title: Is this value a valid year C#
body: <p>What's the best way to check if a string is a valid year using C#?</p>
  
  <p>I currently have a dropdown list that contains the values {'All','2009','2008'} etc, and I want to know whether the selection is one of the dates or the 'All' field.</p>
  
  <p>Currently I'm checking for <code>bool isYearValid = (Year.ToLower() == "all") ? false : true;</code></p>
  
  <p>How do I check whether the value is a valid year so that I don't have to have this hardcoded check for 'All'?</p>