[Mono-list] Collections of generic objects: IDictionary casting problem

[Generic 2006]

Hi all

I have a problem casting collections of generic objects.

I've implemented two collections using the IDictionary<K,V> generic
interface; the values of these two collections are typed using classes
that have a common generic ancestor (Wrapper<InnerType>). I tried to
cast both of them using such common denominator (Wrapper<InnerType>),
but I got both runtime and compiletime exceptions (see below).
Probably I missed some subtlety of the semantics of generics constructs,
but I'd like your opinion.
Many Thanks!

These are the available classes:
class OuterTypes1 : IDictionary<String,OuterType1>{...}
class OuterType1 : Wrapper<InnerSubType1>{...}
class InnerSubType1 : InnerType{...}

class OuterTypes2 : IDictionary<String,OuterType2>{...}
class OuterType2 : Wrapper<InnerSubType2>{...}
class InnerSubType2 : InnerType{...}

class Wrapper<T> where T : InnerType{...}

class InnerType{...}

This is the inheritance hierarchy:
InnerType
+-InnerSubType1
+-InnerSubType2
Wrapper<InnerType>
+-OuterTypes1
+-OuterTypes2

Here it is a sample excerpt from my code:
{
 OuterTypes1 myOuterTypes1;
 OuterTypes2 myOuterTypes2;
  ...

 {
   IDictionary<String,OuterType1> types;
   /*
    This assignment works fine.
   */
   types = (IDictionary<String,OuterType1>)myOuterTypes1;
  }

 {
  IDictionary<String,Wrapper<InnerType>> types;
   /*
    The following assignment throws a compile-time exception:

error CS0266: Cannot implicitly convert type
'System.Collections.Generic.IDictionary<System.String,OuterType1>' to
'System.Collections.Generic.IDictionary<System.String,Wrapper<InnerType>>'.
An explicit conversion exists (are you missing a cast?)

    but I can't figure out why, as Wrapper<InnerType> should be a super
type of OuterType1 (which extends Wrapper<InnerSubType1>),
    as InnerType is a super type of InnerSubType1.
   */
   types = (IDictionary<String,OuterType1>)myOuterTypes1;
  }

  {
  IDictionary<String,Wrapper<InnerType>> types;
   /*
    The following assignment throws a run-time exception:

System.InvalidCastException: Cannot cast from source type to destination
type

    but I can't figure out why, as Wrapper<InnerType> should be a super
type of OuterType1 (which extends Wrapper<InnerSubType1>),
    as InnerType is a super type of InnerSubType1.
   */
   types = (IDictionary<String,Wrapper<InnerType>>)myOuterTypes1;
  }
}