Help refactoring with attention to SRP and OCP

i have collection class representing users in database, called users.  want add load function class data database.  load function later changed implement kind of asynchronous loading and/or ui update on it's progress know want separate class , utilize interface.  here's i'm thinking far:

public  interface  iload     function  load() as  integer  end  interface   public  enum  databasetype     mysql     mssql end  enum   public  class  loadfactory     private  _load as  iload  	public  sub  new (databasetype as  enum .databasetype, connectionstring as  string )         select  case  databasetype             case  databasetype.mysql                 _load = new  mysqlload(connectionstring)             case  databasetype.mssql                 _load = new  mssqlload(connectionstring)         end  select      end  sub       public  function  getinstance() as  iload         return  _load     end  function  end  class   public  class  users     'collection code here       'implement iload interface      private  _load as  iload      public  function  load() as  integer          'to do: load factory constructor values global variables          dim  loadfactory as  new  loadfactory(databasetype.mysql, "myconnectionstring" )         _load = loadfactory.getinstance         return _load.load()     end  function  end  class  

when need load data users collection use:

dim users new users
users.load()

however, later on decide add more load functionality.  specifically, want execute this:

dim users new users
users.load("select * users office = 2")

way accomplish adding functionality load interface, so?

public  interface  iload<br/>     function  load() as  integer <br/>     function  load(byval  selectstatement as  string )<br/>     function  loadall() as  integer <br/> end  interface  

in above example have couple of classes implement iload , wouldn't hard locate , update, i'm wondering if there better way create interface , abstract classes functionality can later incorporated? feedback on code above , thought process appreciated.

in design users class instantiates new loadfactory. that’s not srp nor ocp.

the example below (c#) uses datamapper pattern. might want decouple dal implementations packaging them in separate assemblies , inject them using di framework (for example unity). allows add new databases without need recompile project.

ps: might want take @ object-relational mapping tools.

srp:

the userscollection class represents collection of users.
the mysqldal class retrieves data mysql database.
the mssqldal class retrieves data form sql server database.
the usermapper class moves data between database , user object.

ocp:

the idal interface defines contract data access components implement.

idal {
  datatable getallusers();
  datatable getusers(string statement);
  datatable getbyid(int id);
};

usermapper {

  public usermapper(idal dal) {
    this.userdal = dal;
  }

  userscollection getallusers() {
    userscollection result = new userscollection();
    datatable userstable = dal.getallusers();

    foreach(datarow[] userrow in userstable.rows) {     
      result.add(new user(userrow[0], .., .., ..));
    }
  }
};

mysqldal : idal {
  ..
};

mssqldal : idal {
  ..
};

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