[1] says (I am not an expert here hence I may be wrong but the restriction looks pretty strict): 

Containers implement the IAdaptable interface; extensions are managed by the platform's adapter manager.

See Also:
Platform.getAdapterManager(), IProject, IFolder, IWorkspaceRoot

Restriction:
This interface is not intended to be implemented by clients.

Restriction:
This interface is not intended to be extended by clients.

[1] http://help.eclipse.org/helios/index.jsp?topic=/org.eclipse.platform.doc.isv/reference/api/org/eclipse/core/resources/IContainer.html