title: Select all elements from dictionary when the key is contained in the second dictionary
body: <p>I have two dictionaries:
  <code>Dictionary&lt;DateTime, decimal&gt; d1;</code> and <code>Dictionary&lt;DateTime, decimal&gt; d2;</code></p>
  
  <p>I want two execute a linq query to select from d1 all elements where <code>d2..ContainsKey(d1.key);</code></p>