title: how to retrieve keys only when filter by value
body: <pre><code>ConcurrentDictionary&lt;int, int&gt; dic = new ConcurrentDictionary&lt;int, int&gt;();
  dic.AddOrUpdate(1, 2, (s, i) =&gt; 0);
  dic.AddOrUpdate(2, 3, (s, i) =&gt; 0);
  dic.AddOrUpdate(3, 1, (s, i) =&gt; 0);
  dic.AddOrUpdate(4, 7, (s, i) =&gt; 0);
  </code></pre>
  
  <p>I want to only select keys where the value is greater than 5. How can I do this?</p>