Generic interface & implementation issue

[link9]

Hello all -

I am having an issue trying to implement a generic interface. Essentially, I have several objects in my library that I need to be able to save their state. So, I created an interface to carry this out. The pertinent bits look like this:

public interface IStateSaver
{
  ...
  List<T> GetSavedItems<T>(IPrincipal p_User) where T : IStateSaver;
  ...
}

So that any user can just call to an implementer and say "Hey, give me all of my objects" and a list of those objects will be returned.

My problem is coming when I go to implement the interface. I must be missing something on how to use generics properly in this situation. An example implementer looks like this:

public class Options : IStateSaver
{
   ...
   public List<T> GetSavedItems<T>(IPrincipal p_User) where T : IStateSaver
   {
      List<IStateSaver> output = new List<IStateSaver>();
      ...
      //list population goes here
      ...
      return output;
   }
   ...
}

The compiler is telling me that I:
"Cannot implicitly convert type 'System.Collections.Generic.List<Cmi.Tabulation.IStateSaver>' to 'System.Collections.Generic.List<T>"

Ok, I understand what the message is telling me, but I've used a conditional that says T must be an IStateSaver. So what gives?

What fundamental piece of knowledge am I missing here?

Any input greatly appreciated.

Thanks,
-paul

The answer to getting answered -- faq855-2992

 

[link9]

<smack>
The solution, of course, is to simply make the method return a List<IStateSaver> if that's what you want, rather than the generic T.
</smack>

The reason that this doesn't work is because of a well documented gotcha of Generics, which is Implicit Type Conversion (or a lack thereof, actually). Here's a link if anyone is interested:

http://www.25hoursaday.com/weblog/2007/08/09/CGenericsImplicitTypeConversionHell.aspx

-p

The answer to getting answered -- faq855-2992