C# Returning a generic interface from a factory

Question

I've implemented a Vehicle service that is responsible for servicing vehicles such as cars and trucks:

public interface IVehicleService
{
    void ServiceVehicle(Vehicle vehicle);   
}

public class CarService : IVehicleService
{
    void ServiceVehicle(Vehicle vehicle)
    {
        if(!(vehicle is Car))
            throw new Exception("This service only services cars")

       //logic to service the car goes here
    }
}

I also have a vehicle service factory that is responsible for creating a vehicle service according to the type of vehicle passed in to the factory method:

public class VehicleServiceFactory 
{
    public IVehicleService GetVehicleService(Vehicle vehicle)
    {
        if(vehicle is Car)
        {
            return new CarService();
        }

        if(vehicle is Truck)
        {
            return new TruckService();
        }

        throw new NotSupportedException("Vehicle not supported");
    }

}

The main issue I have is specifically with CarService.ServiceVehicle method. It's accepting a Vehicle when ideally it should accept a Car instead, as it knows that it will only service cars. So I decided to update this implementation to use generics instead:

public interface IVehicleService<T> where T : Vehicle
{
    void ServiceVehicle(T vehicle); 
}

public class CarService : IVehicleService<Car>
{
    void ServiceVehicle(Car vehicle)
    {
        //this is better as we no longer need to check if vehicle is a car

        //logic to service the car goes here 
    }
}

The issue I'm having is how to update VehicleServiceFactory to return the generic version of the vehicle service. I've tried the following but it results in a compilation error as it's unable to cast CarService to the generic return type IVehicleService:

public class VehicleServiceFactory 
{
    public IVehicleService<T> GetVehicleService<T>(T vehicle) where T : Vehicle
    {
        if(vehicle is Car)
        {
            return new CarService();
        }

        if(vehicle is Truck)
        {
            return new TruckService();
        }

        throw new NotSupportedException("Vehicle not supported");
    }

}

Any suggestions would be appreciated.

Answer 1

Simply cast the service to the interface:

return new CarService() as IVehicleService<T>;

You know T is car but the compiler doesn't, its not smart enough to follow the method logic nor is it meant to be; as far as the compiler knows, T can be anything as long as its a Vehicle. You need to tell the compiler, "Hey, I know what I'm doing, T and Car are in fact the same type."