Question

I have

public interface IFoo
{
   IEnumerable<IThingy> Thingies{get;}
}

I want to then be able to do

class Thing1 : IThingy
{
   ...
}
class ImplementFoo : IFoo
{
   List<Thing1> m_things;
   IEnumerable<IThingy> Thingies {get {return m_things;}}
}

ImplementFoo.Thingies returns an IList (which is an IEnumerable) of Thing1s (which are IThings). So in theory this code should work, but it does not. VS suggests a cast in the getter; that compiles but fails at run time. Am I expecting too much of covariance in c# 4?

VS 2010 -> Silverlight 4. Here’s the compile error

Cannot implicitly convert type ‘System.Collections.Generic.List<MyProj.Column>‘ to ‘System.Collections.Generic.IEnumerable<MyProj.IColumn>‘. An explicit conversion exists (are you missing a cast?)

EDIT: People tell me this should work, but it doesnt work in SL4

Answer 1

THis is a difference between SL4 and CLR4. THe IEnumerable interface is not marked ‘out’. Apparently fixed in SL5