how ReadOnlyDictionary from .net 4.5 does not expose .Add method ? (not why)

Question

I am curious how is it possible that ReadOnlyDictionary implements IDictionary and ICollection but it does not expose the methods Add(TKey key, TValue value)

ReadOnlyDictionary<object, object> dictionary;
dictionary.Add(null, null); // compilation error 

IDictionary<object,object> i;
i = dictionary;
i.Add(null, null);// no compilation error

Answer 1

ReadOnlyDictionary explicitly implements the Add methods (both in generic and non-generic form). Something like this:

void IDictionary.Add(Object key, Object value)

You can call it if you cast it to an IDictionary, but it will throw a NotSupportedException according to MSDN.