Cannot Convert from error in simple class structure

I have the following class relationships

    public interface ICapability
    {
    }

    public interface IBaseService<T> where T : ICapability
    {
    }

    public abstract class BaseService<out T> : IBaseService<T> where T : ICapability
    {
       // modified...
       T MapEventToCapability(dynamic eventData, T capability);
    }

    public class SomeCapability : ICapability
    {
    }

    public partial class Service1 : BaseService<SomeCapability>
    {
        public Service1()
        {
        }
    }

    public class ServiceResolver 
    {
        public void Register(BaseService<ICapability> serviceToRegister)
        {
        }
    }

I try to invoke the Register method, passing in a new service1 as shown:

var b = new ServiceResolver();
var c = new Service1();

b.Register(c);

However I get a compile time error on c in the call to Register as follows;

Cannot convert Service1 to BaseService<ICapability>

I assumed that because Service1 is of type BaseService and that since SomeCapability is of type ICapability that this wouldn’t be an issue.

I tried casting to BaseService as well I tried changing the input parameter on Register to be an IBaseService and again casting but then I get a runtime error.

>Solution :

Note the question has been updated since this answer was posted – covariance is no longer an option having added a method which is incompatible

You’ll need to do 2 things to make this work

1. Make Register take IBaseService<ICapability> not BaseService<ICapability>
2. Make IBaseService covariant by marking the generic type as out – this is the same as the reason you can pass a List<Foo> to a merthod which expects an IEnumerable<Foo> as IEnumerable<T> is covariant in a similar manner.

public class ServiceResolver
{
    public void Register(IBaseService<ICapability> serviceToRegister)
    {
    }
}

and

public interface IBaseService<out T>
    where T : ICapability
{
}

Live example: https://dotnetfiddle.net/O0yXa5