title: Way to generate a unique number that does not repeat in a reasonable time?
body: <p>I'm integrating/testing with a remote web service and even though it's the "QA" endpoint, it still enforces a unique email address on <em>every</em> call.</p>
  
  <p>I can think of <code>DateTime.Now.Ticks</code> (e.g. 634970372342724417) and <code>Guid.NewGuid()</code>, but neither of those can be coalesced into an email with max. 20 chars (or can they?).</p>
  
  <p>I suppose it's not that hard to write out to a file a number that contains the last number used and then use <code>email1@x.com, email2@x.com, etc...</code> but if I can avoid persisting state I always do.</p>
  
  <p>Does anyone have a trick or an algorithm that gives something of a short length "guid" that is unique to a reasonably long time period (say a year) that I could use for my email addresses of max length 20 chars with (max length of guid) = 14 = 20 - length of "@x.com"?</p>