db3;
            }
            else if (e.DBKey == DbEnum.db1)
            {
                //此处像不像读写分离？
                var dbkeyIndex = new Random().Next(0, e.AvailableDBKeys.Length);
                e.DBKey = e.AvailableDBKeys[dbkeyIndex]; //重新定向到其他 db
            }
            break;
        case "Insert":
        case "Update":
        case "Delete":
        case "InsertOrUpdate":
            break;
    }
};