title: how ReadOnlyDictionary from .net 4.5 does not expose .Add method ? (not why)
body: <p>I am curious how is it possible that <code>ReadOnlyDictionary</code> implements <code>IDictionary</code> and <code>ICollection</code> but it does not expose the methods <code>Add(TKey key, TValue value)</code></p>
  
  <pre><code>ReadOnlyDictionary&lt;object, object&gt; dictionary;
  dictionary.Add(null, null); // compilation error 
  
  IDictionary&lt;object,object&gt; i;
  i = dictionary;
  i.Add(null, null);// no compilation error
  </code></pre>