[DotNetDevelopment] Question about IList and inheritance

[graphicsxp]

Hello,

I've got two classes, CGMPost and MSMPost, which both inherits from a
class Post.

I have a IList called 'posts' that contains a collection of CGMPost
and MSMPost entities.

If I do that :

List<Post> myPosts = posts as List<Post>;

then myPosts is null (altough posts contains 7 entities of type
CGMPost and MSMPost).

Why is that ?