Generic interface in C#

Question

I have a generic interface and I want to constrain types that this generic parameter can accept. Here is the interface:

public interface IBaseRequestRepository<T> where T : IRequestPackage,
       IRequestDynamicPackage, IRequestHotelOnly, IRequestFlightOnly 
{ 
        IList GetByOccupancyAndTravelDate(int occupancyId, int travelBegYear,
                                          int travelBegDate, int travelEndYear,
                                          int travelEndDate); 
}

But this gives an error:

Error 1 The type 'IRequestPackage' cannot be used as type parameter 'T' in the generic type or method 'IBaseRequestRepository'. There is no implicit reference conversion from 'IRequestPackage' to 'IRequestFlightOnly'.

Any suggestions?

Answer 1

You need to satisfy all generic constraints and not just one.

Thus you can't substitute IRequestPackage into T because it doesn't derive from all the other interfaces.

You can pass in either an interface type that inherits from all the interfaces you specified as a constraint or a class type that implements all these interfaces.