Question

I have a generic interface hierarchy which describes some controllers for other generic types, and I’m having trouble clarifying in my mind why a particular casting scenario is not valid.

Simplified code is as follows;

// 'DTO' interfaces
public interface IBase
{ }

public interface IDerived : IBase
{ }


// 'DTOs'
public class Base : IBase
{ }

public class Derived : Base, IDerived
{ }


// controller interfaces
public interface IBaseController<T> where T : class, IBase
{ }

public interface IDerivedController : IBaseController<IDerived>
{ }


// controllers
public class BaseController<T> : IBaseController<T>
    where T : class, IBase
{ }

public class DerivedController : BaseController<IDerived>, IDerivedController
{ }

Now, the situation I’m having trouble with is this;

IDerivedController x = new DerivedController();
bool is1 = x is IDerivedController;   // true
bool is2 = x is IBaseController<IDerived>; // true
bool is3 = x is IBaseController<IBase>; // false ???

The final line is where my confusion lies. The controller interfaces associate correctly, and the ‘DTOs’. But not both together… ?

Answer 1

See http://msdn.microsoft.com/en-us/library/dd997386 for info on variant generic interfaces, which is the heart of what you’re asking. You must declare that an IBaseController<IDerived> can in fact be used as an IBaseController<IBase> in order for it to be used as such.

For the reason for this, consider trying to use a List<string> as an IList<object> vs trying to use an IEnumerable<string> as an IEnumerable<object>. The IEnumerable conversion is ok because you can use the items as objects without any problems, but you can’t add any object to a List<string>, because then it would contain something besides strings.

If you change it to public interface IBaseController<out T> where T : class, IBase, then x is IBaseController<IBase> becomes true. If the compiler complains that you can’t make T an out parameter, then you can’t do the conversion you’re looking to do (unless you change the interface to make it compatible).