Question

I want to create a delegate pointing to a static function…the static function has a second parameter of type Flight, the Flight class implements ITravelObject interface and the second delegate parameter requires this interface.

The following code doesn’t work. Why is that? Is there any other possible implementation?

namespace CovarianceContravarianceTest
{

    public class Data {}

    public interface ITravelObject
    {
        int ID { get; set; }
    }

    public abstract class BaseObject {}

    public class Flight : BaseObject, ITravelObject
    {
        int ITravelObject.ID {get;set;}
    }

    public static class LetService
    {
        public static void DoSomething(Data da, Flight let) {}
    }

    public delegate void TravelServiceMethodDelegate(Data dataAccess,ITravelObject     travelObject);

    class Program
    {
        static void Main(string[] args)
        {
            TravelServiceMethodDelegate test = new TravelServiceMethodDelegate(LetService.DoSomething);
        }
    }

}

This results in error

No overload for ‘DoSomething’ matches delegate ‘CovarianceContravarianceTest.TravelServiceMethodDelegate’

thanks everyone for answers, I’ve already figured out this isn’t even a covariance/contravariance issue

Answer 1

You are trying to plug this method

public static void DoSomething(Data da, Flight let) {} 

into this delegate

var test = new TravelServiceMethodDelegate(LetService.DoSomething);

This cannot work. Why? Consider the following code:

class CarRide : ITravelObject { }

void GoForACarRide(TravelServiceMethodDelegate travelService) {
    travelService.Invoke(someData, new CarRide());
}

This compiles, because CarRide implements ITravelObject.

However, if I call GoForACarRide(test), it will break, since test points to DoSomething, which expects a Flight.