title: Do using directives hurt performance or increase app size compared to full namespaces?
body: <p>I read somewhere that when you add <code>using System;</code> to your <code>.cs</code> file, the compiler (or maybe something else) adds all <code>System</code> related classes to your .cs file and so maybe it is better to not always add <code>using System;</code> and instead reference your <code>DateTime</code> with its full namespace like this: <code>System.DateTime</code>, for example, if it is easy enough to do and few things reference <code>System</code> it in your .cs file.</p>
  
  <p>Is that true and if so, can that hurt performance or increase the size of the app? If so, I realize that <code>using System;</code> is easier to write and is a convenience and so therefore, you must weigh convenience with performance. It also might be the case that only adding a <code>using System;</code> might not make much difference but when many references are added, maybe it could? Thanks!</p>