title: using namespace
body: <p>what is the difference between   </p>
  
  <pre><code>using System;     
  </code></pre>
  
  <p>and  </p>
  
  <pre><code>using namespace System;    
  </code></pre>
  
  <p>is it the same thing?   </p>
  
  <p>thanks</p>