title: Significance of 'in' 'out' and 'retval' inside parameters in C#
body: <pre><code>Output([out, retval] long* Retval);
  Length([in] long Len);
  </code></pre>
  
  <p>What is the significance of 'out', 'retval' and 'in' in this case ?? </p>
  
  <p>I know the significance of ref and out which is used usually, but not this.</p>
  
  <p>Thanks. </p>