c# inheritance - how to cast from base type to sub type?

Question

Background

I'm not sure my question in the title is actually expressed properly, but here's what I'm trying to do. (I am using a bogus analogy here to represent real code written by the team. We aren't allowed to post actual code. But I have tried to ensure that the concepts I see in our code base have been transferred over to this example)

I have some vehicle records in my database... some of them are cars, others will be trucks etc. I also have some controllers - endpoints for each vehicle type. So I have a Car controller that allows specific methods just for cars.

All of the columns in the database are identical for all types of vehicles. The vehicle DTO class looks like this:

 public class Vehicle
 {
        [Key]
        public int Id { get; set; }
        [Column]
        public int BrandName{ get; set; }
        [Column]
        public string VehicleDetails{ get; set; }

        public IDictionary<string, object> VehicleDetailsJson
        {
          get
          {
            return (IDictionary<string, object>)JsonConvert.DeserializeObject<List<ExpandoObject>>(VehicleDetails).FirstOrDefault();
          }
    }

But in the case of a car, we need additional fields. so we have this:

public class Car : Vehicle
{
  public static List<string> Models {get;} = new()
  {
     "two-seater",
     "four-seater",
     "six-seater"
  }

  public bool doSomethingToTheBrandName()
   {
      //does something to the brand string on the base class.
      return true; 
   }
}

Because all the database columns that are being return are the same, I'm using the vehicle model as a return type in my repository class. This is how I pull the data from the database: (pseudo-ish code)

    public class VehicleRepository: GenericRepository<Vehicle>, IVehicleRepository
     {
      private readonly string encryptionPassword;

     public VehicleRepository(AppSettings appSettings) : base(appSettings)
      {
        encryptionPassword = appSettings.EncryptionPassword;
      }

      public List<Vehicle> GetVehicles(string brandName= "")
      {
        var vehicles = new List<Vehicle>();

            var searchFilter = !string.IsNullOrEmpty(brand )
                               ? $"BrandName = @Brandname"
                               : "BrandName = 'all'";
            vehicles = Retrieve().ToList();
        
        return vehicles ;
    }

Here's the interface for the rpository:

public interface IVehicleRepository : IGenericRepository<Vehicle>
{
    List<Vehicle> GetVehicles(string brandName = "");
}

Problem

Where I'm running into issues is trying to call the repository from a car controller. This is where I'm not quite sure how to design / write the code.

The car controller uses requests and responses like this:

public class CarRequest: Request
{
    public string brandName{ get; set; }

}

public class CarResponse: Response
{
    public List<Car> Cars { get; set; }  //notice we are returning a car, not vehicle.
}

And here's the controller itself:

   public class CarController: ControllerBase
    {
        private readonly AppSettings appSettings;
        private readonly IVehicleRepository vehicleRepository;

    public CarResponse GetCars (CarRequest request)
    {
        try
        {
            var response = new GetCarResponse();
            response.Cars = vehicleRepository.Vehicles("ford");

        }
    }

Naturally, the system fails trying to save the response from the database call because I'm trying convert a vehicle into a car.

Questions

• Which class should be responsible for casting from the base type "Vehicle" to "Car".
• Also in the future, if there are Car specific methods that we need to write in a repository class, should we just do that inside the vehicle repo class?
• Anything else that smells off?

EDIT 1

So my real code actually has an if statement baked in (I'm including the suggested refactors)

public List<T> GetVehicles<T>(string brandName= "", int someFlag)
  {
    var vehicles = new List<T>();
    if (someflag==0)
    {
         var searchFilter = !string.IsNullOrEmpty(brand )
                       ? $"BrandName = @Brandname"
                       : "BrandName = 'all'";
         vehicles = Retrieve().OfType<T>.ToList();
    }
    else
    {
       vehicles.Add(Retrieve(someFlag));
    }
    return vehicles ;
   }

I'm sorry but I'm still kinda new to OO and c#. So I wasn't sure if by TVehicle you meant T. I've seen before. But the error I'm getting on this line:

 vehicles = Retrieve().OfType<T>.ToList();

is

CS0119 'Enumerable.OfType(IEnumerable)' is a method, which is not valid in the given context

and I haven't attempted to fix the Else path but that also shows the following error:

CS1503 Argument 1: cannot convert from 'Widgets.Models.Vehicle' to 'T'

EDIT 2

   public List<TVehicle> GetVehicles<TVehicle>(string brandName= "", int someFlag)
where TVehicle: Vehicle
      {
        var vehicles = new List<TVehicle();
        if (someflag==0)
        {
             var searchFilter = !string.IsNullOrEmpty(brand )
                           ? $"BrandName = @Brandname"
                           : "BrandName = 'all'";
             vehicles = Retrieve().OfType<TVehicle>.ToList();
        }
        else
        {
           vehicles.Add(Retrieve(someFlag));
        }
        return vehicles ;
       }

But I'm still getting the same two errors on the same two lines.

Answer 1

Generally it is a good practice to have a separated repo for each domain entity. So it may worth creating an abstract Vehicle repository and derive other repose from it (not tested):

public abstract class VehicleRepository<TVehicle>: GenericRepository<TVehicle>, IVehicleRepository
where TVehicle:Vehicle
     {
      private readonly string encryptionPassword;

     public VehicleRepository(AppSettings appSettings) : base(appSettings)
      {
        encryptionPassword = appSettings.EncryptionPassword;
      }

      public List<TVehicle> GetVehicles(string brandName= "")
      {
        var vehicles = new List<TVehicle>();

            var searchFilter = !string.IsNullOrEmpty(brand )
                               ? $"BrandName = @Brandname"
                               : "BrandName = 'all'";
            vehicles = Retrieve().ToList();
        
        return vehicles ;
}

and then define TruckVehicle, CarVehicle ,.. inheriting from that class.

Answer 2

One of the solution to this is can be that, we will need to define the method generic so that we can pass the type from calling side something like:

  public List<TVehicle> GetVehicles<TVehicle>(string brandName= "")
  {
    var vehicles = new List<TVehicle>();

    var searchFilter = !string.IsNullOrEmpty(brand )
                       ? $"BrandName = @Brandname"
                       : "BrandName = 'all'";
    vehicles = Retrieve().OfType<TVehicle>.ToList();
    
    return vehicles ;
}

and now on calling side it should be something like:

var cars = GetVehicles<Car>("brand name");