title: how to convert datetime? to datetime
body: <p>I have filed <code>txtb_dateOfService</code> is required to complete the form but if <code>txtb_dateOfService</code> is emapty return null if not  TryParse the date <code>&gt;</code> I had this error I don't know how to fix it</p>
  
  <blockquote>
    <p>The best overloaded method match for
    'System.DateTime.TryParse(string, out System.DateTime)' has some
    invalid arguments</p>
  </blockquote>
  
  <pre><code>    DateTime? dateOfService= null;
      if (string.IsNullOrEmpty(txtb_dateOfService.Text))
      {
          dateOfService = null;
  
      }
      else
          if (DateTime.TryParse(txtb_dateOfService.Text, out dateOfService))
          {
  
          }
  </code></pre>