title: cannot convert from Dictionary to IDictionary
body: <p>I'm trying to create interface with method</p>
  
  <pre><code>void Import(int id, IDictionary&lt;int, IDictionary&lt;int, string&gt;&gt; items);
  </code></pre>
  
  <p>then call this method like</p>
  
  <pre><code>Dictionary&lt;int, Dictionary&lt;int, string&gt;&gt; items = viewModel.Items
    .GroupBy(t =&gt; t.Number)
    .ToDictionary(t =&gt; t.Key, t =&gt; t.ToDictionary(x =&gt; x.LanguageId, x =&gt; x.Translation));
  
  
  Import(id, items);
  </code></pre>
  
  <p>I expected that it should works but got an error</p>
  
  <pre><code>cannot convert from 'System.Collections.Generic.Dictionary&lt;int, System.Collections.Generic.Dictionary&lt;int, string&gt;&gt;' to 'System.Collections.Generic.IDictionary&lt;int, System.Collections.Generic.IDictionary&lt;int, string&gt;&gt;'  
  </code></pre>
  
  <p>Why I can't use interface IDictionary here? Should I cast manually?</p>