title: What makes a using directive unnecessary?
body: <p>What causes the c# compiler to say "using directive is unnecessary" on a using directive?</p>
  
  <p>Presumably current namespace. What else? Is this defined somewhere in the documentation? I couldn't find a definitive answer to a google search.</p>
  
  <p>EDIT to clarify, I'm looking for a list of places where the compiler looks to determine that a directive is not needed.</p>
  
  <p>E.g. 
  current namespace
  project references</p>
  
  <p>I'm not looking for a simple fix (e.g. remove it) I'm trying to understand <strong>why</strong> it happens. Pointing me to any documentation that explains it would be awesome.</p>