派生类型不能隐式转换为基接口

interface IModel {}

class MyModel : IModel {}

interface IRepo<T>
    where T: IModel { }

class Repo : IRepo<MyModel> { }

//EDIT: A smaller example
IRepo<IModel> repo = new Repo(); // Cannot implicitly convert.. An explicit convertion exists. Missing cast?

// Old example:
/*
The type 'Repo' cannot be used as type parameter 'C' in the generic type or method.
'Castle.MicroKernel.Registration.ComponentRegistration<S>.ImplementedBy<C>()'.
==> There is no implicit reference conversion from 'Repo' to 'IRepo<IModel>'.
*/
container.Register(
    Component.For<IRepo<IModel>>()
   .ImplementedBy<Repo>());

但是repo是从irepo派生的,mymodel是从imodel派生的。为什么这个不行?

我尝试在repo上添加隐式运算符,但不允许在接口之间转换。

这是由C 4的co/contra变量解决的吗(不,我不知道我在说什么:)?