Best practice: multiple entities to single entity

Answer 1

Assuming that id represents the member's id, you could create 3 separate functions to handle each type, thus separating your concerns more than they are now.

Example:

[AcceptVerbs(HttpVerbs.Post)]
public JsonResult DeleteMusicStyleByMember(int id)
{
    if (!(Session["MemberLoggedIn"] is Member)) return Json(string.Empty);
    Member member = (Member)Session["MemberLoggedIn"];
    _memberService.DeleteMusicStyle(member, id);
    return SelectedMusicStyles();
}

[AcceptVerbs(HttpVerbs.Post)]
public JsonResult DeleteMusicStyleByBand(int id, int typeid)
{        
    Band band = _bandService.GetBand(typeid);
    _bandService.DeleteMusicStyle(band, id);
    return SelectedMusicStyles();
}

[AcceptVerbs(HttpVerbs.Post)]
public JsonResult DeleteMusicStyleByEvent
    (int id, int typeid)
{
    Event event = _eventService.GetEvent(typeid);
    _bandService.DeleteMusicStyle(event, id);
    return SelectedMusicStyles();
}

Then you would just modify your jquery post to go to the respective methods depending on what you're trying to do.

Answer 2

How would you refactor this code?

1) The code which checks the user is logged in should be moved:

 if (!(Session["MemberLoggedIn"] is Member)) return Json(string.Empty);
    Member member = (Member)Session["MemberLoggedIn"];

This is a cross cutting concern, which should be applied using a security framework, Spring pops to mind as an example.

2) I would avoid using a singleton pattern to represent this use-cases, they can quickly turn into a collection of scripts which when grow large can be difficult to know where to place code. Consider using the Command Pattern instead.

This pattern will allow you to return the results as JSON, XML or any other format based on the interfaces you which your command to conform too.

class DeleteMusicStyleByBandCommand : JsonResultModelCommand, XmlResultModelCommand {

  public DeleteMusicStyleByBand(int id, int typeid) {
     //set private members
  }

  public void execute() {
    ..
  }

  public JsonResult getJsonResult() { .. }

  public XmlResult getXmlResult() { .. }
}

The Command pattern IMHO is much better at representing use-cases than many methods in a Service..