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.

Answer 2

The error message suggests that IRequestPackage does not inherit from IRequestFlightOnly - does it?

Note that the where clause is an AND relationship, not an OR - so your where clause is that T must implement ALL of

• IRequestPackage,
• IRequestDynamicPackage
• IRequestHotelOnly,
• and IRequestFlightOnly

Answer 3

看起来你想要一个IRequest接口或RequestBase抽象类;）

Answer 4

You should use a type that implement all the interfaces specified by the constraint, not just one of them. It's an "AND", not an "OR"...

Answer 5

You should reconsider your type constraints and your program structure. It looks like you are missing a base interface or an abstract base class.