title: How to return DateTime.Today as a FieldNullValue in FileHelpers
body: <p>I'm trying to set the <code>FieldNullValue</code> for a <code>DateTime</code> variable in a <code>FileHelpers</code> class to the Today value (so if when I read the .csv there is no entry, it defaults to Today's date). The code I've tried is:</p>
  
  <pre><code>[FieldOptional]
  [FieldNullValue(DateTime.Today)]
  public DateTime DATE;
  </code></pre>
  
  <p>However I get the error </p>
  
  <blockquote>
    <p>"Error    1   An attribute argument must be a constant expression, typeof
    expression or array creation expression of an attribute parameter
    type".</p>
  </blockquote>
  
  <p>Please can someone explain what I'm doing wrong?</p>