TerritoryControllerAndService.cs

public class TerritoryController : Controller
{
    readonly ITerritoryService _terrService;

    public TerritoryController(ITerritoryService terrService)
    {
        _terrService = terrService;
        _terrService.AddValidationError = x => ModelState.AddModelError("", x);
    }

    public ViewResult SomeAction(int? id)
    {
        // do something with with _terrService
        return View(something);
    }
}

public class TerritoryService : ITerritoryService
{
    public Action<string> AddValidationError { get; set; }
    publc int SomeService(int id)
    {
      // ...
      if(theresAnError)
        AddValidationError("there was an error");
      // ...
      return someInt
    }
}