title: Math with Enums (e.g. DayOfWeek) in C#
body: <p>Why is it that the following code won't work:</p>
  
  <pre><code>endDate.AddDays(7-endDate.DayOfWeek);
  </code></pre>
  
  <p>While this will:</p>
  
  <pre><code>endDate.AddDays(0-endDate.DayOfWeek + 7);
  </code></pre>
  
  <p>?</p>
  
  <p>(By "won't work" I mean results in the following compilation error: "cannot convert from 'System.DayOfWeek' to 'double'")</p>