title: Union vs Unionwith in HashSet
body: <p>What the difference between  <code>HashSet.Union</code>  vs <code>HashSet.Unionwith</code>  when i combine 2 hashsets.</p>
  
  <p>I am trying to combine like this:</p>
  
  <pre><code>HashSet&lt;EngineType&gt; enginesSupportAll = _filePolicyEvaluation.EnginesSupportAll;
          enginesSupportAll = enginesSupportAll != null ? new HashSet&lt;EngineType&gt;(engines.Union(enginesSupportAll)) : enginesSupportAll;
  </code></pre>
  
  <p>what is the best method for this example and why?</p>