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?

CodesInChaos · Accepted Answer · 2011-03-18 08:37:55Z

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.

Stuart · Accepted Answer · 2011-03-18 08:37:03Z

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

MattDavey · Accepted Answer · 2011-03-18 08:54:02Z

2

Looks like you want an IRequest interface or RequestBase abstract class ;)

answered Mar 18, 2011 at 8:54

MattDavey

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Comments

Thomas Levesque · Accepted Answer · 2011-03-18 08:39:28Z

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"...

Falcon · Accepted Answer · 2011-03-18 08:40:26Z

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

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