Question

Lets create some interfaces

public interface ITimeEventHandler
    {
        string Open();
    }

    public interface IJobTimeEventHandler : ITimeEventHandler
    {
        string DeleteJob();
    }

    public interface IActivityTimeEventHandler : ITimeEventHandler
    {
        string DeleteActivity();
    }



    public interface ITimeEvent
    {
        ITimeEventHandler Handler { get; }
    }

    public interface IJobTimeEvent : ITimeEvent
    {
        int JobID { get;  }
    }

Create a class

public class JobTimeEvent : IJobTimeEvent
    {
        public int JobID
        {
            get; internal set;

        }

        public IJobTimeEventHandler Handler
        {
            get; internal set;
        }

    }

My question is .. when implementing an interface which define a base class property why cant the class implementing interface return a derived class type object ??

For ex in class JobTimeEvent, IJobtimeEvent needs a property of type ITimeEventHandler but why IJobTimeEventHandler type is not allowed which derived from ITimeEventHandler

Answer 1

This is a duplicate of

Why C# doesn't allow inheritance of return type when implementing an Interface

The feature you want is called “return type covariance”, and it is a frequently requested feature in C#. It is not supported by the CLR and we have no plans to implement it in C#, sorry!